Verification of Direct Brachytherapy Dosimetry for a Single Seed Implant
A new technique using direct post-implant dosimetry, which does not depend explicitly on brachytherapy seed orientation or position, was explored for a prostate and a breast case. This technique, proposed by E Sajo and ML Williams (SW), uses trace amounts of positron emitters placed in the seed capsule and uses the positron emission tomography image in conjunction with a computed tomography image (PET-CT) to compute the therapeutic dose distribution in the patient. The SW technique could reduce errors in the post-implant dose computations associated with seed localization, seed shadowing and medium heterogeneity. Dose point kernels were obtained using Monte Carlo simulation for a single seed in a breast and prostate geometry. Greens functions were computed for the positron marker and therapeutic photons using Monte Carlo (MC) simulations. Various dose computation options in the MC code MCNP were compared and the best were selected for this project. A single seed was imaged for a prostate phantom and a breast phantom using a PET-CT. The image data was used to obtain dose for the annihilation photons for the experimental seeds. The Sajo-Williams mathematical method was used to compute the therapeutic dose of the seed based on the positron marker dose. The therapeutic dose computed this way was compared to the dose obtained using the Pinnacle3 treatment planning software and to an MCNP benchmark model. For the breast case the comparison showed a good agreement with Pinnacle3, but both under-predicted the dose close to the source with respect to the benchmark. For the prostate case Pinnacle3 somewhat under-predicted the values in the MCNP benchmark, and the SW method appreciably under-predicted the dose near the source. In all cases, farther away from the source where most of the dosimetric interests lie, the agreement is very good.
Advisor:Ravi Rau; Erno Sajo; John Gibbons; Wei-Hsung Wang
School:Louisiana State University in Shreveport
School Location:USA - Louisiana
Source Type:Master's Thesis
Date of Publication:11/14/2006