Quality Metric to Assess Adequacy of Hydrogel Rectal Spacer Placement for Prostate Radiation Therapy and Association of Metric Score With Rectal Toxicity Outcomes

PURPOSE: Although hydrogel spacer placement (HSP) minimizes rectal dose during prostate cancer radiation therapy, its potential benefit for modulating rectal toxicity could depend on the achieved prostate-rectal separation. We therefore developed a quality metric associated with rectal dose reduction and late rectal toxicity among patients treated with prostate stereotactic body radiation therapy (SBRT). METHODS AND MATERIALS: A quality metric consisting of prostate-rectal interspace measurements from axial T2-weighted magnetic resonance imaging simulation images was applied to 42 men enrolled in a multi-institutional phase 2 study using HSP with prostate SBRT (45 Gy in 5 fractions). A score of 0, 1, or 2 was assigned to a prostate-rectal interspace measurement of <0.3 cm, 0.3 to 0.9 cm, or ≥1 cm, respectively. An overall spacer quality score (SQS) was computed from individual scores at rectal midline and ±1 cm laterally, located at the prostate base, midgland, and apex. Associations of SQS with rectal dosimetry and late toxicity were evaluated. RESULTS: The majority of the analyzed cohort had an SQS of 1 (n = 17; 41%) or 2 (n = 18; 43%). SQS was associated with maximum rectal point dose (rectal Dmax; P = .002), maximum dose to 1 cc of rectum (D1cc; P = .004), and volume of rectum receiving ≥100% of prescription dose (V45; P = .046) and ≥40 Gy (V40; P = .005). SQS was also associated with a higher incidence of (P = .01) and highest-graded late rectal toxicity (P = .01). Among the 20 men who developed late grade ≥1 rectal toxicity, 57%, 71%, and 22% had an SQS of 0, 1, and 2, respectively. Men with an SQS of 0 or 1 compared with 2 had 4.67-fold (95% CI, 0.72-30.11) or 8.40-fold (95% CI, 1.83-38.57) greater odds, respectively, of developing late rectal toxicity. CONCLUSIONS: We developed a reliable and informative metric for assessing HSP, which appears to be associated with rectal dosimetry and late rectal toxicity after prostate SBRT.