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Evaluating EUBED as a Metric for HDR Prostate Brachytherapy Plan Quality Assessment Based On In-Silico Tumorlet Analysis

B Leong

B Leong1*, N Kalman1 , C Rogers2 , D Todor1 , (1) Virginia Commonwealth University, Richmond, VA, (2) Barrow Neurological Institute, Phoenix, Arizona


SU-I-GPD-T-2 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: HDR prostate brachytherapy typically employs volumetric dose prescription to the whole prostate, but factors like V100 and D90 are sensitive to variations in source positioning, PTV delineation, and plan optimization strategy. With evidence that prostate cancer consists of multifocal tumorlets, plans that meet the same volumetric prescription dose may have different radiobiological effectiveness. Due to high dose gradients in HDR within regions covered by a prescription isodose, additional metrics like Equivalent Uniform Biologically Effective Dose (EUBED) may provide a more complete description of target coverage and treatment efficacy from a tumorlet perspective.

Methods: A retrospective analysis was performed over 11 treated HDR fractions of varying PTV volumes (23.16cc-140.85cc) in which plans are reoptimized using identical V100 (≤±0.2% variation) and OAR constraints but different V150 (average 5.1% reduction). A software platform (MATLAB) was built to generate 500 in-silico tumorlets of 9 specified volumes (4500 tumorlets/plan) and position them within the PTV using statistical inference. Tumorlets were superimposed over the 3D dose distribution and EUBED was computed for each tumorlet and the whole prostate. BED based on D90 was also evaluated. The prescribed dose is 13Gy/fraction.

Results: Averaged over all plans, the 5.1% reduction in V150 after reoptimization decreased the whole prostate EUBED by 0.54Gy and BED90 by 1.06Gy. However, the reduction in mean tumorlet EUBED of specified volume ranged from 1.37Gy to 2.52Gy for 10cc to 0.1cc tumorlets, respectively. For small tumorlets, the reduction in EUBED after reoptimization tends to increase with PTV volume.

Conclusion: HDR prostate plans of identical source and target geometry with different optimization strategies demonstrate an effect on tumorlet EUBED greater than the whole prostate EUBED. If tumorlets are the true target for HDR brachytherapy, then other metrics may need to be considered for plan evaluation and dose reporting beyond the standard V’s and D’s.

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