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Medical
physics is the application of physics to the diagnosis and treatment of
disease. Clinical radiation therapy involves the use of high doses of
high-energy radiation to kill cancer cells. About 50% of all people with
cancer will be treated with radiation. Modern radiation therapy treatment
machines, called linear accelerators, deliver high doses to the tumor, and
lower doses to the surrounding normal tissue. In other words, the radiation
dose is targeted to the area where tumor cells are located. Radiation oncologists
and medical physicists work together to develop a specific plan of treatment
for each patient utilizing CT scans, MRI scans, ultrasound images; and 3-D
treatment planning computers. Once a
specific plan of treatment has been prescribed, incremental doses of
radiation are delivered each day, Monday through Friday, by the radiation
therapists who operate the linear accelerators. An entire course of treatment
will last between 2 and 7 weeks.
Thermoluminescent Dosimetry
Radioactive
seed sources, such as Iodine 125, have been widely used for interstitial
implants for a number of years in several tumor sites, especially the
prostate. Dosimetry parameters,
including the dose rate constant, radial dose function, and anisotropy
function must be measured for new seed sources to build a three-dimensional
dose distribution model of the seed. Once a three-dimensional dose
distribution model has been developed, a group of seeds can be precisely
placed inside a tumor site in an arrangement that exactly treats the size and
shape of the tumor. Thermoluminescent
dosimeters (TLDs) are ideal for measuring the dosimetry parameters of a
radioactive seed source. Ionizing radiation, such as x-rays, absorbed in
matter produces ionization. Most of
the absorbed energy goes into heat while a small portion is used to break
chemical bonds. In some materials,
such as LiF, a very small fraction of the energy is stored in meta-stable
energy states. Some of the energy can
be recovered later as visible photons if the material is heated. The phenomenon of visible photons being
released by thermal means is thermoluminescence. Thermoluminescent materials can be used as
radiation dosimeters. A common
thermoluminsecent dosimeter is made from LiF formed into the shape of a 1 mm
diameter rod with length 3 mm. These
TLD rods can be positioned in tissue equivalent material (a phantom) in a
precise geometric pattern surrounding the radioactive seed source to measure
the dosimetry parameters of the source.
Research work requires developing appropriate phantoms with the
precise geometrical arrangement of TLDs needed to measure the dosimetry
parameters of the seed source, collecting and analyzing the TLD data to
determine the dosimetry parameters, and then building the three-dimensional dose
distribution model of the source.
Clinical/Research Work
Spent the academic year at the Vanderbilt University Medical Center focusing on clinical radiation therapy and dosimetry research. The clinical component involved training in the development of 3-D treatment plans for cancer patients as well as the operation and testing of the linear accelerators used to deliver the radiation treatments. Specifically, the clinical experience included 3-D treatment planning • dose calculations • Varis database record and verify system operations • development of Infomaker reports to query the Varis database • daily patient dose checks • weekly patient chart checks • Varian Clinac 1800 (4x and 6/10x) and 2100C (6/18x) accelerator operations • accelerator monthly checkouts • operation of a Wellhoffer water phantom • assistance in commissioning a new Varian Clinac 2100C accelerator.
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The dosimetry research
component involved measuring the dosimetry parameters for the I-Plant 3500
seed. The Implant Sciences I-Plant 3500 Iodine-125 seed was being
characterized at Vanderbilt for use in brachytherapy treatment including the
treatment of prostate cancer. Dosimetry parameters, including the dose rate
constant, radial dose function, and anisotropy function, were measured for the
new seed source in order to build a three-dimensional dose distribution
model. Performed the experimental measurements utilizing Thermoluminescent
Dosimeters (TLDs) to measure the dose rate produced by the seed at specific
radii and angles in tissue equivalent material. Subsequently performed the
data analysis needed to construct the 3-D dose distribution model for the new
seed. This 3-D model is now incorporated in treatment planning computers.
Patients first began treatment with the I-Plant 3500 seed in early June 2000.
The results of this work have been published – see reference below.
D.M. Duggan and B.L. Johnson, Dosimetry of the I-Plant Model 3500 Iodine-125 Brachytherapy Source, Medical Physics 28, 661-670 (2001)
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Send comments and questions to:
johnsonb@kwc.edu
Last
modified: 5-2-2007