Quantification Of The Heterogeneity In Hydrogel Rectal Spacer Volume And Impact On Spacer And Rectal Dosimetry

SpaceOAR is FDA approved for use in men with prostate cancer to reduce rectal toxicity. Minimal work has been reported to the real-world heterogeneity of the hydrogel spacer placement, contour delineation, and the dosimetric impact of this diversity. Furthermore, there is concern in men with high risk prostate cancer that the spacer itself could dissect the disease, and little is known about the actual dose received to the spacer. Leveraging the University of Michigan Radiation Oncology Analytics Resource (M-ROAR), a quality improvement project was initiated for men with prostate cancer. In this study, patients with intermediate risk prostate cancer treated with definitive radiotherapy (RT) from 1/2018 to 8/2019 with a contoured rectal spacer were included. Treatment, volumetric, and dosimetric variables of the spacer, rectum, bladder, and PTV were extracted. RT was delivered either as SBRT (7.5-8 Gy x 5 fractions), moderate hypofractionation (moderate hypo; 3 Gy x 20 fractions), or conventional fractionation (1.85 Gy x 42-43 fractions). A total of 176 patients were included in this study (120 SBRT, 31 moderate, and 25 conventional). The median contoured spacer volume was 9.66 cc (range, 2.02-19.79; interquartile range (IQR) 7.47-11.16 cc). There was no correlation with spacer volume over the study period (R2 0.014), nor with rectal volume (R2 0.002), bladder volume (R2 0.009), or PTV volume (R2 0.009). For patients receiving SBRT, the median D0.05cc was 42.58 Gy (range, 39.32-46.22 Gy) and the median Dmin to the spacer was 17.46 Gy (range, 1.92-34.28 Gy). For patients receiving moderate hypo, the median D0.05cc was 64.10 Gy (range, 38.22-65.45 Gy) and the median Dmin to the spacer was 38.14 Gy (range, 7.84-58.80 Gy). For patients receiving conventional RT, the median D0.05cc was 57.84 Gy (range, 5.94-72.46 Gy) and the median Dmin to the spacer was 52.31 Gy (range, 5.24-70.82 Gy). The rectal D0.05cc was also very heterogeneous; SBRT (range, 25.27-41.74 Gy), moderate hypo (range, 36.67-61.54 Gy), and conventional (range, 11.35-80.86 Gy). There was no correlation with absolute spacer volume and any rectal dosimetric variables. There is substantial variation in the contoured rectal spacer volume, despite target delineation being performed by only two physicians. The variability appears to be independent of PTV or adjacent normal tissue volume. We demonstrate that although the spacer usually receives a portion of the prescribed dose, the minimum dose to the spacer in the majority of cases is subtherapeutic and would be unlikely to kill cancer cells displaced by the spacer. The rectum importantly still often received the prescription dose to a small volume (0.05 cc), and this may have implications for further extreme dose escalation, even in the context of a rectal spacer.