Download The Convolution/Superposition Method: A Model

Physics and Imaging
in Radiation Therapy
Giovanni Maria Piacentino
Modulo 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
30 years
Half-Value
Layer (Water)
8.2 cm
Half-Value
Layer (Lead)
6.5 mm
Cs-137
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
B
A
X-rays
Protocol
A: AnteriorPosterior
B: Lateral
Dose
A
B
Distribution
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
Linear Accelerator Waveguide with 4 Cavities
E
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).
Accelerated Electrons
Photon
Beam
Electron
Beam
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
++++++++++++++++++++++++++++++++++++
A
+ +
- +
+ +
+
+
+
+
-----------------------------------
Anode
Cathode
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 neutrallycharged particles.
Animation of Radiation
Interaction
Water
Molecule
Ions
Created
+-
++-
Photon Interacts,
Setting in Motion a
Fast Electron
Formation of Radicals and
Long Lived Products
+-
OH-
OH
H2
+
H+
H2O-
Hydroxyl
Radical
H
H2O2
H2O
Hydrogen
Radical
Hydrogen 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 crosssection 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
Tumor seen
only on MRI.
a) Axial CT
b) Axial MRI
c) Coronal CT
d) Coronal MRI
MRI Angiography (MRA)
AVM
Nidus
Arteriol-venous
malformations (AVM)
are often treated with
radiation therapy.
The nidus of
malformed vessels is
clearly visualized
using MRA.
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
Tomotherapy
X-Ray Beam
Binary MLC Leaves
Binary Multileaf
Collimator
Tomotherapy
Binary Multileaf
Collimator
Helical Fan Beam
Helical
Scanning
Tomotherapy
Binary Multileaf
Collimator
Helical Fan Beam
Helical
Scanning
Megavoltage
(MV) Detector
Tomotherapy
Binary Multileaf
Collimator
MV
Scan
Helical Fan Beam
Helical
Scanning
Megavoltage
(MV) Detector
UW Clinical Helical Tomotherapy Unit
Siemens
Linac
GE CT
Detector
Siemens
RF
System
GE
Gantry
May 2000 at UW Physical Sciences Laboratory, Stoughton WI
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
Mesothelioma
Movie ROI slice 27
Dose Rate
Cumulative Dose
Slice 27
50 %
80 %
90 %
Slice 31
50 %
80 %
90 %
Slice 36
50 %
80 %
90 %
MVCT vs.
kVCT for the
Rando
Phantom
MVCT Obtained
On the UW
Tomotherapy
Benchtop Unit
Volume Rendering of
Rando Phantom Scans
Helical MVCT
Siemens Hi-Q kVCT
3-D Imaging
Adaptive Radiotherapy
Determine the
Dose Delivered
Intensity-Modulated
Treatment
Optimized
Planning
MV CT
Imaging
Modify the
Delivery
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.