ROC7080: RADIATION THERAPY PHYSICS LABORATORY

LAB 1 : FILM DOSIMETRY

GROUP I SPRING 2014

KEVIN JORDAN

GRADUATE STUDENT, RADIOLOGICAL PHYSICS KARMANOS CANCER CENTER

WAYNE STATE UNIVERSITY

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Table of Contents

I. Introduction ................................................................................................................. 3 II. Laboratory Procedures & Methods ......................................................................... 3

II.a Quality control of densitometers ....................................................................................... 3 II.b Calibration of film dosimetry system ............................................................................... 3 II.c Measurement of beam profiles .......................................................................................... 3 II.d Manual calculation of beam parameters .......................................................................... 3 II.e Measurement of output and depth dose curve for small electron cutout ...................... 4

III. Questions & Discussion ............................................................................................ 4

III. Appendix A: Experimental Data ............................................................................. 10 IV. Appendix B: Sample Calculations ......................................................................... 19

V. Appendix C: References ......................................................................................... 19

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I. Introduction

In this laboratory, the uses and mechanisms of film dosimetry have been explored. The proceeding sections can be found to include Laboratory Procedures & Methods, Questions & Discussion along with the appendices, which include the experimental data, sample calculations and references.

II. Laboratory Procedures & Methods

II.a Quality control of densitometers Due to not having any test strips available, the group was not able to conduct Laboratory Procedures & Methods Part A.

II.b Calibration of film dosimetry system Relevant Information: Film Type: Kodak EDR2 Film Processor Type: Kodak OMAT RPX M6B Processing Temperature on processor readout: 34.8°C A linear accelerator was used to irradiate Kodak EDR2 films between 45 – 1200 cGy. Recall that one has 1cGy/MU at 100cm SSD and 10x10 field with photons with dmax being at 2.7 cm for this 15MV photon energy. Note that three times the specified dose was used for this section due to V films not being available in the clinic during the laboratory. The resulting Optical Density (OD) and ODnet versus dose plots, respectively, can be found in Figure 4 and Figure 5, based on the recorded raw data from Table 1. Calibration Curve, Raw OD Data

II.c Measurement of beam profiles

Table 2 can be found to have respective MU values for the open and wedge 15MV fields, based on the hand calculations found in Appendix  B:  Sample  Calculations.

II.d Manual calculation of beam parameters Please see Table 3 and Table 4 below for the manual calculation of these values.

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II.e Measurement of output and depth dose curve for small electron cutout

Please see Table 6, Figure 10, and Figure 11 below for the measurement values for output and depth dose for small electrons.

III. Questions & Discussion

Q1: Why might you anticipate significant variations in measurements within the same density step using the scanning densitometer?

The laboratory did not have the materials to do this question from Section A of the lab methods, but one could hypothesize potential issues and/or variations in light output from the scanning densitometer could cause measurement variation. Additionally, the base+fog within that region could vary between small steps within the film.

Q2: Using this curve, determine the dose range that you would prefer to perform film dosimetry in and state why.

One would want to utilize the steepest portion of the plots in order to provide the smallest error and deviation between optical densities and therefore dose. A larger deviation can and will occur in the least steep areas of the curve, thus causing significant variability in the potential dose.

Q3: Why don’t we just plot profiles of optical density and get our flatness, symmetry and field sizes from this?

Because the OD curve is relative and not absolute one would not want to plot OD profiles and get flatness and symmetry. In order to ensure the most accurate, absolute dose distribution one needs to have a standard of calibration to normalize against, like the group did by calibrating against a known standard within the RIT scanning densitometer software. With this accuracy, the flatness, field size and symmetry values are obtained appropriately based off a normalized calibrated standard. Also, optical density could be looked at as a line measurement, while flatness and symmetry could be looked at as a slice or plane.

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Q4: Write the formula you are using to calculate beam flatness (there are several) and specify where it comes from. The formula for beam flatness can be found in Figure 1 below. The formula can

be found to be represented by:

𝐹𝑙𝑎𝑡𝑛𝑒𝑠𝑠 =  𝐷!"# − 𝐷!"#𝐷!"# + 𝐷!"#

𝑋  100

Figure 1. Beam Flatness Equation & Parameters

Q5: Are your measured flatness values acceptable? What criteria did you use to make this judgment? Where do these criteria come from?

The typical requirement for beam flatness is less than 3% unflatness, which is specified in TG45. The results of the PDD curves all show to have a unflatness less than 3% so they are within the acceptable range.

Q6: Your physician has written the prescription based on dmax and d90% values found in the treatment machine data book (which are, of course, for a large field). What advice will you give to the physician after analyzing your data?

Due to small cutout opening based on the given energy, 9MeV, there is a loss of lateral scatter equilibrium (LSE). The approximate rule of thumb of MeV/3 then one

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would have 3cm or greater needed to establish LSE. With the cutout radius being smaller than that, the output factors are changed. When the field is large enough for a given energy, the electrons on the periphery can not make into region of interest or measurement point, thus for every electron near the measurement point that scatters in, there is one that scatters out. However, with small field sizes those scatter electrons on the periphery can easily make it to the point of measurement and the in scatter will cause more charge to be deposited in that area, and thus more surface dose; LSE is disrupted and the dmax and d90 values are shifted to left on the PDD curve. From Table 6 and based of the analysis from Figure 10 and Figure 11, one can see that there is a 193% depth over estimation at dmax and 120% depth over estimation at d90, and thus the physician should consider these true values and not trust the TPS data based on 10x10 open field output factors.

Q7: It turns out that one of your colleagues measured this cutout last night using a p-p ion chamber in a water phantom. He measured ionization in the chamber as a function of depth using identical setup parameters, but his curve looks different than yours. Why?

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This curve generated is a Percent Depth Ionization Curve (PDI) and thus still needs to be converted to Percent Depth Dose (PDD). Recall that for photons, the charge collected is approximately proportional to Dose in water (Dw). However, for electrons, this is not true:

𝐷! 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠  !"#!#"$%#&'(

 𝐿𝜌  𝑎𝑛𝑑  𝑃!"

Therefore, these changes need to be accounted for a function of depth.

Q8: What is the definition of “optical density”? “Net optical density”? Optical density (OD) is the measurement of blackness or opacity of the film. It is

based on the following equation and is measured using a densitometer. 𝑂𝐷 = 𝑙𝑜𝑔 !!

!

The usable range for OD is up to about 3, beyond that point the film becomes

black beyond recognition. Naturally, without any radiation exposure, film already has some blackening; the base, which attenuates some light, and the fog, which are developed silver (Ag) grains. This base + fog value tends to be approximately 0.2. The ODnet, therefore is:

𝑂𝐷𝑛𝑒𝑡 = 𝑂𝐷𝑟𝑎𝑤 − 𝑂𝐷(𝑏𝑎𝑠𝑒 + 𝑓𝑜𝑔) Q9: You realize that you have lost one of your films from this lab and must irradiate a new one. Can you use the same dose response curve to evaluate it or do you have to make a new one? Suppose you haven’t processed any of the old films yet. If you process them all together, can you use the new film with the dose response curve constructed from the old films?

One would need to make a new dose response curve, all curves should be done under the same batch of film and reference conditions. It would not be recommended to do the later either, as the reference conditions under which the new film exposures were taken will still be different from the reference conditions under which the previous days exposures were taken. The nominal procedure would be to take all films on the same day, reference conditions and batch of film in order to be most accurate in the normalization with the dose response curve. Q10: How does the measured open field photon profile change with depth? Why do these changes occur? Recall that photons have a very definitive profile in which there is buildup region; initializing at a particular dose based on the particular energy and begins to deposit

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energy once the surface of interest is encountered. From there, the photon dose will gradually increase to a maximum, within this region being the buildup region. Beyond, the buildup region there will be an approximately proportional decrease in dose until the photon has exhausted all such interactions at particular depth, based on the energy and material(s) encountered in its path. Q11: How do the inplane and crossplane flatness compare? What affects the flatness of a linear accelerator field? How can it be adjusted? The crossplane and inplane flatness are quite comparable to each other; differing only by 0.8% in the 2.7 centimeter deep field and 0.3% in the 10 centimeter deep field. Please see Table 4 for these values in III.  Appendix  A:  Experimental  Data.

The flattening filter along with the positional and angular steering coils are responsible for the flatness of a linear accelerator field. The flatness is dynamically adjusted in real time through the monitor chamber and servo electronics to adjust the position and angular deviation to meet the proper flatness tolerance; a pictorial example can be found in Figure 2 below.

Figure 2. Monitor Chamber & Steering

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Q12: How do the inplane and crossplane penumbra compare? Based on this comparison, which jaw do you think is the inner jaw (closest to the source) in the treatment head?

The inplane (vertical) penumbra is much greater on one side than that of the crossplane (horizontal) penumbra’s one side. The large magnitude inplane penumbra of this given side is closest to the source, thus the smaller magnitude penumbra is the outer jaw and thus farthest from the source. Please see Figure 3 below for a visual representation of this. Recall that decreasing the source to collimator distance (SDD) increases the penumbra, and increasing the SDD decreases penumbra; they are inversely proportional.

Figure 3. Penumbra Physics & Radiation Therapy

Q13: How does the magnitude of the “hot spot” under the wedge change with depth? Why might this change occur? The hot spot occurs due to the thin edges of the wedge and the magnitude of these hot spots increases with wedge angle and field size (Khan pp. 194). One can easily see in

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Figure 6 and Figure 7 how the normalized output starts at 120% and then linearly decreases with increasing thickness of the wedge.

III. Appendix A: Experimental Data Table 1. Calibration Curve, Raw OD Data

Table 1: Calibration Curve Data

Dose (cGy) Optical Density Net Optical Density0 0.20 0.00

45 0.53 0.3390 0.98 0.78

150 1.58 1.38225 2.20 2.00300 2.76 2.56390 3.25 3.05480 3.63 3.43600 3.89 3.69900 4.11 3.91

1200 4.18 3.98

Table 2. Hand Calculations for the Measurement of Beam Profiles

Measurement of Beam ProfilesFilm Wedge Depth in Phantom MU Delivered (15MV)

1 NONE 2.70 46.502 NONE 10.00 53.003 45 2.70 88.004 45 10.00 100.00

Table 3. Width Measurements (NO Wedge)

Table 2: Width Measurements

Field Width (cm) Penumbra(cm) Field Width (cm) Penumbra Width (cm)1 (2.7cm deep) 1.77 197 12.1281 8.9712 12.1794 1.13922 (10cm deep) 1.74 193 12.1324 9.1492 12.1922 1.4596

Crossplane (Horizontal)

Film Central Axis NOD Central Axis Dose (cGy)

Inplane (Vertical)

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Table 4. Flatness Measurements (NO Wedge)

Table 3: Flatness Measurements

Maximum Minimum %Unflatness Maximum Minimum %Unflatness1 (2.7cm deep) 101 98 1.61% 104 99 2.42%2 (10cm deep) 102 98 1.67% 103 99 1.95%

FilmInplane (Vertical) Crossplane (Horizontal)

Table 5. Wedge Measurements

Table 4: Wedge MeasurementsMaximum Symmetrically Opposite MaximumPosition (cm) Dose Position Dose (cGy)

3 (2.7cm deep) 1.63 180 -3 310.3152 3 310.31524 (10cm deep) 1.72 191 -3 325.52 3 325.52

Film Central Axis NOD Central Axis Dose (cGy)

Table 6. Electron Cutout Measurements

Table 5: 9 MeV Electron Cutout MeasurementsMaximumOD Dose (MU)

Cutout 1.99 200 0.41 1.2510x10 OPEN 2.01 200 0.79 1.5

overdose% 193% 120%

D90% dose (cm)Film Depth of dmax (cm)

Figure 4. 15MV, 100SSD, 10x10, OD vs Dose (cGy)

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

0 200 400 600 800 1000 1200 1400

OD

Dose (cGy)

15MV Photons, 2.7cm =dmax, 100SSD, 10x10: OD vs. Dose (cGy)

Series1

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Figure 5. 15MV, 100SSD, 10x10, ODnet vs Dose (cGy)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0 200 400 600 800 1000 1200 1400

OD

Dose (cGy)

15MV Photons, 2.7cm =dmax, 100SSD, 10x10: ODnet vs. Dose (cGy)

Series1

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Figure 6. 15MV, 2.7 cm deep, with Wedge

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Figure 7. 15MV, 10 cm deep, with Wedge

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Figure 8. 15MV, 2.7cm deep, NO Wedge

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Figure 9. 15MV, 10 cm deep, NO Wedge

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Figure 10. 9MeV Electrons, 10x10 Applicator, with Cutout

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Figure 11. 9MeV Electrons, 10x10 Applicator, NO Cutout (OPEN)

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IV. Appendix B: Sample Calculations

V. Appendix C: References AAPM,.TG45, Report 47.

Burmeister, Jay. Radiation Dosimetry Coursepack. 2014 Rakowski, Joe. Radiation Therapy Physics Course Notes. 2014