Presentation on theme: "Physics and Imaging in Radiation Therapy"— Presentation transcript:
1 Physics and Imaging in Radiation Therapy Giovanni Maria PiacentinoModulo di radioterapia
2 95% of Radiation Therapy is to Treat Cancer X-ray of a Crab
3 Outline Introduction Brachytherapy Production of External Beam RadiationQuantifying the Amount of RadiationInteraction of Radiation with MatterConventional Radiotherapy Treatment Processes3-D Imaging for Radiation Oncology3-D Conformal Radiation TherapyTomotherapyConclusions
4 BrachytherapyLiterally, 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.
5 Common Photon Sources for Brachytherapy IsotopeHalf-LifeHalf-ValueLayer (Water)Layer (Lead)Cs-13730 years8.2 cm6.5 mmAu-1982.7 days7.0 cm3.3 mmIr-19274 days6.3 cm3.0 mmI-12559 days2.0 cm0.02 mmPd-10317 days1.6 cm0.01 mm
6 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.
8 Production of External Beam Radiation Block diagram of a linear accelerator. A magnetron or klystron produces radio waves.
9 Animation of Electron Acceleration in a Linac Linear Accelerator Waveguide with 4 CavitiesEThe 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.
10 Treatment head configuration for megavoltage photon (A) and electron beams (B). Accelerated ElectronsPhotonBeamElectronBeam
11 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.
12 Measuring Ionization with an Ion Chamber Anode-+--+-+A+-CathodeRadiation 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.
13 Photon Beam Penetration with Depth Notice the low dose near the surface and nearly an exponential fall-off with depth.
14 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.
15 Interaction of Radiation with Matter Interaction of neutrally-charged particles.
16 Animation of Radiation Interaction Water MoleculeIons Created+ -+ -+ -Photon Interacts, Setting in Motion a Fast Electron
17 Formation of Radicals and Long Lived Products + -OHOH-2HH2O2H2O-H2O+H+HHydroxylRadicalHydrogenRadicalHydrogen and Hydrogen Peroxide Products
18 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.
20 A treatment simulator has an x-ray tube to image the patient in treatment position.
21 Image from a treatment simulator to determine the shape of the treatment field.
22 Tracing out the shape of the field to make custom blocks to shield normal tissue.
23 The custom block is fabricated out of low-melting temperature heavy metal alloy (mainly lead)
24 The custom block is mounted is attached to the linac to treat the field.
25 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.
26 Computed Tomographic Scanning Photograph of a cross-section through a human abdomen.CT scan through the same section.
27 3-D VisualizationVolume 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.
28 Magnetic Resonance Imaging Abdominal MRI. From the National Library of Medicine Visual Human Project.
29 Comparison Between CT and MRI Tumor seenonly on MRI.Axial CTAxial MRICoronal CTCoronal MRI
30 MRI Angiography (MRA)AVM NidusArteriol-venous malformations (AVM) are often treated with radiation therapy. The nidus of malformed vessels is clearly visualized using MRA.
31 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.
53 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.
54 ConclusionsRadiation 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.