Investigation of bladder dose and volume factors influencing late urinary toxicity after external beam radiotherapy for prostate cancer

Background: We sought to identify the bladder dose–volume factors associated with an increased risk of late urinary toxicity among prostate cancer patients treated with radiotherapy. Methods and Materials: This retrospective analysis included data from 128 prostate cancer patients treated on protocol with 2 Gy/fraction to 46 Gy followed by a boost to 78 Gy. The endpoint for this analysis was Grade 1 or greater late genitourinary (GU) toxicity occurring within two years of treatment. The Lyman-Kutcher-Burman, mean dose, threshold dose, and hottest volume models were fitted to the toxicity data using the maximum likelihood method. Results: Model fits based on dose–volume histograms tended to fit the toxicity data better than models based on dose–wall histograms. The hottest volume (hotspot) model was found to be the best-fitting model investigated. The best fit was for the hottest 2.9% of bladder (95% CI, 1.1–6.8%). This model has an area under the receiver operating characteristic curve of 0.74. The hotspot model separated the patients into clinically meaningful subgroups with ∼25% of the patients who received