Rectal spacers in high-dose-rate-brachytherapy: Optimizing peripheral zone radiation delivery

The utilization of rectal spacers (RS) in prostate cancer treated with definitive radiotherapy has been shown to provide rectal dose sparing, decreased rectal toxicity, and improved gastrointestinal quality-of-life metrics. This dose-sparing effect presents an opportunity for dose escalation to the prostate, potentially enhancing clinical and biochemical outcomes while minimizing adverse effects. Although the influence of RS on overall prostate dosimetry has yielded mixed findings, the specific impact on radiation delivery to the predominantly tumorigenic peripheral zone (PZ) remains largely unexamined. This study aims to evaluate the effects of RS on PZ dosimetry in prostate brachytherapy.

A single-institution, retrospective analysis was conducted on patients who received two-fraction high-dose-rate brachytherapy (HDR-BT) for localized prostate cancer. Dosimetric parameters and biochemical outcomes were compared between consecutive patients who received intraoperative RS (RS+) and those who did not (RS-). Additional subset analysis was performed stratifying by prostate size. Comparisons were performed using the Mann-Whitney U test. Dosimetric parameters were assessed for the prostate, planning target volume, PZ, and organs at risk (OAR).

Between January 2020 and July 2024, a total of 92 patients who underwent HDR-BT were identified, of whom 46 (50%) were RS+. The contoured volumes of the prostate and PZ were comparable between the RS- and RS+ cohorts, with a median of 38.5 vs. 34.96 cm³ (p = 0.4475) and 11.25 vs. 10.25 (p = 0.1964), respectively, for the prostate and PZ. However, median PZ D₉₀ was significantly higher in RS+ patients compared to RS- (RZ+ 115.65% vs. 110.25%, p < 0.0001). The increase in PZ D₉₀ in the RS+ group was more pronounced in patients with smaller prostates (<40 cm³: RS+ 115.97% vs. 108.87%, p < 0.0001). Regarding organs at risk, the RS+ group exhibited a reduction in rectal D_max (p < 0.0001), V₂₀ (p = 0.0002), D₈₀ (p < 0.0001), and D_2cc (p < 0.0001); a decrease in urethral D_max (p = 0.0017) and V₁₁₀ (p = 0.0002); and an increase in bladder V₇₅ and D₈₀ (p < 0.0001).

As expected, RS HDR-BT successfully reduced rectal radiation exposure in this cohort. RS use is associated with increased radiation delivery to the PZ, potentially augmenting ablative dosing to the primary site of disease and the most likely zone for potential microscopic intraprostatic spread, as well as decreased urethral doses. This enhanced dose distribution within the PZ, with simultaneous improved urethral sparing, supports the incorporation of RS in HDR-BT. Whether this dosimetric enhancement results in long-term biochemical control or improved overall oncologic outcomes remains an ongoing area of investigation.