Medical Physics Seminar – Monday, April 7, 2014
Energy Based Brachytherapy Dosimetry
Martha Malin (student of Dr. Larry DeWerd)
Research Assistant, Dept of Medical Physics, UW-School of Medicine & Public Health, Madison, WI - USA -
New model-based dose calculation algorithms (MBDCAs) for brachytherapy treatment planning compute patient-specific dose distributions by propagating the fluence emitted by the brachytherapy source through a CT image of the patient. This is a fundamental change from the current TG-43 brachytherapy dosimetry protocol which superimposes dose distributions from homogenous water phantoms on the patient anatomy. With this fundamental change, new ways are needed to quantify source strength and the anisotropic emission of photons from the encapsulation of sources. This work addressed both these needs. First, this work determined the feasibility of directly measuring the angular fluence leaving the encapsulation of the source as a way to measure the anisotropic emission of photons. Second, by proposing a new source strength metric, emitted power (EP), which more directly works with MBDCAs for low-energy brachytherapy sources. Emitted power is the energy per unit time leaving the encapsulation of the source. As no instrument existed capable of measuring the small power levels of these sources, this work developed and built a calorimeter operating at cryogenic temperatures to provide absolute measurements of EP. EP measurements were made for several source models.