Session Item

To identify sub-regions of the bladder associated with different patient-reported late persistent urinary symptoms after RT for prostate cancer using Dose-Surface Maps (DSMs).

Patients (pts) from a prospective, multi-institutional trial, including pts treated with radical RT with conventional/moderate hypofractionation were considered. One item from CIQ-SF (incontinence) and 3 items from IPSS (frequency, urgency, weak stream) were considered to assess persistent symptoms scored by adapting a previously introduced methodology (LAPERS). We defined persistent symptoms as mild or moderate/severe if the median score over late follow-ups (FUPs) was ≥1 or ≥2, respectively; in addition, a median score worse than the baseline (BL) value was considered to split pts that worsened after RT. The minimum FUP was 5 years, and only pts with at least 3 late FUPs were included.
The analysis was conducted considering as end-points all persistent symptoms and only the ones that worsened at FUP compared to BL.
The bladder DSMs were generated by an automatic dedicated tool written in Python, starting from the CT scans/dose cubes of treatment planning. The maps were opened in their anterior surface, aligned and normalised both in left-right (LR) and cranial-caudal (CC) directions to compare pts and apply voxel-wise analysis (VWA) spatially. Specifically, for L-R direction, we normalized to the maximum bladder extent within the population. In the CC direction, we: (a) aligned all maps at the most caudal slice, (b) kept the most caudal 1.5 cm fixed (as a surrogate of the bladder neck, considering this part of the bladder fixed and not influenced by bladder filling), (c) normalised the remaining bladder surface to the CC extension of the median bladder height (6 cm in this population). In this way, “small” bladders were stretched to the median and “large” bladders were shrunken to the same median. Doses were converted to EQD2 (a/b=1Gy, previously found for late urinary symptoms) at the pixel level. A pixel-wise t-test was used to highlight anatomical areas with significant differences between the average dose maps in the groups with/without toxicity.

384/539 pts had complete FU data and DSM. Table 1 reports toxicity rates. Fig.1 shows DSMs and summary results of the VWA. For severe urinary frequency, the larger dose differences (4-8Gy) were placed posteriorly, including the upper part of the bladder neck. The bottom part of the neck and the cranial bladder were highlighted for moderate/severe urinary urgency (differences 4-7Gy). The severe weak stream was associated with the anterior bladder. We found no specific significant regions for the other symptoms.

We proposed a more consistent anatomic-based normalization of bladder DSM. We found specific subregions associated with late persistent severe frequency, moderate/severe urgency and severe weak stream.
Severe late frequency and moderate/severe urgency confirmed the areas identified by Palorini (RO 2015) for acute toxicity.
The study was funded by ERA PerMed Network, Reference Number ERAPERMED2020-110 ERP-2020-23671125