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

Abstract 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 (2.5–2.65Gy/fr, 70–74Gy) 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 50–70Gy at 5–7mm 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 5–12mm from the base (2Gy equivalent doses=80–82Gy, α / β =3–5Gy) 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.