Quality Metric to Assess Adequacy of Hydrogel Rectal Spacer Placement for Prostate Radiotherapy and Association of Metric Score With Dosimetric and Gastrointestinal Toxicity Outcomes.

PURPOSE/OBJECTIVE(S) Hydrogel rectal spacer placement (HSP) has been shown to minimize rectal dose during prostate cancer radiotherapy, yet its potential benefit for modulating rectal toxicity could in large part depend upon the quality of prostate-rectal separation achieved from HSP. We therefore developed a practical quality metric to evaluate HSP, validating its association with rectal dose reduction, as well as physician- and patient-reported rectal toxicity using a prospective dataset of men treated with prostate stereotactic body radiotherapy (SBRT). MATERIALS/METHODS Using the axial T2-weighted MRI images from a small pilot cohort of men with HSP who had been treated at our institution with prostate SBRT, a hydrogel spacer quality metric was devised from prostate-rectal space thickness (PRST) measurements. A score of 0, 1, or 2 was assigned to a PRST of < 0.3 cm, 0.3-0.9 cm, or ≥1 cm, respectively; an overall spacer quality score (SQS) was computed from individual scores (total of 9) at rectal midline and 1 cm to the right and left, located at the level of the prostate base, midgland, and apex. This quality metric was then applied to MRI simulation scans of 43 men with low/intermediate risk prostate cancer who had been enrolled on a multi-institutional phase II study to assess safety and efficacy of HSP during prostate SBRT. Associations between SQS and rectal dosimetry, late gastrointestinal (GI) toxicity, and EPIC bowel quality of life were quantified in these men treated with 45 Gy in 5 fractions. RESULTS Among this cohort, the majority had a SQS of 1 (n = 18; 42%) or 2 (n = 18; 42%). SQS was associated with maximum rectal point dose (rectal Dmax; P = 0.001), maximum dose to 1 cc of rectum (D1cc; P = 0.003), and volume of rectum receiving ≥100% of prescription dose (V45; P = 0.03). For those with SQSs of 0, 1 and 2, the median rectal Dmax (cGy) was 4683, 4600, and 3921, respectively. SQS was associated with incidence of late GI toxicity (P = 0.03) and highest late GI toxicity score (P = 0.02). Among the 20 men who developed late GI toxicity, 57%, 67% and 22% with Grade ≥1 had a SQS of 0, 1, and 2, respectively. Men with SQSs of 2 compared to 0-1 had 1.36 times lower odds of an increased rectal Dmax (95% CI: 1.14-1.63); and 1.20 times lower odds of an increased rectal D1cc (95% CI: 1.07-1.34). Men with SQSs of 2 versus 0-1 had 3.30 times lower odds of a higher late GI toxicity score (95% CI: 1.18-9.28). While evidence of a significant difference in baseline EPIC bowel summary score between SQSs was not observed (P = 0.07) as expected, men with SQSs of 2 compared to 0-1 had 3.52 times higher odds (95% CI: 0.91-13.64) of a superior bowel summary score 12 months from prostate SBRT. CONCLUSION We developed a reliable, easy to use, and informative metric for assessing HSP, which appears to be associated with rectal dosimetry and late GI toxicity in men treated with prostate SBRT.