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Radiation TherapyTechnical Aspects
Introduction
Treatment modalities for cancer:
Surgery – Radiotherapy – Chemotherapy
Radiotherapy – 50% of patients
About 60% of all tumor patients can be
considered to be potentially curable.
(Malignant localized tumor, no metastatic
disease)
The aim: to deliver a radiation dose, to kill all
tumor cells.
Difficulties: OAR are located close to the tumor
The double goal of radiation therapy:
• Increase the dose to the target volume !
• Decrease the dose to healthy tissue !
1. better tumor control
TCP – Tumor Control Probability
2. decrease of side effects
NTCP – Normal Tissue Complication
Probability
These means higher probability of patient cure.
CONVENTIONALCONFORMAL
CONVENTIONALCONFORMALIMRT
The course of radiotherapy
Radiotherapy treatment chainRadiotherapy treatment chainRadiotherapy treatment chainRadiotherapy treatment chainRadiotherapy treatment chain
I. Patient Immobilization
High TU dose, low dose at OARs
Sophisticated delivery techniques very steep
dose gradient between target and the organs at risk
patient immobilization is a crucial issue.
Setup errors – underdosage in the target,
overdosage in the normal tissue.
General Considerations
1. Definition of Target Volumes (ICRU 50, ICRU 62)
GTV (Gross Tumor Volume) – the clinically evident
target volume, visible in diag. images.
CTV (Clinical Target Volume) – GTV + margin
(containing micr. population of tumor cells)
PTV (Planning Target Volume) – accounts for setup
uncertainties, organ motion and deformation
The PTV should have a high probality of containing the CTV during the whole treatment
IMMOBILIZATION
2. Sources of Uncertainties
a. Patient setup uncertainties
b. Organ motion and deformation
3. Design Requirements
General intention to reduce the CTV – PTV safety margins.
High reproducibility.
Compatible with the imaging modalities.
Practical and easy to use.
Comfortable for the patient.
Immobilization Techniques
Head targets: Invasive fixation – radiosurgery
total dose – in a single fraction
Overall setup uncertainty < 1mm
INVASIVE FIXATION
Non-invasive fixation
bite blocks and/or face masks
mask – individually fabricated for each patient
thermoplastic material, polyurethane foam, self-hardening Scotch-cast bandages.
MASK OF SCOTCH –CAST BANDAGES
Extracranial Targets
much more challenging
rotations around the body axis
target position may change within the body
organ motions
Several possible solutions
Vacuum pillows
Evacuated flexible bags
Self-hardening bandages – thermoplastic sheets, bandages.
Decreasing of movements caused by breathing
Breast Treatments
one of the most complicated problems
fixation of opposite side mamma
position of arms
VACUUM PILLOW
VACUUM PILLOW PRODUCTION
VACUUM FIXATION SYSTEM
SELF-
HARDENING
BANDAGES
ABDOMINAL
COMPRESSION
DEVICE
II. Imaging
Imaging for therapy planning serves the following purposes• A target volume (TU) and the organs at risk are the basis for therapy
planning. 3D patient model beam directions are optimized.
• The dose is calculated based on CT data. DVHs can be plotted for the tumor and the organs at risk.
• A 3D-anatomical model is also required for positioning of the patient at the therapy device.
A 3D-model is normally obtained using X-ray computed tomography (CT).
Functional imaging (MRI, PET, SPECT)
useful for the definition of tumor, allows the visualization of microscopic
disease outside the region of highest cell density.
1. X-ray Computed Tomography (CT)
2. Magnetic Resonance Imaging (MRI)
3. Nuclear Medicine SPECT (Single Photon Emission Tomograph)
PET (Positron Emission Tomograph)
III. Tumor Localization
Before image data can be used for radiation treatment planning relevant structures have to be identified
– Which structures are important?
– How structures and volumes can be delineated?
– How can be combined different modalities?
1. Volume Definition
Two different kinds of structures are important
– The target volume
– The organs at risk (OAR),which have to be spared.
ICRU Report 50 (1993) és ICRU Report 62 (1999)
(International Commission on Radiation Units and Measurements)
GTV, CTV, PTV
– GTV – Gross Tumor Volume
– CTV – Clinical Target Vomume (GTV+margin)
– PTV – Planning Target Vomume (CTV+margin)
IMPORTANT STRUCTURES
VOLUMES in ICRU 50
2. Image segmentation
Segmentation – the process of distinguishing relevant structures/volumes from the background.
1 slice – 2D segmentation
More than 1 slice – 3D segmetation
Segmentation in radiation treatment planning
_- delineating the PTV
– delineating the organs at risk
– delineating the surface contour of the body
Manual segmentation
Semiautomatic segmentation algorithms
Automatic segmentation algorithms
Two groups of segmentation algorithmsRegion-based approaches (find an area of similar properties)
Edge detection algorithms (look for sudden changes)
3. Image Registration
Image sequences of various modalities are used:
CT, MRI, PET, SPECT
A definite relation is necessary between the picture elements (pixels).
Registration : methods which are able to calculate these relations (transformations)
Eg. At least 3 corresp. pairs of points transformation
matrix calculation correlation between the two sequencies.
Manual registration: user interaction
Semiautomatic registration: partly user interaction
Automatic registration: do not require any user interaction
Scope of transformation: global and local
Geometrical properties: rigid –elastic transformation
Image fusion – Display of registered image sequences
PARALLEL DISPLAY
SLIDING WINDOW
INTERACTIVE MATCHING
IV. 3D Treatment Plannig
The goal of planning: to find the optimal treatment plan.
Based on 3D model of patient anatomy.
- to find the optimal beam directions.
- to form the beam shape exactly to the tumor
shape (to minimize the dose to healthy tissues).
- to accurately calculate the physical dose distribution.
The 3D patient model is based on
- 3D tomographs (CT,MR,PET)
- 2D slices 3D (image cube)
3D Model
3D patient model
3D navigation
- Contouring transformation to a 3D model
(interpolation)
The Radiotherapy Planning Cycle
- a series of beams are applied in order to concentrate the
dose on the target volume.
- the beams (dose) are superimposed on the target.
- the healthy tissues can be kept below tolerance lewels.
The 3D model
Dose distributions
THE PLANNING CYCLE
Acquisition of CT(MR,PET)image sequences
Definition of tumor, target volume and organs at risk
Definition of treatment parameters
Virtual therapy simulation
Dose calculation
Evaluation of dose distribution
Patient treatment
Optimization
1. Definition of Beam Directions
- Spatial relations between target volume and organs at risk!
- Main criterion: the target volume is enclosed completely by the beam without enclosing any organs at risk. If it is not possible, to minimize the volume of organ at risk covered by beam.
Tools: Beam's Eye View (BEV)
The planner views the 3D-model from the position of the radiation source.
Interactive Beam’s Eye ViewBeam’s Eye View
Beam's Eye View
Beam's Eye View
BEAM 1 BEAM 2 BEAM 3
Beam's Eye View
BEAM 1 BEAM 2 BEAM 3
Beam's Eye View
BEAM 1 BEAM 2 BEAM 3
Beam's Eye View
- Observer's View
presents the 3D model from an arbitrary
point of view.
Helps to minimize that subvolume where the single
beams overlap.
- Spherical View
Observer’s ViewObserver’s View
Spherical View
Observer’s View
Observer’s View
Spherical View
2. Additional Treatment Parameters
Irradiation directions and beam shapes
A series of other parameters:
- Selection of radiation type: fotons - generally
electrons – superficial tumors
protons, heavy ions
- energy of radiation, beam quality
- beam modifying devices
bloks, wedges, compensators, dynamic
collimátors etc.
It is possible to shape the 3D dose distribution
to better match to the form of target volume.
3. Dose Calculation
- A dose calculation algorythm calculates the
expected dose distribution using the beams
specified.
4. Evaluation of Treatment Plans
Several alternative configurations can be compared
and the most suitable one used.
Qualitative Evaluation of Dose Distribution
Biological Models : TCP, NTCP
Forward Planning, Inverse Planning
Forward and Inverse Planning
- 3D dose distribution: isodose surfaces
- Isodose distribution from slice to slice:
isodose lines, color wash
- Dose-Volume-Histograms
DVH – a simple way of displaying the 3D dose
distribution.
DVHs usually are displayed as cumulative histograms
showing the fraction of the total volume receiving doses
up to a given value.
3D isodose distributions
2D isodose distributions
Dose-Volume- Histograms
V. Patient Positioning
Substancial role in radiation therapy:
Fixation and immobilization of patient
Absolute positioning at the irradiation device.
1. Step: Definition of patient-fixed coordinates.
2. Step: Image acquisition.During planning target point is calculated in patient-fixed coordinates.
3. Step: Patient positioning at the irradiation device, immediately before treatment.(To move the target point to the isocenter)
Patient-fixed coordinate system
Patient-fixed coordinate system
Image acquisition and target point coordinates
Positioning at the irradiation device
Positioning at the irradiation device
2. Portal Films and Electronic Portal Imaging
Films: provide information for repositioning of patients, from fractions to fractions.
Electronic portal imaging devices (EPID): real time images, time efficient patient positioning.
Types:Fluoroscopic systems, scintillation screen – camera.
Ionization chamber arrays, e.g.256 x 256.
Patient positioning based on external markers and anatomical points . (Final control.) Comparison of :
Simulator images Port images
DRR Port images
The Beamview system
X-ray images and electronic ports
VII. The Treatment
A. Treatment modalities
1. Linear Accelerators (Linacs)
Basic idea : to accelerate electrons in the field of electromagnetic wave travelling in a waveguide.
Principle of an elementary linac: X-ray tube
A high voltage of several MVs means a big insulation problem or a great tube size.
Instead of high-voltage a series of smaller voltage are applied.(These fields are produced by microwaves)
Linac
Increase of electron speed with energy
Concept of an elementary accelerator
Microwave cavities
electron oscillations in the wall,
acceleration of electrons in the cavities.
Travelling-wave Accelerator:
a series of microwave cavities of a length equal to one-quarter wavelength.
for a 10 MeV electron beam 125 cm length
at higher energies standing wave waveguide
Principle of electron acceleration
Travelling wave acceleration
Travelling wave acceleration
Gyorsító cső metszete
Standing wave accelerator:
the RF energy reflected at both ends creating a standing wave.
The length of each cavities equal one-quarter wavelength.
Half of the cavities have zero field all times, they can be moved off the beam axis.(Shortened standing wave tube)
2. Accelerator Major Subsystems
structure - gantry.
RF source (magnetron or klystron), modulator, circulator, waveguides, electron gun, AFC system, cooling system, gas system, vacuum system,
treatment head: bending magnet, target, primary collimmator, flattening filter, monitor chamber, secondary collimator.
Generation of a standing wave
Standing wave acceleration
Shortened standing wave tube
Shortened standing wave tube
Shortened tube
3. Multi-Leaf-Collimators (MLCs)
In clinical radiotherapy it is often necessary to produce irregular shaped fields.
Two possibilities: beam shaping with blocks
use of multi-leaf-collimators (MLCs)
Integrated MLCs:
medium size and large fields (up to 40x40 cm2)
Accessory MLCs: e.g.for stereotactic conformal therapy, micro-MLCs, small fields (10x10 cm2)
Integrated MLC
Accessory type micro-MLC
Accessory type micro-MLC
Accessory type micro-MLC
Beam shaping with micro-MLC
Important features
Maximum field size – 40x40 cm2, 10x10 cm2
Leaf resolution (leaf width) – 1 cm, 2-3 mm
Maximum overtravel
How far a leaf can be moved over the midline
Operating modes
Static: Dynamic:
Focusing properties and penumbra
MLC in static mode
MLC in dynamic mode
B. Treatment Procedures
(conformal therapy)
1. Convencional (classical) conformal RT
The basic problem:
- PDD is an exponential decreasing function of depth. The dose is higher close to the surface than at the depth of tumor.
The solution:
- using more fields
- to tailor the beams to the shape of target volume.
the conformity of dose distribution can be increased.
Conformity:
- a 3D dose distribution should follow the tumor shape while sparing the OARs.
Four-field treatment technic
How can be conformity increased?
Higher number of beams
Optimization of beam directions
Optimization of beam energy (photons))
Application of a MLC.
Smaller leaf width.
More than one target point.
Moving bean irradiations.
The Limits of Conventional Conformal RT.
Conformal and homogeneous dose distribution cannot be obtained in all cases.
-Difficult to find good directions.
-Beam overlap in the case of high number of beams.
Micro-MLC
Tumor and OARs
Multifield radiation
2. Intensity Modulated Radiation Therapy (IMRT)
Solution: IMRT-technique.
Basis: generation of intensity modulated fields and to treat with these fields.
(Fig. Shows a beam arrangement with seven IMFs).
How to deliver these fields?
Step-and-Shoot technique (superposition of irregurarly shaped and partial overlapping field components.)
Sliding Window technique, or dynamic MLC(independently moving leaves during radiation)
Physical Compensators (absorbing material)
The principle of IMRT
Intensity modulated fields
Step-and-Shoot technique
Dynamic MLC technique
a, Step-and-Shoot technique
(static, Bortfeld-Boyer technique)
Generally an IMF is the superposition of irregularly shaped and partial overlapping field components.
Terms: intensity map, channel, intensity level, field component (subfield, segment)
Close-in technique
Sweep technique
Close-in: leaves are moving in both directions
Sweep: leaves are moving in one direction only
Step-and-Shoot Close-in technique
Technical terms
in IMRT
Step-and-Shoot Close-in technique
Step-and-Shoot Close-in technique
One leaf pair
Step-and-Shoot Sweep technique
Step-and-Shoot Sweep technique
Step-and-Shoot Sweep technique
One leaf pair
Step-and-Shoot Close-in and Sweep technique
b, The Dynamic Technique
An analog-limiting case of the sweep technique, called sliding window technique.
Leaf position accuracy is very important.
Shorter treatment time High complexity
No problems with low dose fields Verification too.
c, Physical compensators
Compensator: an absorbing material with variable thickness. The prescribed intensity is produced
by the thickness of the matter.
+ −
Dynamic IMRT technique
Small positioning error– large dose error
Physical
compensator
Physical compensator
Some specifics:
- Every intensity map – individual compensator (labour intensive)
- Divergence - layers
- High spatial resolution.
- Faster, than the step-and-shoot. (treatament time)
- IMFs without MLC.
- Mold material.
VII. Clinical Radiation Dosimetry
1. Principles
Definition of Dose: the absorbed dose is the energy absorbed in the dm mass element divided by the dm.
Clinical dosimetry, water absorbed dose to water
Radiation types and fields:
wave or particle
Two types of radiations play a major role in radiology.
- photons: X-rays or gamma rays, with energies in the range of keV and higher. Zero rest mass.
- electrons: they have a rest mass and a negative charge. Electron states or nucleus transition (beta-rays)
(Gy – Gray)
Radiation field: a part of the space where rays or particles are moving.
Flux density: the number of particles which cross through a small perpendicular plane per unit time.
Energy Transfer by Photons and Electrons
Photons: - photoelectric effect
- Compton effect
- Pair production
These interaction processes release secondary electrons, which again interact with the matter.
Electrons: - collisions with the atoms or electrons.
- radiative processes (Bremsstrahlung production)
Inelastic collisions with the electrons in the atomic shell lead to excitation and ionization of the atom.
Photoelectric effect
Compton effect
Pair production
2. Measurement of Dose
A variety of physical and chemical effects can be used.
Ionization in gas ionization chamber
proportional counter
Geiger-Müller counter
Ionization in solid semiconductor crystal
Luminescence TLD
Chemical effects photografic film
chemical dosimeter
Thermal effects calorimeter
Absolute Measurement
Farmer-type ionization chamber + water phantom
1. Positioning 2. Connection 3. Measurement 4. Calculation 5. Corrections
1. Positioning
2. Connection
3. Measurement
4. Calculation
ND,w – calibration factor, SSDL ref. cond.
5. Corrections
k = kρ·ks ·kp ·kQ e.g.
Relative dose measurement
Chamber positioning in water phantom
Connection to the electrometer
Measurement with the electrometer
Control source for density correction
Air density correction
3. Phantoms
A measurement of absorbed dose is performed within an absorbing medium called phantom.
Standard phantoms
Water phantom: TBA (Therapy Beam Analyzer)
Anatomical phantoms: Alderson-Rando phantom
IMRT phantoms
4. Dose verification
A dose verification test is required to guarantee, that the radiation applied to the patient is exactly the same as simulated and calculated by the computer.
Steps of the verification:
a, Virtual treatment of an appropriate phantom (plan transfer to the phantom, calculation, dose distr.)
b, Irradiation of the phantom measurement
c, Comparison of the calc. and meas. results.
Standard phantom
Water phantom
Alderson-Rando phantom
IMRT phantom
Comparison of plan and measurement