SAHLGRENSKA ACADEMY

EVALUATION OF ECLIPSE TREATMENT PLANNING SYSTEM FOR CALCULATION OF RADIATION DOSES TO PATIENTS TREATED WITH TOTAL BODY IRRADIATION AT EXTENDED TREATMENT DISTANCE

Linnéa Karlsson

Thesis: 30 hp

Program: Medical Physics Programme

Level: Second Cycle

Semester/year: Autumn 2018

Supervisors: Roumiana Chakarova,

Kerstin Müntzing,

Caroline Adestam Minnhagen

Examiner: Magnus Båth

Abstract Thesis: 30 hp

Program: Medical Physics

Level: Second Cycle

Semester/year: Autumn 2018

Supervisors: Roumiana Chakarova,

Kerstin Müntzing,

Caroline Adestam Minnhagen

Examiner: Magnus Båth

Keyword: total body irradiation, TBI, treatment planning system,

TPS, Eclipse

Purpose: The aim of this study was to evaluate the current treatment planning system

(TPS) for external radiotherapy at Sahlgrenska University Hospital (SU),

Eclipse, for total body irradiation (TBI) at extended treatment distance.

Theory: External radiotherapy is used for treatment of different cancer diseases. High

energy photons are usually directed towards the tumour. When planning for the

radiotherapy treatment, most commonly, a computer-based TPS is used.

Situations, where the whole body needs to be irradiated, are called total body

irradiation. There are many parameters that differ between each hospital

performing TBI. For example, prescribed dose, number of fractions, dose rate,

source-to-skin distance (SSD) and delivery technique. The TPS used at SU is

not validated for TBI, which is performed at extended treatment distance.

Method: Absolute doses, percental depth dose (PDD), profiles, off-axis values and

transmission at two different SSDs (350 cm and 460 cm) for two linear

accelerators, TrueBeam and Clinac iX were studied in phantom geometries

created in Eclipse version 13.6.23. The clinical field currently used, as well as

other field setups, was investigated, including multileaf collimator (MLC) fields

and fields defined by jaws, smaller than the clinical field. Results from earlier

measurements and Monte Carlo simulations of the clinical field were compared

with the results from Eclipse. Additional measurements were performed in a

solid water phantom for different field sizes and corresponding Eclipse data

were evaluated. Dose distribution comparisons between Monte Carlo

simulations and Eclipse for patients previously treated with TBI in Gothenburg

were performed. The possibility to improve the homogeneity of the dose

distribution in patients was investigated by implementing the field-in-field

technique in Eclipse.

Results: The absolute doses for the clinical field, determined in Eclipse, at 10 cm depth

were up to 4.3 % higher than the measured at SSD = 460 cm and 0.1 % higher

than measured at SSD = 350 cm. The PDD in Eclipse compared to measured

PDD was in good agreement. The cross-section area of the phantom,

perpendicular to the beam axis, had a larger effect on the dose deviation at 10

cm depth, than the depth of the phantom. Profiles at extended SSD > 275 cm

showed oscillations with increased amplitude related to increased SSD. For

smaller fields defined by jaws, the doses differences were 1.9 % higher in

Eclipse than the corresponding measurements. If the fields were defined by

MLCs instead, the deviations increased to 3.0 % at SSD = 350 cm and to 4.5 %

at SSD = 460 cm. Patient doses calculated in Eclipse varied compared to the

Monte Carlo calculated doses. Both higher and lower deviations were observed

up to 4 % for Dmean when dose-volume-histogram for the body was studied. The

field-in-field technique was feasible, but the planning strategy was highly

dependent on the individual patient size and anatomy features.

Conclusion: Eclipse overestimates the dose at SSD = 460 cm and shows good agreement

with the expected dose at SSD = 350 cm. The profiles in Eclipse show

oscillations for SSDs larger than 275 cm, which implies that the dose

distribution in a patient at extended SSD in Eclipse is not correct. The

difference between calculated and measured doses is affected by the definition

of the fields and the SSD used. Further investigations are needed before Eclipse

can be used for treatment planning at extended treatment distance.

Populärvetenskaplig sammanfattning

Ungefär 50 % av de cancerdrabbade patienterna i Sverige får någon gång strålbehandling.

Extern strålbehandling innebär i de flesta fall att högenergetiska fotoner riktas mot områdena

som ska behandlas. Vid helkroppsbestrålning (TBI) är målet att ge stråldos till hela kroppen.

Syften med TBI är bland annat att undertrycka kroppens egna immunförsvar och skada de

elakartade tumörcellerna. Tillfällen då TBI kan användas är inför stamcellstransplantationer,

för patienter med olika typer av blodcancer och vid vissa immunologiska sjukdomar. Det

finns olika tekniker att genomföra extern strålbehandling och TBI, beroende på vilka resurser

och kunskaper som finns på respektive sjukhus. En vanlig teknik för TBI är att ha ett längre

avstånd mellan behandlingsmaskinen och patienten, source-to-skin distance (SSD), än för

patienter som behandlas med konventionell strålbehandling. Patienten kan då stå upp eller

ligga ner under behandlingen. Det finns en rad olika parametrar som måste väljas vid TBI, till

exempel vilken doshastighet och hur behandlingen ska delas upp i fraktioner. Dessa val

baseras på vilken biologisk effekt som ska uppnås i patienten.

Dosplanering inför strålbehandling kan göras manuellt eller med ett dosplaneringssystem

(TPS), ett program som används för beräkning av stråldos. Eclipse, det system som används

på Sahlgrenska Universitetssjukhus, fungerar för SSD upp till 130 cm, vid längre avstånd är

det ännu inte validerat.

Syftet med denna studie var att utvärdera Eclipse, för beräkning av stråldoser till patienter

som behandlas med TBI. För att göra detta studerades dosplaneringssystemet med förlängt

SSD då det är den teknik som används. Två olika typer av linjäracceleratorer användes med

var sitt SSD, 460 cm och 350 cm. I Eclipse skapades fantomgeometrier med olika storlekar.

Resultat från Eclipse jämfördes med tidigare mätningar som genomförts. Mätningar i olika

fältgeometrier genomfördes med syftet att kunna förbättra den metod för TBI som används nu

och fälten jämfördes mot Eclipse. Dessutom analyserades Monte Carlo-simuleringar av

patienter som fått TBI på Sahlgrenska och dessa jämfördes med det aktuella

dosplaneringssystemet.

Vid studie av dosen på 10 cm djup sågs en variation beroende på fantomgeometri som

använts i Eclipse samt även vilket SSD som används. Vid jämförelse mellan procentuella

djupdoskurvor (PDD) ser de ut på samma sätt i Eclipse som vid mätningar. Dosprofilen längs

med fantomet hade en oscillerande form som inte var förväntad. Det blev mer markant med

ökat avstånd och berodde även på vilka metoder som fältet formats på. Mätningarna gav lägre

doser än Eclipse.

Skillnaden i medeldos till patienten mellan Eclipse och Monte Carlo-simuleringar

varierade från patient till patient men var för det flesta inom ± 4 %. Undersökningen med

tilläggsfält resulterade för vissa patienter i en mer homogen dosfördelning, för andra var det

svårare att uppnå en homogen dosfördelning med denna metod. Patientens kroppsform

spelade roll för hur bra dosfördelningen blev.

Table of content Background and Theory ........................................................................................... 1

1.1 Techniques of performing total body irradiation .............................................. 2

1.2 Treatment planning and evaluation .................................................................. 4

1.3 Aims .................................................................................................................. 4

Methods and Material ............................................................................................... 4

2.1 The current TBI method in Gothenburg ........................................................... 4

2.2 Dose calculations in Eclipse ............................................................................ 6

2.3 Previous measurements and Monte Carlo calculation ...................................... 7

2.4 Validation measurements ................................................................................. 7

2.5 Studies of Eclipse accuracy in phantom geometry ........................................... 9

2.5.1 Absolute doses ............................................................................................... 10

2.5.2 PDD ............................................................................................................... 10

2.5.3 Profiles and off-axis values ........................................................................... 10

2.5.4 Monte Carlo comparison ............................................................................... 11

2.5.5 Other studies of Eclipse ................................................................................. 11

2.6 Retrospective dose distribution ...................................................................... 11

2.7 Dose planning ................................................................................................. 11

Results ....................................................................................................................... 12

3.1 Validation measurements ............................................................................... 12

3.2 Studies of Eclipse accuracy in phantom geometry ......................................... 13

3.2.1 Absolute doses ............................................................................................... 13

3.2.2 PDD ............................................................................................................... 14

3.2.3 Profiles and off-axis values ........................................................................... 16

3.2.4 Monte Carlo comparison ............................................................................... 18

3.2.5 Other studies of Eclipse ................................................................................. 19

3.3 Retrospective dose distribution ...................................................................... 19

3.4 Dose planning ................................................................................................. 21

Discussion ................................................................................................................. 22

4.1 Measurements ................................................................................................. 22

4.2 Eclipse accuracy ............................................................................................. 22

4.3 Patient cases and dose planning ...................................................................... 23

4.4 Limitations ...................................................................................................... 23

Conclusion ................................................................................................................ 24

Acknowledgement .................................................................................................... 24

References ................................................................................................................. 25

Appendix .......................................................................................................................

1

Background and Theory The knowledge about radiation and its applications in medicine has been used since

Wilhelm Röntgen took an x-ray image of his wife’s hand in 1895 by letting her hold the hand

in front of a photographic plate and then irradiating it [1]. Applications have included both to

diagnose and to treat patients.

External radiotherapy treatment (RT) with photons is a sort of therapy with ionizing

radiation, where high energy photons are used to irradiate malignant cells in order to kill them.

The radiation is damaging the DNA in the cells either directly or indirectly by generating free

radicals [2]. Almost fifty percent of patients diagnosed with cancer are being treated with

external RT either to cure, reduce pain or to increase the survival time [3]. The treatment goal

is to deliver the prescribed dose to the tumour target while sparing normal tissue and organs at

risk (OAR). Therefore a patient specific treatment plan is designed. [4].

Total body irradiation (TBI) is a technique and a type of external RT, where the whole body

is the target. Purposes of TBI are immunosuppression and/or to kill malignant cells. In 1990

Joseph E. Murray and E. Donnall Thomas received the Noble Prize in Physiology or Medicine

for their research in organ and cell transplantations. One of their discoveries was that total body

irradiation reduced the probability to reject the transplanted organ [5]. An early, unethical study

on the effects of TBI was made in the 1960s by Defence Atomic Support Agency in the United

States of America. The purpose of this study was to investigate the acute effects of radiation

and the effects for their army troops [6]. Diseases, where TBI is a possible treatment method,

include myeloma, leukaemia, Hodgins’s lymphoma and immunodeficiency. TBI is widely used

prior to bone marrow transplantation and stem cell transplantation.

Bone marrow is soft tissue in bone cavities. Its functions are to produce blood cells and

store fat. There are two types of bone marrows, red (blood-producing stem cells) and yellow

(fat, bone and cartilage-producing cells). With increasing age, the red bone marrow is replaced

with yellow fat tissue [7]. Stem cells in the bone marrow and other blood production sites in

the body, divide themselves into one of the three types of blood cells in the body, leukocytes,

erythrocytes and thrombocytes [8]. Cancers that originate from the bone marrow are for

example myeloma, lymphoma and leukaemia [7].

In many of the above-mentioned cancer types, stem cell transplantation can be an optional

treatment. Alternatively, bone marrow transplantation can be used, depending on where the

cells are taken from. Stem cells can be taken either from the patient or from a donor. Before

receiving the transplanted cells, the patient must undergo a pre-treatment. This can be done

either by chemotherapy and/or by irradiation. The purpose of the pre-treatment is

immunosuppression and sometimes to kill the malignant cells, which is called conditioning in

medicine [9]. Graft-versus-host disease (GVHD) can be a severe complication to bone marrow

transplantation and other types of transplantations. The cause of GVHD is immuno-cells of the

transplanted organ which attack cells in the body they have been transplanted to [10].

TBI may be combined with chemotherapy. Studies have shown that this combination is

more efficient than each of them separately, for instance, in treatment for leukaemia. One

advantage of TBI compared to chemotherapy is that it affects the central nervous system where

the chemotherapy is ineffective [11, 12].

2

Although advantageous, there are risks and complications with the TBI treatment. Acute

effects include nausea, skin irritation and interstitial pneumonitis (IP). Long-term effects may

be cataract and thyroid complications [13]. Long-term complications can appear some months

after treatment up to several years later, depending on dose, other medication and the type of

complication. For example, secondary cancer has a latency period of many years [14].

Complications related to the lungs are mainly IP and increases in risk if GVHD occurs. Cataract

is common but can be reduced with fractionated treatment [13]. Both in men and women, TBI

has been shown to affect the gonad function. Most common is total or partial gonad failure but

in a few cases, the gonads are not affected at all [15].

To reduce the risk of acute effects, such as IP, the dose rate is usually low and commonly

under 10 cGy/min. The treatment is fractionated to increase the total dose and reduce the risk

of complications. Total doses are usually in the range of 8 and 12 Gy [16]. According to

Swedish studies from 2003, there are about sixty people each year in Sweden treated with TBI

[17].

1.1 Techniques of performing total body irradiation There are numerous methods that can be used for TBI. Early techniques for TBI included a

sweeping field or multiple radiation sources. For example, Co-60 or Cs-137 was placed around

the patient, to create a homogenous dose distribution. Standing position with irradiation

posterior and then anterior (AP/PA technique) or lateral irradiation from both sides (standing,

sitting or lying) with extended source-to-skin distance (SSD) has for a long time been the most

common ways to perform TBI. Energies can be in the range of 0.6 MV to 25 MV depending

on photon source and treatment setup [16]. Figure 1 is presenting many of the early and

traditional setups of performing TBI or half body irradiation.

Figure 1. Some possible ways to deliver TBI. Redrawn from [16].

a. Four sources. b. Two horizontal beams. c. Two vertical beams. d. Source scans horizontally. e. Half body, direct and

oblique fields. f. Direct horizontal, long SSD.

b. a.

c.

.

d.

e. f.

3

In the standing AP/PA position, lung blocks as compensators, are commonly used to shield

the lungs. Usually, an immobilization stance is used to support the patient and to guarantee the

same position every treatment. In the lateral technique, the patient can either lay on the back

with the arms used as lung shields, or on the side. Otherwise, lung blocks can be used [16]. A

schematic picture of TBI with linear accelerator and standing position is presented in Figure 2.

To create a long SSD the gantry is tilted down, or the couch is removed, and the patient is

placed on the floor. With an extended SSD the total body are being covered with one static

field. If the energy is above 1,25 MeV, a screen is used in front of the patient as an electron

generator to increase the superficial dose [16].

A common treatment method to achieve a more uniform dose distribution is the field-in-

field technique. This has also been suggested for TBI to create a more uniform dose to the whole

body [18].

Figure 2. Schematic picture of TBI with standing technique at extended SSD.

As the techniques for external RT evolves, so does the range of TBI techniques. Intensity-

modulated radiotherapy and volumetric arc therapy have the benefits of giving a conform dose

to the target volume and reduce the dose to surrounding tissues and are alternatives for TBI in

some cases. One technique with rotating and modulated delivery is helical tomographic RT,

where a linear accelerator mounted on a helical computed tomography (CT)-gantry is used. The

patient is lying on a moving couch that travels along the machine. The field of view in helical

tomographic RT is less than the volume to cover with TBI. Therefore, adjacent fields are used

and this needs to be done with high precision [19]. One main concern about the use of

techniques where fields are not covering the whole patient at the same time, i.e. intensity

modulated, spliced or sweeping fields, are circulating cells. This means that cells may escape

from the irradiation and are therefore at risk of not receiving the prescribed dose [16].

Many of these new techniques may be used to specifically deliver the dose to the bone

marrow and/or to the lymphatic system, such as total marrow irradiation (TMI), total lymphatic

irradiation or total marrow and lymphatic irradiation. The main advantage with these new

techniques is the ability to reduce the dose to OAR, such as lungs and eyes. TMI may be an

alternative method prior to bone marrow transplantation in the cases where modulated RT is

available for TBI [19]. All modalities are not clinically relevant to all types of TBI since they

serve different purposes and require different apertures and knowledge.

4

1.2 Treatment planning and evaluation Treatment planning and dose calculation, i.e. defining fields and calculating monitor units

(MU) for a particular dose distribution can be performed in different ways. In general, a plan is

generated, and the corresponding dose distributions are calculated in a treatment planning

system (TPS). One of the ways of determining the quality of the treatment plan is calculating

the dose homogeneity. Accepted values of dose variations in the target region for external

radiotherapy, are –5 % to +7 % of the prescribed dose [4]. In TBI, the recommended dose

homogeneity in the body can be up to ±10 % [4, 20].

Evaluation of the treatment plan can be done by using a dose-volume-histogram (DVH).

Dose distribution both to OAR and target can be evaluated [21].

A TPS is usually configured to reproduce the accelerator beam at isocentre distances (up to

130 cm from the accelerator target). The ability to perform dose calculations in TPS Eclipse

(Varian Medical System, Inc. Palo Alto) with extended SSD has recently become available. A

study by Lamichhane et al. on the use of Eclipse calculations for SSD longer than 3 m indicated

that the dose distribution agreed well compared to measured values. However, they did not

recommend Eclipse for absolute dose or MU calculations [20]. Studies of TPS based planning

and field-in-field for TBI or TMI have shown better dose homogeneity and reduced dose to

OAR [18, 22].

The accuracy of dose calculations at long treatment distance needs to be further studied.

1.3 Aims The ultimate goal is to improve dose determination and dose homogeneity for TBI at

Sahlgrenska University hospital (SU). The aims of this study were experimental and

theoretical validation of the accuracy of the Eclipse TPS in phantom and patient geometries

for TBI geometry. Furthermore, to investigate the possibilities to improve the current TBI

technique.

Methods and Material

2.1 The current TBI method in Gothenburg The current TBI method used at SU in Gothenburg, is the lateral technique with two lateral

15 MV fields applied to a patient lying on a couch designed for TBI geometry at extended SSD.

Before treatment, a planning CT is performed with 8 mm resolution. The dose is prescribed to

a reference point defined in the centre of the patient, at the widest point of the hips. The patient

width is calculated from the planning CT. Water tanks are placed above the head and under the

feet to compensate for the loss of scattering which would occur otherwise, as displayed in

Figure 3.a and b. Water tanks are also placed on the sides of the head, and in between the legs,

to create a rectangular shape around the body, with the tissue equivalent density. The size of

the rectangle is matched with the width that corresponds to the reference point.

A bolus may be used to compensate for the different widths around the patient, for instance

around the throat, to produce a more uniform dose. Styrofoam, with the thickness measured

from the couch to the bottom of the lung, shown in Figure 4.a, is placed under the shoulders, to

prevent the shoulders and upper arms from shielding the spinal cord. The arms are kept together

on the stomach to shield the lungs and reduce the lung dose. Since the dose maximum is reached

a few centimetres inside of the skin a plexiglass screen with a thickness of 1.6 cm is used in

5

front of the patient, placed in a fix holder at the long axis of the couch, to generate electrons

and improve the superficial dose.

The distance from the source to the centre of the patient, source-axis distance (SAD), is 480

cm, represented by the laser in Figure 3.b. The gantry is rotated towards the patient and the

collimator head is rotated to produce the largest possible field diagonally with the jaws at 38×38

cm2. Multi-leaf collimators (MLCs) and blocks of lead are used to form a maximum field size,

seen in Figure 4.b, which is 15×43 cm2 at isocentre, at extended SSD the field is covering the

total patient. All patients are treated with the same field size and 15 MV photons. The number

of MU per field for the treatment is calculated using the equation

MU = K ∙ td

2 ∙

100

PDDpm∙ SSDcorr (Equation 1)

where K is the number of monitor units (MU) required to deliver 1 Gy at 10 cm depth, td is the

dose prescribed for one fraction, PDDpm is the percentage depth dose curve measured behind

the plexiglass normalized at 10 cm depth, and SSDcorr is the correction factor for SSD,

dependent on the patients’ width compared to the reference of 40 cm. The number of MU that

is calculated correlates to the deposited dose to water, not dose to tissue or media.

Figure 3. Patient positioning during TBI and with the lasers shown in red. a. The green part represents the Styrofoam.

b. The positioning compared to the laser through the patient centre, also called SAD. The figures are from SU and have

been approved for use in this publication.

A Monte Carlo (MC) simulation is performed with the number of MU and the planning CT,

to visualize the dose distribution and dose homogeneity. If the simulation indicates dose

inhomogeneities, these are reduced before treatment with additional water equivalent material.

Lasers customized for treatment at extended SSD are used to position the centre of the patient

at SAD, going through the nose and umbilical plane. An additional laser is used for the reference

point to verify the position in craniocaudal-direction. Lasers and patient positioning are shown

in Figure 3.a and b.

The dose rate is set to 300 MU per minute on the machine, which corresponds to a dose rate

of 11 cGy/min at 10 cm depth in the patient. The extended SSD reduces the dose rate, due to

the inverse square law, and enlarges the field size. The most common fractionation scheme is

2.75 Gy per fraction, one fraction per day and four fractions in total.

a.

b.

6

An in-vivo dosimetry system is used to monitor the delivered dose to the patient each

fraction. In Gothenburg, diodes are used, and their places are shown in Figure 5. These points

are the reference point, at the neck and auditory canal.

The above-described method is used at a Varian TrueBeam (TB) linear accelerator (Varian

Medical Systems Inc). At SU there is also a possibility to use a Varian Clinac iX linear

accelerator (Varian Medical Systems Inc) with SSD = 350 cm, SAD = 365 cm, reference width

30 cm and dose rate 19 cGy/min at 10 cm depth. When using a shorter SSD the field becomes

shorter and the technique, in that case, is limited to shorter patients.

Figure 4. a. The thickness required of the Styrofoam is shown with the red line [23].

b. The clinically used field. The collimator is rotated to create the longest possible field. The MLCs are shown in green and

blocks in the darker shade of grey. The figures are from SU and have been approved for use in this publication.

Figure 5. The red lines are showing the left and right position of the diodes during the first treatment fraction. The

lowest line is in this case also used as the reference plane. The figure is from SU and has been approved for use in this

publication.

2.2 Dose calculations in Eclipse In Eclipse, different calculation algorithms may be used to determine the dose. The Analytic

Anisotropic Algorithm (AAA) is a convolution/superposition method that is currently used for

clinical calculations at SU. The patient is represented as water with different densities and the

a

.

b

7

calculated dose is given as dose to water. Beam characteristics consist of analytical sources

with parameters fitted to Monte Carlo simulations and measurements for a specific machine.

Contributions from primary photons, extra-focal photons, electron contamination and scattered

photons are calculated separately [24]. Optional in Eclipse is also Acuros XB as a calculation

algorithm, which uses a model-based algorithm that analytically solves the linear Boltzmann

transport equation. It takes into account the tissue composition and calculates doses to media

and water [25]. Acuros XB has superior accuracy compared to AAA in heterogeneous

geometries, such as lungs [26].

For calculations and comparisons, it is important to understand how the grid size in the

calculation matrix is defined by the different algorithms. The AAA uses a grid that is

divergent perpendicular to the beam axis. The grid size is specified at SAD = 100 cm.

Therefore, the grid size at an extended SSD will be expanded perpendicular to the beam axis

and will remain constant in beam direction. The grid size chosen in Acuros XB is the size that

will be calculated in the volume and does not diverge [24].

2.3 Previous measurements and Monte Carlo calculation Measured data obtained previously during TBI commissioning at SU have been used in

the current work. The data include a calibration factor (number of MU for 1 Gy at 10 cm

depth along beam axis), depth dose distributions along the beam axis as well as off-axis doses.

All values were obtained behind plexiglass with the 15 MV clinical field. From these

measurements, the factors; K, PDDpm and SSDcorr in Equation 1 were determined.

The phantom for these measurements was a water-filled phantom of size 20×20×20 cm3

used with additional water cans placed on the sides of the phantom. For TrueBeam at SSD

460 cm, an electrometer from Janus Engineering (Sindelfingen, Germany) together with

PTW-Freiburg Semiflex ionization chambers TB31010, and ionization chamber NE2571

(Phoenix Dosimetry Ltd, Sandhurst, United Kingdom) were used. At the Clinac iX,

electrometer from Janus Engineering, ionization chamber NE2571 and SSD 350 cm were

used.

Dose distributions obtained earlier by the Monte Carlo method were used to evaluate the

accuracy of the dose distribution in patient geometry. The MC simulations were based on the

BEAMnrc/DOSXYZnrc code package. MC accelerator model was validated for 15 MV Clinac

iX at extended treatment distance for the clinical field. The patient was represented by nine

tissues. The dose distribution was given in term of dose-to-medium and was imported in Eclipse

[27].

2.4 Validation measurements The measurements were performed with a solid water phantom of size 24×30×30 cm3 at

both Varian Clinac iX linear accelerator and Varian TrueBeam (TB). For the iX machine, an

initial measurement in the reference geometry with 120 MU at SSD 90 cm, field-size 10x10

cm2 and 300 MU/min, was performed to receive a value from the electrometer that correlated

to the reference calibration setup 1 Gy at 10 cm depth, with gantry and collimator rotation 0°.

Correction for daily output was performed based on dosimetry measurements that were

performed two weeks before and after this study. The rest of the measurements were performed

at SSD 350 cm, SAD = 365 cm, with the plexiglass placed 10 cm in front of the phantom, 900

MU and same dose rate as before. All values were collected 10 cm in the phantom. The setup

is presented in Figure 6.

8

The clinically used field with MLCs and blocks was measured. Three fields formed by the

jaws with sizes 15×40; 5.5×40 and 2.7×40 cm2 at isocentre, were measured. The first field had

the same size as the clinical field but was created by jaws. The latter two fields correspond to

field-sizes of 20 cm and 10 cm at SAD = 365 cm in the anterior-posterior (AP) direction,

schematically presented in Figure 7. MLC fields with sizes 2.7×40; 2.7×10 and 2.7×19 cm2,

were also investigated. These fields corresponded to field-lengths 36.5 cm and 69 cm at SAD

in craniocaudal direction and 10 cm in AP direction, as in Figure 8.

Off-axis values were studied with the jaw field size 15×40 cm2, by moving the phantom 5,

10 and 15 cm in the gantry-target direction.

Figure 6. Placement of the phantom and ionization chamber at the TBI setup. The green laser indicates the SAD length.

Figure 7. Schematic picture of the fields that were 20 cm and 10 cm high in SAD. Note the direction of the field

compared to the gantry and patient placement.

Figure 8. Schematic picture of the fields that were 10 cm high in SAD and with varying length in the craniocaudal

direction. Note the direction of the field compared to the gantry and patient placement

9

The detector was a Farmer ionization chamber (PTW Freiburg) and used without build-up

material. For read-out, an electrometer (Fluke Biomedical, Advanced Therapy Dosimeter) was

used.

Measurements were also performed at TB but with SSD = 460 cm and SAD = 480 cm. To

correlate the electrometer readout for these measurements to the dose, an initial measurement

was performed with the gantry rotated 270° and dose rate 300 MU/min, collected at SSD = 90

cm. Jaw shaped fields were 15×40; 4.2×40 and 2×40 cm2. In similarity to measurements at the

iX machine the latter two fields are corresponding to 20 cm and 10 cm high fields in AP

direction, at SAD. MLC shaped fields were 2×40; 2×14.3; 2×10 and 2×5 cm2. Their lengths in

craniocaudal direction were 69 cm, 48 cm, and 24 respectively at SAD. Also measured at TB

was a field 9.6 cm high and 24 cm long at SAD. The off-axis measurements for TrueBeam were

performed at 5, 10, 15, 20, 30 and 40 cm. Water cans were placed on both sides of the phantom,

with the size 22 cm high, 12 cm wide and 18 cm deep. The setup is presented in Figure 9. The

equipment used at the TrueBeam was a Farmer ionization chamber (PTW Freiburg) and

electrometer E5 (Newport).

The analysis was performed by comparing the mean of the collected values (nC) for each

field relative to each geometry and settings in Eclipse with AAA, version 13.6.23 (Varian

Medical System, Inc. Palo Alto) and with a grid size of 0.25 cm. When the off-axis at SSD =

460 cm was studied, an additional phantom was created with the additional scattering material

included. To correlate the measured values to the dose, the daily output was measured at the

TB right before the study.

Figure 9. Measurement setup when measuring off-axis values at SSD 460 cm.

2.5 Studies of Eclipse accuracy in phantom geometry Phantoms were created with a CT value of -7 HU (defined as water) together with an

additional plexiglass wall with a CT value of 330 HU in Eclipse TPS version 13.6.23. The

plexiglass was defined as support material and placed 10 cm in front of the phantom or, as close

to 10 cm as possible. Figure 10 demonstrates the representation of the coordinates x, y and z.

10

Figure 10. Definition of the parameters used when the phantoms were created.

Two different machine set-ups were used: Varian TB linear accelerator and Varian Clinac

iX linear accelerator. Their calibration factors were 2703 MU/Gy for TB with SSD 460 cm and

1545 MU/Gy for Clinac iX with SSD 350 cm commissioned with the clinically used field. The

clinical field and its settings at SU were used for all phantom sizes at both machines, including

SSD, dose rate and MU for 1 Gy at 10 cm depth according to Equation 1. The algorithm used

was AAA (grid size of 0.25 cm), the doses at 10 cm depth were also calculated with Acuros

XB (grid size 0.3 cm), dose to media. All calculations ran with heterogeneity corrections on.

The analysis was performed in Excel 2013 (Microsoft Office).

2.5.1 Absolute doses For the study of absolute doses phantoms of sizes 20×20×20; 30×30×30; 40×40×40 and

40×40×150 cm3 were created. Doses were collected at 10 cm depth in the central beam axis for

the above-mentioned phantoms, for both SSDs with the clinical settings.

Doses without plexiglass were calculated. To determine the impact of the distance between

the source and the phantom, the phantoms were calculated with SAD = 365 cm and SAD = 480

cm. Doses were collected at 10 cm depth.

A more precise impact of the dose due to phantom sizes was studied by creating additional

phantoms with the sizes 20×30×30; 24×30×30; 20×40×40 and 30×40×40 cm3. Their doses were

calculated for the two SSDs with AAA grid size of 0.25 cm.

2.5.2 PDD The PDDs were obtained along the beam axis for the phantoms. The values were

normalized at 10 cm depth. Values for the PDDs at six different depths (0.5 cm; 2 cm; 5 cm;

10 cm; 15 cm and 20 cm) were also collected in the phantoms.

2.5.3 Profiles and off-axis values Off-axis values were collected in a phantom of size 20×20×40 cm3 with the same CT values

as before and with plexiglass, by moving the phantom 10; 20; 30 and 40 cm laterally (plus 60

cm at SSD = 460 cm) and compared with previous measurements. All values were normalized

at the phantom centre for the comparisons.

Profiles were collected at the different SSDs and phantoms. A more detailed study of the

profiles was performed since the profiles appeared to behave in an unexpected way. The

phantom of size 40×40×150 cm3 at the TB machine, with different SSDs of 250 cm, 275 cm,

350 cm, 460 cm and 500 cm was used. All profiles were normalized to the centre of the phantom

and calculated with AAA and a grid size of 0.25 cm. The profile at SSD = 460 cm was also

11

calculated with AAA, of 0.1 cm grid size and studied with fields determined by jaws and blocks

separately. A profile with Acuros XB and a grid size of 0.3 cm was collected for the phantom

size 40×40×40 cm3. This was the largest phantom that could be calculated with Acuros XB and

for comparison, that field was also calculated with AAA with grid sizes of 0.1 cm and 0.25 cm.

2.5.4 Monte Carlo comparison The PDD curve and profile at SSD = 460 cm was also compared with a previous MC-

simulation with a phantom of size 46×21×150 cm3. The MC-simulation was performed with

the purpose to validate MC as a method for TBI compared with commissioning data. Therefore,

the plexiglass was taken into account in the phase space. This phantom was also created in

Eclipse and simulated with the iX machine. The profile of the phantom was studied at 10 cm

depth. The profile from Monte Carlo simulations was generated by dividing the phantom into

two equally sized sections and taking the mean values of each section. The profile was

normalized at the phantom centre. Differences in the fluctuations were investigated by

calculating the deviation of the dose and the fluctuation in each point. Fields in Eclipse were

created by blocks to make the situation most similar to the Monte Carlo created beam

configuration. For comparison, a profile from a field shaped by jaws with the same field size as

the clinical field was studied.

2.5.5 Other studies of Eclipse The transmission at extended SSD was studied in Eclipse by closing all MLC leaves in one

bank, outside the field, then closing them in the other bank. The mean dose from both these

calculations was divided with the dose from an open field of size 10×10 cm2. The phantom was

20×20×20 cm3.

Doses with different distances between the plexiglass and the phantom were also calculated.

These plexiglass to phantom distances were 2 cm, 4 cm, 10 cm and 15 cm. The phantom of size

20x20x20 cm3 and the clinical field was used.

2.6 Retrospective dose distribution Dose distributions calculated with AAA, were used for retrospective dose distribution of

nine patients who already had undergone TBI at SU. Their treatment plans were also simulated

by the MC technique described earlier. These patients were of different age, gender and body

shape. The iX machine was the most similar to the MC-code, therefore these were compared.

The AAA with a grid size of 0.25 cm and an automatic segmentation method for defining the

lungs was used.

The DVH parameters studied were Dmean (mean dose to the patient), Dmax (maximum dose to

the patient), and V95% for the total body (the volume of the body that receives 95 % of the

prescribed dose). For the lungs, Dmean and Dmax were evaluated. A visual comparison of the dose

distribution of different anatomical locations in the body was performed.

2.7 Dose planning Minor investigations of individual treatment planning for patients who have been treated

with TBI at SU were performed. The goal was to find a method with a dose homogeneity within

±10 %. The treatment planning was performed in Eclipse and results from this study were

considered. Therefore, additional plans with fields defined only by jaws and fields defined by

12

MLCs were used. The evaluation was performed by studying the dose distribution within ±10

%.

Results

3.1 Validation measurements Measurements with the currently used treatment setup and with different field-sizes are

shown in Table 1 and Table 2. The measured values are corrected for dose output which was

1.001 on the iX machine and 1.008 on the TB. The mean difference between calculated and

measured doses were 1.8 % higher at SSD = 350 cm, and 2.0 % higher at SSD = 460 cm, if the

fields were created by jaws. The mean difference for fields created by MLCs at SSD = 350 cm

was 3.0 % and at SSD = 460 cm 4.4 %.

Table 1. Doses from Eclipse and measurements for s.w phantom of size 24×30×30 cm,3 SSD = 460 cm, TrueBeam, and 900

MU. The doses were collected at 10 cm depth at the central axis.

Field size Dose with AAA

(Gy)

Measured

(Gy)

Difference

(%)

15×43 cm2 - blocks and MLC 0.355 0.3475 2.1

15×40 cm2 - jaws 0.359 0.3544 1.3

4.2×40 cm2 - jaws 0.34 0.3303 2.9

2×40 cm2 - jaws 0.318 0.3127 1.7

2×40 cm2 - MLC 0.335 0.3202 4.6

2×14.3 cm2 - MLC 0.334 0.3200 4.4

2×10 cm2 - MLC 0.333 0.3194 4.2

2×5 cm2 - MLC 0.331 0.3168 4.5

Table 2. Doses from Eclipse and measurements for s.w phantom of size 24×30×30 cm3, SSD = 350 cm, Clinac iX, and 900

MU. The doses were collected at 10 cm depth at the central axis.

Field size Dose with AAA

(Gy)

Measured

(Gy)

Difference

(%)

15×43 cm2 - blocks and MLC 0.597 0.5834 2.3

15×40 cm2 - jaws 0.604 0.5988 0.9

5.5×40 cm2 - jaws 0.587 0.5738 2.3

2.7×40 cm2 - jaws 0.558 0.5456 2.3

2.7×40 cm2 - MLC 0.575 0.5573 3.2

2.7×19 cm2 - MLC 0.572 0.5550 3.1

2.7×10 cm2 - MLC 0.572 0.5562 2.9

Values off-axis for SSD = 460 cm are presented in Figure 11, where extra scattering material

was added on the sides of the phantom, both when measured and evaluated in Eclipse. Off-axis

measurements for SSD = 350 were performed without adding extra scattering material and are

shown in Figure 12.

13

Figure 11. Off-axis values measured with TrueBeam at SSD = 460 cm and calculated values in Eclipse for the same

distances. Doses were collected at 10 cm depth and normalized at central beam axis.

Figure 12. Off-axis values measured with Clinac iX at SSD = 350 cm and calculated values in Eclipse for the same

distances. Doses were collected at 10 cm depth and normalized at central beam axis.

3.2 Studies of Eclipse accuracy in phantom geometry

3.2.1 Absolute doses Doses calculated in Eclipse for different phantoms at SSD = 460 cm are presented in Table

3. For SSD = 350 cm the doses are presented in Table 4. The doses were collected at 10 cm

depth and the number of MU that was used is for 1 Gy at 10 cm depth. Doses were also

calculated with Acuros XB if possible. The dose to water gave the same results as the calculated

dose to media. The tables are also presenting the impact of using plexiglass in front of the

phantom.

99

101

103

105

107

109

0 5 10 15 20 25 30 35 40 45 50Rel

ativ

e d

ose

co

mp

ared

to

the

centr

e

Distance off-axis [cm]

Off-axis at SSD = 460 cm

Measured Eclipse

100

101

102

103

104

0 2 4 6 8 10 12 14Rel

ativ

e d

ose

co

mp

ared

to

the

centr

e

Distance off-axis [cm]

Off-axis at SSD = 350 cm

Eclipse Measured

14

Table 3. Doses at SSD = 460 cm, calculated by Eclipse with AAA and Acuros XB dose to medium, 2703 MU.

Phantom Dose with

AAA (grid

size 0.25 cm)

Dose with

Acuros XB

(grid size 0.3 cm)

Dose with AAA

(grid size 0.25 cm)

without plexiglass

Difference in dose

between plexiglass

compared to

without plexiglass

40×40×150 cm3 1.085 Gy - 1.128 Gy - 4.0 %

40×40×40 cm3 1.078 Gy 1.065 Gy 1.119 Gy - 3.8 %

30×30×30 cm3 1.066 Gy 1.059 Gy 1.108 Gy - 3.9 %

20×20×20 cm3 1.043 Gy 1.040 Gy 1.090 Gy - 4.5 %

20×40×40 cm3 1.077 Gy 1. 069 Gy 1.120 Gy - 4.0 %

30×40×40 cm3 1.077 Gy 1.071 Gy 1.121 Gy - 4.1 %

20×30×30 cm3 1.064 Gy 1.054 Gy 1.109 Gy - 4.2 %

Table 4. Doses at SSD = 350 cm, calculated by Eclipse with AAA and Acuros XB dose to medium, 1545 MU.

Phantom Dose with

AAA (grid

size 0.25 cm)

with

plexiglass

Dose with

Acuros XB (grid

size 0.3 cm) with

plexiglass

Dose with AAA

(grid size 0.25 cm)

without plexiglass

Difference in dose

between plexiglass

compared to

without plexiglass

40×40×150 cm3 1.044 Gy - 1.085 Gy - 3.9 %

40×40×40 cm3 1.037 Gy - 1.076 Gy - 3.8 %

30×30×30 cm3 1.025 Gy 1.015 Gy 1.066 Gy - 4.0 %

20×20×20 cm3 1.001 Gy 0.998 Gy 1.045 Gy - 4.5 %

20×40×40 cm3 1.035 Gy 1.022 Gy 1.076 Gy - 4.0 %

30×40×40 cm3 1.036 Gy 1.028 Gy 1.077 Gy - 4.0 %

20×30×30 cm3 1.022 Gy 1.010 Gy 1.066 Gy - 4.3 %

Calculated doses with fixed SADs are presented in Table 5. All phantoms were calculated

with plexiglass and AAA with grid size of 0.25 cm and the number of MU for 1 Gy in the

reference geometry.

Table 5. Doses at 10 cm in the phantom calculated with Eclipse AAA, for the two SADs used a SU.

Phantom SAD = 365 cm.

Dose at 10 cm

depth.

SAD = 480 cm.

Dose at 10 cm

depth.

40×40×150 cm3 1.073 Gy 1.085 Gy

40×40×40 cm3 1.066 Gy 1.078 Gy

30×30×30 cm3 1.025 Gy 1.046 Gy

20×20×20 cm3 0.974 Gy 1.002 Gy

3.2.2 PDD PDD curves for SSD = 460 cm and SSD = 350 cm are presented in Figure 13 and Figure

14, normalized at 10 cm depth, both with and without plexiglass in front. In these figures, the

15

phantom of size 40×40×150 cm is used. The PDD for all other phantom sizes can be studied in

Appendix A. The PDDs are collected with Eclipse, AAA with a grid size of 0.25 cm and

compared with previous measurements. Values for PDD were also collected at six points in the

phantoms with normalization at 10 cm shown in Table 6 and Table 7.

Figure 13. PDD for the phantom with size 40×40×150 cm3. Both with and without plexiglass and measured data with

plexiglass.

Figure 14. PDD for the phantom with size 40×40×150 cm3. Both with and without plexiglass and measured data with

plexiglass.

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40Rel

ativ

e d

ose

dis

trib

uti

on [

%]

Depth in the phantom [cm]

PDD normalized at 10 cm. SSD = 460 cm

With plexiglass Without plexiglass Measured

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40Rel

ativ

e d

ose

dis

trib

uti

on [

%]

Depth in the phatnom [cm]

PDD normalized at 10 cm. SSD = 350 cm

Measured With Plexiglas Without Plexiglas

16

Table 6. Comparison of PDD values at six depths in the phantoms in Eclipse with measured values at SSD = 460 cm.

PDD comparison, normalized at 10 cm [%]. SSD = 460 cm.

Phantoms in Eclipse 0.5 cm 2 cm 5 cm 10 cm 15 cm 20 cm

20×20×20 122.21 121.57 113.75 100 86.66 30×30×30 119.13 120.10 113.10 100 87.74 76.33

40×40×40 118.13 119.30 112.60 100 87.98 76.90

40×40×150 118.37 118.73 112.18 100 88.35 77.50

Measured values 117.66 118.9 111.32 100 86.83 75.45

Table 7.Comparison of PDD values at six depths in the phantoms in Eclipse with measured values at SSD = 350 cm.

3.2.3 Profiles and off-axis values Off-axis values for each of the two used SSDs compared with commissioning data is

presented in Figure 15 and Figure 16. All values were normalized to 100 % at off-axis value =

0 cm.

Figure 15. Off-axis values for TB with SSD 460 cm compared with previous measurements.

100

101

102

103

104

105

106

0 10 20 30 40 50 60 70

Rel

ativ

e d

ose

off

axis

Distance off-axis [cm]

Off-axis at SSD = 460 cm

Measured Eclipse

PDD comparison, normalized at 10 cm depth [%]. SSD = 350cm.

Phantoms in Eclipse 0.5 cm 2 cm 5 cm 10 cm 15 cm 20 cm

20×20×20 cm3 123.07 122.63 114.38 100 86.26 46.71

30×30×30 cm3 119.90 121.00 113.59 100 87.26 75.50

40×40×40 cm3 118.91 120.30 112.12 100 87.61 76.22

40×40×150 cm3 118.81 119.54 112.70 100 87.96 76.78

Measured values 118.50 120.50 113.60 100 88.10 76.40

17

Figure 16. Off-axis values for Clinac iX with SSD = 350 cm compared with previous measurements.

Profiles for different SSDs are presented in Figure 17 with phantom 40×40×150 cm3. All

profiles are normalized at the centre of the phantom, beam axis at 75 cm and produced with

2703 MU, Eclipse AAA with the grid size of 0.25 cm. For SSD = 460 cm, the largest dose

difference due to the amplitude was 3.8 % and with SSD = 350 cm around 1 %. Profiles with

different field configurations are presented in Figure 18. If the grid size = 0.1 cm was used, the

dose variation was 4.5 %. The dose variation if only jaws were used was 2.5 %. If only blocks

were used to create the field the amplitude was 4.3 %. Profiles collected from Eclipse with the

same phantom size are also presented, collected at 10 cm depth, with fields defined only by

jaws and by blocks together with MLCs. The influence of different algorithms is shown in

Figure 19, with both Acuros XB and AAA for the phantom of size 40×40×40 cm3.

Figure 17. Profiles collected with different SSDs for TB. All values are normalized to the centre of the phantom. Beam

axis at 75 cm.

100

101

102

103

104

105

106

0 5 10 15 20 25 30 35 40

Rel

ativ

e d

ose

off

axis

Distance off-axis [cm]

Off axis at SSD = 350 cm

Measured Eclipse

0.95

0.97

0.99

1.01

1.03

1.05

1.07

1.09

0 20 40 60 80 100 120 140

Rel

ativ

e d

ose

off

axis

Off axis distance [cm]

Profiles for different SSDs

SSD= 250 cm SSD= 350 cm SSD= 460 cm SSD= 500 cm

18

Figure 18. Profiles collected with different field definitions for TB. All values are normalized to the centre of the

phantom. Beam axis at 75 cm.

Figure 19. Profiles with different calculation models in Eclipse. Beam axis at 20 cm.

3.2.4 Monte Carlo comparison For SSD = 460 cm, the MC-simulated PDD is presented in Figure 20 and compared to PDD

with Clinac iX. Calculations with Monte Carlo generated the profile presented in Figure 21.

The statistical fluctuations of the Monte Carlo simulation were within ± 2 %. Geometrical

symmetry was used to improve the statistical accuracy by taking mean values of the dose for

the voxels on both sides of the beam axis.

0.95

0.97

0.99

1.01

1.03

1.05

1.07

1.09

0 20 40 60 80 100 120 140Rel

atie

do

se c

om

par

ed t

o t

he

centr

e o

f th

e p

hat

no

m

Off axis distance[cm]

Profiles with different field definitions

SSD= 460 cm, grid size 0.1 cm SSD= 460 cm, jaws only

SSD=460 cm, ordinary field SSD=460 cm, block only

100

102

104

106

108

110

112

0 5 10 15 20 25 30 35 40

Rel

ativ

e d

ose

of

the

pre

scri

bed

[%

]

Off axis distance [cm]

Profile with different calculation algorithms at SSD = 460 cm

AAA- grid 0.25 cm AAA- grid 0.1 cm Acuros XB- grid 0.3 cm

19

Figure 20. PDD curves from Monte Carlo simulations and Eclipse at SSD = 460 cm

Figure 21. Profiles determined from Monte Carlo simulation and Eclipse with two different field definitions. Beam axis

at 75 cm.

3.2.5 Other studies of Eclipse The MLC transmission in Eclipse for SSD = 460 cm was 1.66 % and the previously

measured was 1.69 % (SSD = 90) at commissioning. For SSD = 350 cm the transmission in

Eclipse was 1.42 % and measured at SSD = 90 cm 1.4 %. This means that even if calculations

are performed at extended SSD, the transmission factor is constant.

The distance between the plexiglass and the phantom did not influence the dose at 10 cm

depth in Eclipse (presented in Appendix B).

3.3 Retrospective dose distribution Differences in dose distribution between Eclipse calculations and MC-simulations were

visible in some anatomical locations but not in others. The most common region of agreement

was the reference plane, as shown in Figure 22. Differences were noticeable in other locations,

0

20

40

60

80

100

120

0 50 100 150 200 250 300 350 400Rel

ativ

e ab

sorb

ed d

ose

[%

]

Depth in water or phantom [mm]

PDD, Eclipse compared to Monte Carlo. Normalized at 10 cm.

Monte Carlo Eclipse

0.95

0.97

0.99

1.01

1.03

1.05

1.07

1.09

0 20 40 60 80 100 120 140

Rel

ativ

e d

ose

co

mp

ared

to

the

centr

e o

f th

e

phan

tom

Lenght of the phantom [cm]

Profiles from Monte Carlo simulations and Eclipse

Eclipse jaws Eclipse MLC and block Monte Carlo

20

for example, the lungs, stomach and shoulders, as shown in Figure 23. The dose distributions

calculated by Eclipse and Monte Carlo are shown to the left and right respectively. In these

figures green is representing good agreement with the prescribed dose.

Figure 22. Dose distributions calculated by Eclipse (left) and Monte Carlo (right) for a child at the reference plane. The

range of the colour scale is ± 15 % of the prescribed dose.

Figure 23. Dose distributions calculated by Eclipse (left) and Monte Carlo (right) for a child in the stomach. The range

of the colour scale is ± 15 % of the prescribed dose.

The DVHs for each patient, are presented in Table 8. Values labelled 1 are collected for the

whole body and values labelled 2 are for the lungs. The mean difference in the mean dose to

the body between AAA and MC for all patients was 1.86 %, which is within the statistical

fluctuations of MC. The mean difference in mean lung doses was 0.2 % lower with AAA than

with MC. Maximum doses were in general higher with MC compared to Eclipse. The

differences in V95% doses were between 1 % and 13 %.

21

Table 8. DVH parameters for the total body (1) and lungs (2) calculated with Eclipse and Monte Carlo simulations. Doses

are presented in Gy.

Patient Dprescribed V95% 1

AAA

V95%1

MC

Dmean1

AAA

Dmean1

MC

Dmax1

AAA

Dmax1

MC

Dmean2

AAA

Dmean2

MC

Dmax2

AAA

Dmax2

MC

Man 1 2.75 95% 92% 3.04 3.04 4.04 4.86 2.74 2.74 2.39 3.66

Man 2 2.75 92% 80% 2.91 2.79 3.87 4.19 2.87 2.82 3.33 3.41

Man 3 2.0 90% 93% 2.15 2.17 3.04 3.37 2.00 2.07 2.38 2.75

Woman

1 2.75 96% 91% 3.05 2.95 4.16 4.14 2.99 2.92 4.01 4.08

Woman

2 2.0 97% 96% 2.29 2.20 3.25 3.39 2.27 2.26 2.64 2.62

Child 1 2.75 92% 82% 2.94 2.91 3.76 4.18 2.93 2.92 3.38 3.71

Child 2 2.75 87% 77% 2.87 2.80 3.63 3.77 2.80 2.79 3.02 3.15

Child 3 2.75 92% 87% 2.77 2.72 3.23 3.46 2.78 2.75 3.10 3.11

Child 4 2.75 86% 87% 2.75 2.75 3.24 3.51 2.68 2.72 3.02 3.25

3.4 Dose planning For small size patients, homogeneous dose distributions were created with fields defined by

jaws and MLC fields separately. For patients with wide shoulders or variations in width, it was

more difficult to create homogeneous dose plans, especially if only jaws were used. The most

apparent problem was to deliver the prescribed dose to the spine without receiving too high

doses (more than 15 %) in the lungs or to the skin. Example of one additional field is shown in

Figure 24.

Figure 24. One example of an additional field, suitable for the field-in-field technique, that was studied in Eclipse.

22

Discussion

4.1 Measurements Based on the current method for TBI at SU, an optimal technique to increase the dose

homogeneity to the patients would be the field-in-field technique. The accuracy of the Eclipse

TPS for smaller fields, defined by jaws or MLCs, were therefore measured.

The off-axis measurements were collected with different settings for the two SSDs

because we had to handle different problems. The difference between off-axis values in

Eclipse and measured values, normalized at the central beam axis, was higher at SSD = 460

than SSD = 350 cm.

4.2 Eclipse accuracy Larger phantoms resulted in higher doses at 10 cm depth because more photons interact

with matter and contribute to the dose. The cross-section area perpendicular to the beam axis

was important for the dose. Seen for example in the long phantom. The thickness of the phantom

is changing the back-scatter and the dose increases with more phantom-mass behind the point

of measurement to a certain thickness, but it did not affect the doses much at 10 cm depth in

this study. For treatment planning to a patient at extended SSD, this means that the calculated

dose in Eclipse is more dependent on the length and height than the thickness of the patient.

Acuros XB resulted in lower doses compared to AAA, both because of the difference in

grid size and because they are based on different radiation transport methods. The grid size

definition may affect the dose differences at different SSD with AAA since the grids diverge.

The commissioning in TBI geometry was performed with one phantom and when the same

phantom size in Eclipse was used, the dose at SSD = 350 cm agreed and became 4.3 % higher

at SSD = 460 cm. At 10 cm depth, the mean difference in absolute doses between the studied

SSDs in Eclipse was 3.9 % for all phantoms.

As Lamichhane et al. demonstrated with SSD = 400 cm, field sizes 5×5 cm2 and 40×40 cm2,

energy 6 MV, version of Eclipse 11.0.47 and a grid size of 0.25 cm, the differences varied in

measured and calculated doses between 4.9 % in a homogeneous phantom and up to 27.6% for

the heterogeneous phantoms [20]. The differences in the current study were in the same

magnitude as the homogeneous phantom but not the same percent, which was expected since

different settings were used. Studies with heterogeneous phantoms need to be further

investigated at extended SSD.

The study of off-axis values in Eclipse was performed with the phantom size 20×20×40 cm3

which is the phantom most geometrically similar to the one used for commissioning.

The difference in off-axis values between the two SSDs compared with commissioning data

was greater at longer SSD. Profiles started to oscillate at SSDs over 275 cm, which we could

find no physically explanation to. Therefore, further investigations regarding the shape of the

profile were performed. The long phantom was used so that the interesting pattern could be

visible. As the SSD increased, the dose difference became higher and at SSD = 460 cm it was

3.8 %. If treatment planning is to be performed in Eclipse at extended SSD this needs to be

taken in to account. If treatment planning improves the dose homogeneity to the patient more

than the dose differs in the profiles, it may be acceptable to perform treatment planning in

Eclipse. Fields defined by jaws reduced the fluctuations compared with fields created by MLC

23

and/or blocks. No matter if the blocks were completely covering the MLCs or not, the same

shape occurred. There may be something in the source definition which is valid for normal SSD

that cannot be calculated properly at extended SSD. We could not find any previous studies of

profiles at extended treatment distance that noticed this shape.

The comparison with a Monte Carlo simulation showed that the PDD and profiles were in

good agreement with Eclipse. It is assumed that the profile at extended SSD looks like a profile

at any distance. One explanation to why the profiles looks different at extended SSD can be the

algorithms are not customized for calculations at extended SSD.

The transmission was not able to be measured at extended SSD because of the weak signals.

When the SSD is increased, the transmission should become smaller. In this study the

transmission is constant, which could be explained by that it is a constant value given to Eclipse.

The distance between the phantom and the plexiglass did not impact the dose at 10 cm depth

in the phantom, in Eclipse, but may influence the measurements. Investigations of this should

be performed via measurements. Especially because in a patient geometry, the distance between

the patient and plexiglass is changed over the patient length.

4.3 Patient cases and dose planning The difference in dose between Eclipse and MC simulations varied from patient to patient.

No rescaling factor valid for all patients was obtained. Most of the mean dose variations were

within -1 % to +4 % to the total body but varied more if dose to lungs were compared. One

explanation could be that AAA calculates dose to water. The MC fluctuations in dose are ±2

%, most of the changes in patients were in this interval. The differences in V95% indicates that

the dose deposition points differ.

No standard model for the field-in-field technique that could be used for all kinds of patients

was found. All TBI patients required individual treatment planning and optimization of the

number of fields and MU per fields. To treat with individual fields requires careful positioning

of the patient. If possible, the extra fields should be created by jaws and the limitations with

MLCs need to be considered before it is used at extended SSD. The positioning of the patient

at the planning CT varied in the patient material and because of that, it was even more difficult

to find a standard model of dose planning in Eclipse. If treatment planning in Eclipse will be

further investigated there should be a method for patient positioning so that the evaluation will

be more consistent.

4.4 Limitations Accuracy limitations in Eclipse includes the ability to draw plexiglass of exactly 1.6 cm.

Sometimes it becomes slightly smaller or wider (±0.1 cm). Due to the geometrical limitations

of the CT set, the distance between phantom or body and plexiglass has not always been 10.0

cm but placed as close to 10 cm as possible. This was not affecting the results in Eclipse, as

shown in Appendix B.

The default grid size for AAA of 0.25 cm was used. With the smallest grid size (0.1 cm) the

dose remained almost the same, the calculation time increased, and due to much data, the

program was slowed down a lot. With Acuros XB the biggest grid size was used because the

dose could not be calculated due to exceeding the calculation memory. For many of the

calculations, Acuros XB was not doable. Since the grid sizes of AAA and Acuros XB are

24

defined in different ways Acuros XB would be superior to AAA at extended SSD if the dose

distribution or body is inhomogeneous, because it results in smaller grids at the extended SSD.

The dose collection was performed by taking values from a specific point, if further studies

will be performed, the dose to the sensitive volume of a theoretical ionization chamber outlined

in Eclipse may be studied.

All calculations and comparisons were performed with Eclipse version 13.6.23. A new

version of Eclipse TPS was introduced after this study was completed which in an initial study

showed improved profiles at extended SSDs.

Conclusion Eclipse overestimated the absolute dose at 10 cm depth up to 4.3 % at SSD = 460 cm and

by 0.1 % at SSD = 350 cm, if the same settings and phantom size were used as under TBI

commissioning. The PDDs obtained in Eclipse, normalized at 10 cm depth, were in good

agreement with the corresponding commissioning data. Eclipse profiles orthogonal to beam

axis showed dose deviations due to oscillations. The deviations increased with SSD; from 1%

at SSD = 350 cm to 3.8 % at SSD = 460 cm.

The measured doses from small fields were 1.9 % higher than the corresponding doses in

Eclipse if the fields were defined by jaws. The difference for fields with the same openings

but defined by MLCs increased from 3.0 % at SSD 350 cm to 4.4 % at SSD 460 cm.

The average difference in the mean doses to the body, studied with DVH, for the nine

patients was 1.84 % higher in Eclipse than obtained by the MC method. The difference was

within 0.2 % for the mean dose to lungs. All the mean results from this comparison were

within the statistical accuracy of 2 % of the MC method.

Using MLCs to create field-in-fields was complicated but often lead to a better dose

homogeneity. The recommended dose homogeneity of ±10 % is yet not achieved in

Gothenburg with the clinical technique but may be achieved with additional fields. The

fluctuations in the profiles may be acceptable since they are less than the desired dose

homogeneity.

Further investigations of Eclipse at extended treatment distance, are needed especially for

inhomogeneous phantoms.

Acknowledgement I would like to thank my supervisors Roumiana Chakarova, Caroline Adestam Minnhagen

and Kerstin Müntzing for your help, guidance and support through this work.

I would also like to thank the other employees at Therapeutic Radiation Physics at

Sahlgrenska University Hospital for helping me with Eclipse and other problems that have

occurred throughout this work.

25

References 1. TheNobelFoundation. MLA style: Wilhelm Conrad Röntgen -Biographical. Nobel

Media AB 2018. . [cited 2018- 09-25]; Available from:

<https://www.nobelprize.org/prizes/physics/1901/rontgen/biographical/>.

2. Baskar, R.et al., Biological response of cancer cells to radiation treatment. Frontiers

in molecular biosciences, 2014. 1: p. 24-24.

3. Jaffray, D.A.et al., Radiation Therapy for Cancer, in Cancer: Disease Control

Priorities, H.P. Gelband H, Sankaranarayanan R, Editor. 2015, The International Bank

for Reconstruction and Development/The World Bank: Washington DC. 14. 14.

4. Podgorsak, E.B., Radiation Oncology Physics. 2005, Vienna: International Atomic

Energy Agency.

5. NobelMedia. The Noble Prize in Physiology or Medicine 1990. [cited 2018- 09-25];

Available from: https://www.nobelprize.org/prizes/medicine/1990/press-release/.

6. Markowitz, G., The Treatment: The Story of Those Who Died in the Cincinnati

Radiation Tests. Journal of American History, 2003. 90(1): p. 300-301.

7. McLarnon, A. Bone Marrow. British Society for immunology. [cited 2019- 03-26];

Available from: https://www.immunology.org/public-information/bitesized-

immunology/organs-and-tissues/bone-marrow.

8. Dean, L., Blood and the cells it contains, in Blood Groups and Red Cell Antigens.

2005, National Center for Biotechnology Information: Bethesda, Maryland, USA.

9. Ishibashi, N.et al., National survey of myeloablative total body irradiation prior to

hematopoietic stem cell transplantation in Japan: survey of the Japanese Radiation

Oncology Study Group (JROSG). Journal of radiation research, 2018. 59(4): p. 477-

483.

10. Spiryda, L.B.et al., Graft-versus-host disease of the vulva and/or vagina: diagnosis

and treatment. Biology of Blood and Marrow Transplantation, 2003. 9(12): p. 760-

765.

11. Kahn, F.et al., The Physics of Radiotherapy. Vol. 4th edition. 2009: Lippincott

Williams & Wilkins. Chapter 18.

12. Wills, C.et al., Total body irradiation: a practical review. Appl Rad Oncol, 2016.

5(2): p. 11-17.

13. Thomas, O.et al., Long-term complications of total body irradiation in adults. Int J

Radiat Oncol Biol Phys, 2001. 49(1): p. 125-31.

14. Ng, A.K.et al., Secondary malignancies across the age spectrum. Seminars in

radiation oncology, 2010. 20(1): p. 67-78.

15. Couto-Silva, A.C.et al., Final height and gonad function after total body irradiation

during childhood. Bone Marrow Transplantation, 2006. 38: p. 427.

16. VanDyk, J.et al., The Physical Aspects Of Total And Half Body Photon Irradiation.

American Association of Physicists in Medicine, 1986. 17.

17. Strålbehandling vid cancer. The Swedish Council on Technology Assessment in

Healtcare. 2003 [cited 2018 -10-01]; Available from:

https://www.sbu.se/contentassets/8b90c3494eb143e883225e5669603571/kapitel_3.pd

f.

18. Onal, C.et al., Evaluation of field-in-field technique for total body irradiation. Int J

Radiat Oncol Biol Phys, 2012. 83(5): p. 1641-8.

19. Aydogan, B.et al., Linac-Based Intensity Modulated Total Marrow Irradiation (IM-

TMI). Technology in Cancer Research and Treatment, 2006. 5(5).

20. Narottam, L.et al., Going the distance: validation of Acuros and AAA at an extended

SSD of 400 cm. Journal of applied clinical medical physics, 2015. 17: p. 63 -73.

26

21. Metcalfe, P.et al., The Physics of Radiotherapy X-rays and Electrons. Vol. 2. 2007,

Madison, Wisconsin, USA: Medical Physics Pub.

22. Sanchez-Nieto, B.et al., A CT-aided PC-based physical treatment planning of TBI: a

method for dose calculation. Radiother Oncol, 1997. 42(1): p. 77-85.

23. Sahlgrenska University Hospital: Department of Therapeutic Radiation Physics.

24. Sievinen, J.et al., AAA photon dose calculation model in Eclipse. Varian Medical

Systems, 2005.

25. Faille, G.W. et al. Acuros XB advanced dose calculation for Eclipse treatment

planning system. Varian Medical Systems, 2010.

26. Bush, K.et al., Dosimetric validation of Acuros XB with Monte Carlo methods for

photon dose calculations. Med Phys, 2011. 38(4): p. 2208-21.

27. Chakarova, R.et al., Monte Carlo optimization of total body irradiation in a phantom

and patient geometry. Phys Med Biol, 2013. 58(8): p. 2461-9.

Appendix

Appendix A

PDDs for SSD = 350 cm and SSD = 460 cm are shown in Figure 25 and in Figure 26

respictively, for all phantom sizes. Normalized at 10 cm depth.

Figure 25. PDDs for all phantoms used in this study at SSD = 350 cm.

Figure 26. PDDs for all phantoms used in this study at SSD = 460 cm.

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35 40 45

Rel

ativ

e d

ose

[%

]

Depth in phantom [cm]

PDD for SSD = 350 cm

Phantom 20×20×20 Phantom 30×30×30 Phantom 40×40×40

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40 45

Rel

ativ

e d

ose

[%

]

Depth in the phantom [%]

PDD for SSD = 460 cm

Phantom 20×20×20 Phantom 30×30×30 Phantom 40×40×40

Appendix B

The calculated impact of phantom to plexiglass distance with a phantom of size 20×20×20

cm3 at SSD 460 cm, presented in Table 9.

Table 9. The impact on doses at 10 cm depth when the distance between the phantom and plexiglass was changed.

Distance Dose

2 cm 1.043 Gy

4 cm 1.043 Gy

15 cm 1.043 Gy