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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