LAUSR

PURPOSE The purpose of the study was to investigate the impact on dose distribution and radiobiological metrics of common high-dose-rate vaginal brachytherapy treatment parameters and to analyze multiinstitutional data for clinically significant impact on outcomes in early-stage endometrial cancer. METHODS AND MATERIALS Treatment plans were created for all combinations of prescription parameters and used to quantify the dosimetric impact of each parameter and to estimate the dose delivered using common voxel-integrated radiobiological metrics. A rating system, based on risk grouping from GOG and PORTEC trials, was used to consolidate staging information into a cancer "aggressiveness" measure. Correlations between the rating, toxicity, disease recurrence, and plan parameters were investigated. RESULTS When prescribing to 5 mm depth, the variation caused by the diameter was very large across all dose metrics, ranging from 51% to 175% increase with the most divergence in BEDmax. For surface prescription, changing the cylinder diameter from 4 cm to 2 cm caused the dose metrics of BEDmin, Dmin, and gBEUD (a = -3) to increase by 117%, 67%, and 52%, respectively. Prescription to 5-mm depth caused changes across all dose metrics of 260% compared with surface prescription for a 2-cm cylinder. Deeper prescription point (p = 0.005) and longer treatment length (p = 0.01) were correlated with increased stenosis rates. No correlation between recurrence and any plan parameter was found. CONCLUSIONS Dramatic differences in dose distributions arise by small variations of plan parameters, with large impact on rates of vaginal stenosis, but no clear relation with local recurrence. To help radiation oncologists interpret the magnitude of these effects for their patients, we created a tool that allows comparison between dose and fractionation parameters.