First application of a pixel-wise analysis on bladder dose-surface maps in prostate cancer radiotherapy

Purpose: To develop a method for investigating local dose effects on the bladder after prostate cancer radiotherapy based on dose-surface maps (DSMs). Background and purpose: DSMs of patients included in a prospective study (DUE01) were generated by virtually cutting bladder contours at the points intersecting the sagittal plane passing through its center-of-mass: maps were laterally normalized and aligned at the posterior inferior point. The average DSMs of patients with/without toxicity, the DSMs of differences and t statistic were used to select regions better discriminating patients with toxicity. A total of 72 patients with no/mild urinary symptoms before radiotherapy and who were treated with moderate hypo-fractionation (2.5-2.65 Gy/fr, 70-74 Gy) were considered, and the endpoint was an International Prostate Symptoms Score (IPSS) >= 15 at the end of therapy (IPSSend >= 15, n = 25/72). Results: The DSMs of patients with/without toxicity were significantly different (p < 0.05). The percentage of bladder circumference receiving >50-70 Gy at 5-7 mm from the base was associated with an IPSSend >= 15 (odds ratios: 1.03-1.07). Different patterns were recognized for specific symptoms. With frequency/urgency, a quasi-threshold effect on the absolute posterior dose at 5-12 mm from the base (2 Gy equivalent doses = 80-82 Gy, alpha/beta = 3- 5 Gy) was observed. Conclusions: Local-dose effects for acute symptoms were detected in a group of patients treated within a moderately hypo-fractionated protocol. The results for frequency/urgency were consistent with a threshold effect on the trigone. (C) 2016 Elsevier Ireland Ltd. All rights reserved.