Rectal sparing in prostate radiotherapy with combination-brachytherapy and hydrogel spacer

Though some techniques that facilitate rectal sparing such as brachytherapy and intensity modulated radiotherapy (IMRT) have been examined in detail, technical aspects of hydrogel spacer (HS) have been studied less exhaustively. We examined HS quality metrics and approaches to placement for superior dosimetric outcomes.

A single site retrospective review of radiation plans was conducted for patients who received combination-brachytherapy (CBT) with 90 Gy low-dose-rate implant followed by external beam radiotherapy (45 Gy/25 fractions) with operating room (OR) placed HS (2017-2021). A randomly selected set of patients that received CBT without HS over the same time period was used for comparison. Dosimetric outcomes included D1cc and D5% rectum. Dose gradients were quantified. Student's t-test was used for statistical comparisons.

Sixty patients (30 with and 30 without HS) who received CBT for prostate cancer were examined. Those with HS had lower mean D1cc [65.31 Gy (SD = 13.53)] and D5% [53.20 Gy (SD = 10.18)] compared to those treated without HS [91.67 Gy (SD = 8.31) and 75.00 Gy (SD = 8.45), respectively, p < 0.001]. Patients with superior HS (average thickness ≥1 cm; n = 12) had lower mean D1cc [58.49 Gy (SD = 13.25, p = 0.026)] and D5% [48.69 Gy (SD = 9.85, p = 0.049)] than those with thinner HS. When dose gradients were considered, HS spanning the interface between the prostate and perirectal tissues to a thickness ≥1 cm can reduce rectal maximum dose to 50-60 Gy.

Through effective use of CBT and HS, extreme rectal dose restriction is possible. The goal for HS placement should be thickness ≥1 cm from base to apex.