LAUSR

PURPOSE/OBJECTIVE(S) Previously a phase III trial of a hydrogel rectal spacer during prostate RT found decreased toxicity and a clinically significant improvement in bowel quality of life (QOL) at 3-years by Expanded Prostate Cancer Index (EPIC). We performed a secondary analysis to identify men less likely to benefit. MATERIALS/METHODS Clinical and dosimetric data for the 222 patients enrolled on the *** trial were analyzed. The volume of rectum treated to 70 Gy (V70) as well as the QUANTEC rectal dose goals were utilized as surrogates for clinical benefit and plan quality. Mean bowel QOL was assessed at 15- and 36-months post treatment and the likelihood of 1x (5 pt) or 2x (10 pt) minimally important difference (MID) changes were assessed. RESULTS Rectal V70 was correlated with physician scored toxicity (p=0.033) and was utilized as a surrogate for plan quality. There was no correlation between prostate volume and rectal V70 (r=0.077). Rectal V70 pre- and post-hydrogel was 13/3% for the smallest prostates (<40 cc) and 12/2% for the largest (>80 cc). The relative reduction in rectal V70 of 78% did not vary by pre-spacer V70, but the absolute reduction was greater for higher V70. All spacer plans met the 5 QUANTEC rectal dose constraints while 92% of control plans met all constraints. At 3-years, those not meeting all QUANTEC goals had 15.0 pt (stdev 15.1) decline, control pts meeting QUANTEC goals had 4.0 (9.5) pt decline and spacer had +0.5 (7.6) (p<0.01). Previous surgery was not correlated with QOL (p=0.8). Across prognostic groups including: age, body mass index, previous surgery, target volume, or quality of radiation plans there was no statistically significant heterogeneity in the relative benefit of spacer in decreasing the risk of 1x or 2xMID declines. CONCLUSION There was little heterogeneity in the likelihood of spacer reducing the risk of declines in bowel QOL across clinical and dosimetric variables. Even for the >95% of plans meeting QUANTEC rectal criteria hydrogel spacer provided potentially meaningful benefit.