Focal or Whole-Gland Salvage Prostate Brachytherapy with Iodine Seeds with or without a Rectal Spacer for Post-Radiotherapy Local Failure: How Best to Spare the Rectum?

Abstract

Salvage Prostate Permanent Implant (sPPI) for post-radiation local failure provides high rates of biochemical control. The cumulative dose delivered to the prostate and the rectum when the dose from the primary irradiation is taken into account is still unknown. We reviewed the post-implant CT-based dosimetry of 18 selected patients who underwent sPPI with 125I seeds for isolated biopsy-proven local failure several years after External Beam Radiation Therapy (EBRT). Ten patients had whole-prostate sPPI and 8 patients had multi-parametric MRI-based focal sPPI. In eight patients, hyaluronic acid (HA) gel was injected into the prostate-rectum space. The Biologically Effective Dose (BED) of the primary EBRT and the BED of the sPPI were calculated using several α/β ratios and t1/2. The median D90 was 65.0 Gy [12.0 - 107.2] and the median prostate V100 was 45.3% [14.4 - 98.3]. The median D90 and the median V100 for the index lesion were 162.0 Gy [139.4 - 199.1] and 96.1% [88.4 - 100.0], respectively. The median cumulative BED after EBRT+sPPI for the prostate and the rectum were higher in patients treated with whole-gland sPPI than in patients treated with focal sPPI (313.5 Gy2 vs 174.4 Gy2, p = 0.06 and 258.1 Gy3 vs 172.6 Gy3, p < 0.01, respectively). The median D0.1 cc for the rectum was significantly lower in patients who had HA gel: 63.3 Gy [29.0 - 78.3] than in patients with no gel: 83.9 Gy [34.9 - 180.0] (p= 0.04). When we combined BED from primary prostate EBRT and BED from sPPI, we found that cumulative prostate and rectum BEDs were lower with focal sPPI than with whole-gland plans. D0.1cc delivered to the rectum was significantly lower after sPPI with HA gel, while there was no difference between focal or whole-gland plans.