Efficacy of a rectal spacer with prostate SABR - first UK experience

OBJECTIVE: This study assessed the use of implanted hydrogel rectal spacers for SABR-VMAT patients, investigating practicality, dosimetric impact, normal tissue complication probability (NTCP) and early toxicity.

METHODS: Data from the first 6 patients treated within a prostate SABR and rectal spacer trial were examined to determine spacer insertion tolerability, resultant changes in treatment planning and dosimetry and early toxicity effects. CT scans acquired prior to spacer insertion were used to generate SABR plans which were compared to post-insertion CT plans. Plans were evaluated for target coverage, conformity, and organs at risk doses with NTCPs also determined from resultant dose fluences. Early toxicity data was also collected.

RESULTS: All patients had successful spacer insertion under local anaesthetic with maximal grade 1 toxicity. All plans were highly conformal, with no significant differences in CTV dose coverage between pre- and post-spacer plans. Substantial improvements in rectal dose metrics were observed in post-spacer plans, e.g. rectal volume receiving 36Gy reduced by ≥42% for all patients. Median NTCP for grade 2+ rectal bleeding significantly decreased from 4.9% to 0.8% with the use of a rectal spacer (p = 0.031). To date, 2 episodes of acute grade 1 proctitis have been reported following treatment.

CONCLUSION: The spacer resulted in clinically and statistically significant reduction in rectal doses for all patients. Advances in knowledge: This is one of the first studies to investigate the efficacy of a hydrogel spacer in prostate SABR treatments. Observed dose sparing of the rectum is predicted to result in meaningful clinical benefit.