Quantifying geometric and dosimetric advantage from the use of rectal spacing in HDR prostate brachytherapy

Rectal spacers are used during prostate external beam radiation therapy to increase rectal-prostate separation, thereby reducing rectal dose and toxicity. However, their role in high-dose-rate (HDR) brachytherapy remains less understood. This paper presents (i) a cohort study evaluating the dosimetric impact of rectal spacers in HDR prostate brachytherapy and (ii) a simulation study assessing how rectal-prostate separation affects dosimetry.

We retrospectively analyzed 157 patients treated with 15 Gy HDR prostate boost brachytherapy. Patients were divided into preimplant spacer (n = 49) and nonspacer (n = 108) cohorts. Prostate and rectal dose-volume histogram (DVH) metrics were compared, and rectal-prostate separation was measured on intraoperative transrectal ultrasound. A custom linear programming optimizer simulated plans with separations of 0.5-12 mm in 30 nonspacer cases to evaluate resulting dose metrics.

Mean (± standard deviation) rectal-prostate separation was 7.8 ± 1.4 mm with spacer versus 4.0 ± 1.4 mm without spacer. Rectal dose was significantly reduced with spacer (mean D_2cc 7.4 ± 1.0 Gy vs. 9.2 ± 1.0 Gy; p < 0.001), while prostate coverage remained similar (mean D₉₀ 16.4 ± 0.3 Gy vs. 16.3 ± 0.3 Gy; p = 0.071). Simulations showed that separations > 3 mm consistently achieved prostate D₉₀ > 16 Gy and V₁₀₀ > 95%, and separations > 4 mm produced a marked reduction in rectal dose.

Rectal spacers significantly reduce rectal dose in HDR prostate brachytherapy. Patients with rectal-prostate separations of at least 4 mm are most likely to benefit.