Physics and Imaging in Radiation Therapy

Physics and Imaging in Radiation Therapy

Giovanni Maria PiacentinoModulo di radioterapia

95% of Radiation Therapy is to Treat Cancer

X-ray of a Crab

Outline

• Introduction• Brachytherapy• Production of External Beam Radiation • Quantifying the Amount of Radiation• Interaction of Radiation with Matter• Conventional Radiotherapy Treatment Processes• 3-D Imaging for Radiation Oncology • 3-D Conformal Radiation Therapy• Tomotherapy• Conclusions

Brachytherapy

• Literally, close-up therapy, it is the treatment of cancer with radioactive sources.

• The oldest form of radiation therapy and the most often used until the advent of megavoltage (energies of more than 1 million electron volts) photon beams.

• Naturally occurring radium, discovered by Marie Curie, dominated the practice until artificial radioactive sources introduced.

• Modern practice:– intracavitary treatments for GYN malignancies. – permanent implants of radioactive seeds, for example, for

prostate cancer.– intraluminal treatments with radioactive beta sources.– superficial sources for superficial lesions.

Common Photon Sources for Brachytherapy

Isotope Half-Life Half-ValueLayer (Water)

Half-ValueLayer (Lead)

Cs-137 30 years 8.2 cm 6.5 mm

Au-198 2.7 days 7.0 cm 3.3 mm

Ir-192 74 days 6.3 cm 3.0 mm

I-125 59 days 2.0 cm 0.02 mm

Pd-103 17 days 1.6 cm 0.01 mm

Brachytherapy Processes

• Image the patient with anterior-posterior and lateral x-rays.

• Given the prescription, determine the source strengths to use.

• Place the applicators into the patient.• Load the sources into the patient.

– In high-dose rate (HDR) brachytherapy the patient returns for several treatments.

– In low-dose rate (LDR) the patient is treated as an in-patient.

– Short-lived radioactive seeds are left in permanently.

Intracavitary Brachytherapy

Protocol X-rays

Dose

Distribution

A

A

B

B

A: Anterior- Posterior

B: Lateral

Production of External Beam Radiation

Block diagram of a linear accelerator. A magnetron or klystron produces radio waves.

Animation of Electron Acceleration in a Linac

E

Linear Accelerator Waveguide with 4 Cavities

The vectors indicates the electric field, E, (volts / cm) in the linac waveguide. The maximum strength of the electric field ~200,000 V/cm travels with the electron bunches traveling down the waveguide.

Treatment head configuration for megavoltage photon (A) and

electron beams (B).

Photon

BeamElectron

Beam

Accelerated Electrons

Quantifying the Radiation Dose

• The unit of radiation dose is the Gray, which is an SI unit equal to 1 Joule of energy absorbed per kilogram of matter.

• The energy absorbed is in the form of ionization and atomic excitation of matter.

• Both the ionization and excitation of matter can be measured.

• The most common measuring systems are the ion chamber for accurate quantification, thermoluminescent dosimeters (TLD) for convenience and radiographic film for spatial resolution.

Measuring Ionization with an Ion Chamber

++++++++++++++++++++++++++++++++++++

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

+

+ -

-

- +

-

+

-+ -+

-

+

-

+

--

+

Anode

Cathode

A

Radiation ionizes air in an ion chamber. Negative ions migrate toward the anode and positive ions towards the cathode. Ions reaching the electrodes cause current to flow in the circuit. Recombination of ions in flight reduces the measured current.

Photon Beam Penetration with Depth

Notice the low dose near the surface and nearly an exponential fall-off with depth.

Electron Beam Dose Distribution

A is the image of the beam on a sheet of radiographic film. B is the isodose plot (lines of equal dose). Notice the high dose on the surface and the finite range of the electron beam.

Interaction of Radiation with Matter

Interaction of neutrally-charged particles.

Animation of Radiation Interaction

+ -+ -

+ -Water Molecule

Photon Interacts, Setting in Motion a Fast Electron

Ions Created

Formation of Radicals andLong Lived Products

+ -

+ H+

OH

H2O- H

OH-

H2OH H2O2 2

Hydroxyl

Radical

Hydrogen

RadicalHydrogen and Hydrogen Peroxide Products

Conventional External Beam Radiotherapy Treatment Process

• Conventional radiotherapy planning relies on 2-D images from conventional planar x-rays which delineates well the position of bony anatomy but is not useful for visualizing soft-tissue.

• Often opposing beam directions are used.• The boundary of the field is determined from the planar

x-rays.• The field shape determines the shape of custom-made

blocks that need to be fabricated.• The patient is treated from each beam direction daily.• Process is very labor intensive, and without rigorous

quality assurance, may be prone to errors.

Fixation used for head and neck treatment.

A treatment simulator has an x-ray tube to image the patient in

treatment position.

Image from a treatment simulator to determine the

shape of the treatment field.

Tracing out the shape of the field to make custom blocks to

shield normal tissue.

The custom block is fabricated out of low-melting temperature heavy metal alloy (mainly lead)

The custom block is mounted is attached to the linac to treat the

field.

3-D Imaging for Radiation Oncology

• Cormack and Houndsfield won the Nobel prize in Medicine for the invention of the computed tomography (CT) scanner.

• A CT scan is a representation of the patient’s electron density (# electrons/volume). Density differences as little as 0.3% can be detected.

• The magnetic resonance imaging (MRI) scanner is becoming the most important diagnostic tool in medicine.

• MRI can produce a variety of images related to the amount of hydrogen nuclei present and the coupling of their nuclear spins to surrounding matter.

Computed Tomographic Scanning

Photograph of a cross-section through a human abdomen.

CT scan through the same section.

3-D Visualization

Volume rendered image of a head and neck representation obtained from fast CT using a contrast agent. The neck nodes are clearly visible as is the vasculature.

Magnetic Resonance Imaging

Abdominal MRI. From the National Library of Medicine Visual Human Project.

Comparison Between CT and MRI

a) Axial CT

b) Axial MRI

c) Coronal CT

d) Coronal MRI

Tumor seen only on MRI.

MRI Angiography (MRA)

Arteriol-venous malformations (AVM) are often treated with radiation therapy. The nidus of malformed vessels is clearly visualized using MRA.

AVM Nidus

3-D Conformal Radiation Therapy (3-D CRT)

• 3-D CRT relies on obtaining a 3-D representation of the patient from CT or MRI.

• It is much easier to plan the delivery of oblique and non-opposed beam directions.

• The beams can be much better delineated with respect to soft-tissue boundaries.

• Modern accelerators have multileaf collimators which produce irregular field shapes without having to cast heavy metal blocks.

• Treatment verification is still a problem.

CT for planning the radiation treatments.

CT slices forming a patient representation.

The tumor and sensitive structures are outlined.

The beam directions and boundaries are chosen to treat the tumor and

avoid sensitive structures.

Shaping a field with a multi-leaf collimator system.

Modern treatment unit.

Verification with a radiograph obtained using the treatment beam.

Tomotherapy

Binary MultileafCollimator

Tomotherapy

Binary MLC Leaves

X-Ray Beam

Binary MultileafCollimator

HelicalScanning

Tomotherapy

Helical Fan Beam

Binary MultileafCollimator

Megavoltage(MV) Detector

Tomotherapy

HelicalScanning

Helical Fan Beam

Binary MultileafCollimator

Megavoltage(MV) Detector

MVScan

Tomotherapy

HelicalScanning

Helical Fan Beam

UW Clinical Helical Tomotherapy Unit

Siemens Linac

GE CTDetector

Siemens RF System

May 2000 at UW Physical Sciences Laboratory, Stoughton WI

GE Gantry

Clinical Installation Finished

January 16, 2001 at UW Radiotherapy Clinic

Patient Treatments To Start Soon

Status May 3, 2002•Acceptance testing complete, i.e., specifications verified.•Treatment planning beam data commissioning complete.•FDA 510(k) cleared.•UW Animal Subjects Committee approval.•Megavoltage CT scans obtained on client dogs.•UW Investigational Review Board palliative protocol approved.•Final integration tests underway.

Mesothelioma Case

Dose Rate Cumulative Dose

MesotheliomaMovie ROI slice 27

Slice 27 Slice 31 Slice 36

50 %80 %90 %

50 %80 %90 %

50 %80 %90 %

MVCT vs. kVCT for the Rando Phantom

MVCT ObtainedOn the UWTomotherapyBenchtop Unit

Volume Rendering of Rando Phantom Scans

Helical MVCT Siemens Hi-Q kVCT

Adaptive Radiotherapy3-D Imaging

OptimizedPlanning

MV CTImaging

Intensity-Modulated Treatment

Determine theDose Delivered

Modify theDelivery

Imagine if Radiation Were A Drug• It could target arbitrarily-defined anatomic sites. • It would cause little damage to normal tissue away

from the tumor.• The site of its action could be verified precisely.• Its side effects were well known.• It could be non-invasively measured in small

quantities.• It would make other drugs more potent.• Drug tolerance would not develop. • Saving hundreds of thousands of people a year in

the U.S., it would surely be considered our most important drug.

Conclusions• Radiation therapy can be broadly classified into

brachytherapy and external beam radiotherapy.• Linear accelerators are used to treat patients with

photon (megavoltage x-rays) and electron beams. • Photons and electrons behave quite differently when

they interact with matter.• Conventional radiotherapy uses 2-D images for planning

the treatments.• 3-D imaging using CT and MRI provides a representation

of the patient used for planning conformal radiation delivery.

• The accurate verification processes of tomotherapy will form the basis for adaptive radiotherapy.