Hyaluronic acid (HA) rectal spacers (RS) are used to increase rectoprostatic separation during external beam radiotherapy (EBRT) to lower rectal dosimetry and acute/late GI toxicity. Quality of the implant is likely to influence dosimetric and toxicity outcomes. Purpose of this study is to assess implant quality using modified SQS (mSQS) and FV scores achieved in men with localised prostate cancer (PCa) treated with moderately hypofractionated EBRT who underwent stabilized HA RS implant, and its impact on acute and late GI toxicity.

We reviewed 100 consecutive men with localised PCa treated with moderately hypofractionated EBRT (60Gy/62Gy in 20 fractions) and underwent HA RS insertion, treated between June 2020 - September 2022. Acute and late GI toxicity were assessed using CTCAEv5. Implant quality was assessed using mSQS (recto-prostatic separation (RPS)), FV score (symmetry) and RS rectal wall infiltration (RWI) evaluated on post implant T2-weighted MRI axial slices. RWI was independently assessed by an MRI prostate specialist radiologist. SQS, RWI and FV scoring were performed according to prior publications by Grossman et al and Fischer-Valuck et al. mSQS scores were used to stringently account for our planning target volume (PTV) expansions (mSQS score 2 = ≥10mm RPS, mSQS score 1 = 6-9mm RPS, mSQS score 0 = ≤5mm RPS).

Mean age was 74.6 years (SD 6.0 y); 54 patients had unfavourable intermediate risk PCa, 33 had favourable intermediate risk PCa and 9 had low risk PCa. 88 patients were treated to 60Gy in 20 fractions, while 12 patients were treated to 62Gy in 20 fractions. Of the RS implants, 98% were deemed “very easy” by the injector and 2% were deemed “easy”. Overall mSQS score of 1 (“excellent”) was achieved in 74 patients, score of 2 in 24 patients, and a score of 0 occurred in just 2 patients. 97% of the injectors successfully shaped HA to achieve symmetrical rectal spacing with FV score 1. Seven acute G1 GI toxicity, and 5 late G1 GI toxicity were recorded. There was no ≥G2 acute or late GI toxicity. No statistically significant association was identified between FV and SQS scores with acute or late GI toxicity incidence. Only 1 patient experienced implant RWI and the RWI was reversed with hyaluronidase, with no significant sequelae reported. He was subsequently reimplanted successfully with HA RS and completed EBRT uneventfully. Median follow-up was 1.68 years (IQR: 1.28-2.23), with only one regional recurrence reported.

The majority (97%) of patients who underwent HA RS implant had easily achieved high quality RS (both modified SQS scores 1-2 and FV score 1). This translated to low rectal doses and low incidence of acute and late G1 GI toxicities. Stabilised HA RS may be considered in patients undergoing hypofractionated EBRT to the prostate.