Radiation Dose to the Rectum With Definitive Radiation Therapy and Hydrogel Spacer Versus Postprostatectomy Radiation Therapy

Purpose: Management options for localized prostate cancer include definitive radiation therapy (RT) or radical prostatectomy, with a subset of surgical patients requiring adjuvant or salvage RT after prostatectomy. The use of a peri-rectal hydrogel spacer in patients receiving definitive RT has been shown to reduce rectal doses and toxicity. However, in the postprostatectomy setting, a hydrogel spacer cannot be routinely placed. Therefore, we sought to compare rectal dosimetry between definitive RT with a hydrogel spacer versus postoperative RT. Methods and Materials: We identified patients with prostate cancer who underwent conventionally fractionated RT. Rectal dosimetry was evaluated between 2 groups: definitive RT with a hydrogel spacer (79.2 Gy, group 1) and postoperative RT (70.2 Gy, group 2). Rectal dosimetry values were tabulated and compared using Mann-Whitney U test. We implemented a Bonferroni correction to account for multiple comparisons (threshold P < .005). Linear regression analysis evaluated predictors of candidate rectal dose-volume parameters. Results: We identified 51 patients treated during years 2017 to 2018; 16 (31%) and 35 (69%) patients were included in groups 1 and 2, respectively. The rectal volume receiving ≥65 Gy (V65) was significantly lower in group 1 (median, 2.1%; interquartile range, 0.9%-3.1%) than in group 2 (10.7%, 6.6%-14.5%) (P < .001). Use of a hydrogel spacer in the definitive setting was independently associated with lower V65 (P < .001). Similar results were found for V60, V55, V50, and V45 (P < .005 for all). Conclusions: Rectal dosimetry is more favorable for definitive RT (79.2 Gy) with a hydrogel spacer compared with postoperative RT (70.2 or 66.6 Gy). This may inform shared decision-making regarding primary management of prostate cancer, especially among patients at high risk of needing postoperative RT after prostatectomy.