AbstractID: 6830 Title: Dosimetric Effects of Planning Techniques and Source Calibration
Accuracy of Prostate Implants
For a given prostate implant, different planning techniques in selection of source strength and optimization methods can be used. We evaluate the dosimetric effects of different planning techniques, including automatic versus manual optimization; variations of number of seeds for fixed total activity; and source calibration erors. Optimized seed locations were randomly perturbed to simulate post-implant dose calculations. TRUS images of three patients implanted with I-125 seeds for curative treatment, with volumes of 20-cc, 40-cc, and 58-cc respectively were used in this study. Planned seed distributions were determined by using either automatic optimization (VariSeed TPS) or manual optimization, both aiming at achieving better than 95% prostate coverage, less than 150% Rx. dose to urethra, less than 100% Rx. dose to rectum, and acceptable doses to surrounding normal tissue. Seed strengths were varied for given total activities resulting in +/- 20% change in numbers of seeds. DVH’s of prostate, urethra, rectum and surrounding normal tissues were calculated. We find that choice of optimization method or number of seeds per implant change V
100
of prostate by less than 5%. However there were significant increases of V
150
for the prostate, urethra and surrounding normal tissue. Deviation of source strength from planned source activity by more than 10% resulted in significant over or under coverage of the prostate, as well as higher or lower than acceptable doses to urethra and surrounding normal tissue.