Rectum and Bladder Toxicity in Postoperative Prostate Bed Irradiation: Dose–Volume Parameters Analysis

SIMPLE SUMMARY: Postoperative radiotherapy can be associated with both early and late toxicity. In this retrospective analysis, we aimed to assess the prevalence of adverse events and their association with several dose–volume parameters. The results of our study showed that both rectum wall and rectum parameters within the range of 30–65 Gy presented significant association with acute ≥G2 gastrointestinal toxicity. On the other hand, the occurrence of late ≥G2 gastrointestinal toxicity was significantly associated only with rectum wall parameters, including V35–V54, V66–V69, and V71. Finally, the risk of experiencing late ≥G2 genitourinary adverse events was significantly associated with all bladder wall parameters between 30–54 Gy and all bladder parameters between 30–53 Gy. However, there was a clinically relevant risk of adverse events even in patients who adhered to commonly used dose constraints. This supports the conclusions of recent evidence showing that an early-salvage approach is preferable due to its non-inferior oncological outcomes. ABSTRACT: Although prostate cancer treatment is increasingly effective, its toxicities pose a major concern. The aim of our study was to assess the rate of adverse events (AEs) and the prognostic value of dose–volume histogram (DVH) parameters for the occurrence of treatment toxicity in patients treated with post-prostatectomy prostate bed radiotherapy (RT). The AEs were scored according to the CTCAE v.5.0. The rectum and bladder were contoured according to the RTOG Guidelines. The DVH parameters were assessed using data exported from the ECLIPSE treatment-planning system. Genitourinary (GU) and gastrointestinal (GI) toxicity were analysed using consecutive dose thresholds for the percentage of an organ at risk (OAR) receiving a given dose and the QUANTEC dose constraints. A total of 213 patients were included in the final analysis. Acute grade 2 or higher (≥G2) GU AEs occurred in 18.7% and late in 21.3% of patients. Acute ≥G2 GI toxicity occurred in 11.7% and late ≥G2 in 11.2% of the patients. Five patients experienced grade 4 AEs. The most common adverse effects were diarrhoea, proctitis, cystitis, and dysuria. The most significant predictors of acute ≥G2 GI toxicity were rectum V47 and V46 (p < 0.001 and p < 0.001) and rectum wall V46 (p = 0.001), whereas the most significant predictors of late ≥G2 GI AEs were rectum wall V47 and V48 (p = 0.019 and p = 0.021). None of the bladder or bladder wall parameters was significantly associated with the risk of acute toxicity. The minimum doses to bladder wall (p = 0.004) and bladder (p = 0.005) were the most significant predictors of late ≥G2 GU toxicity. Postoperative radiotherapy is associated with a clinically relevant risk of AEs, which is associated with DVH parameters, and remains even in patients who fulfil commonly accepted dose constraints. Considering the lack of survival benefit of postoperative adjuvant RT, our results support delaying treatment through an early salvage approach to avoid or defer toxicity.