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Effect of Hydrogel Spacer On High and Moderate Rectal Doses for Prostate SBRT

A Powers*, R Sheu, Y Lo, R Stock,Mount Sinai Medical Center, S Blacksburg, NYU-Langone Winthrop New York, NY

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Hydrogel spacer is a validated approach to minimize rectal toxicity in patients with prostate cancer undergoing definitive IMRT. We sought to characterize the dosimetric impact hydrogel placement has on salient parameters for Rapid Arc SBRT prostate plans.


Methods: 55 consecutive patients with low-intermediate prostate cancer were treated with definitive LINAC-based prostate SBRT monotherapy. The prescription dose of 3625cGy. 15 (28.3%) patients had hydrogel spacer placed prior to treatment planning. Patients were treated based on dose objectives established by ongoing RTOG protocols. The mean CTV dose was 3754.9cc’s and the mean Rectal V3600cGy, V3440cGy, and V3000cGy were 1.2cc’s, 2.4cc’s, and 5.5cc’s, respectively. The mean Bladder V3700cGy was 3.9cc’s. The Pearson chi-square test and ANOVA were used to compare patient groupings.


Results: Patients with hydrogel spacer had significantly lower mean Rectal V3600cGy (0.9 vs. 1.3cc, p=.001), Rectal V3440cGy (1.7 vs 2.7cc, p<.001), and Rectal V3000cGy (3.8 vs. 6.1cc, p<001). Those with hydrogel spacers had a higher likelihood of a Rectal V3600cGy <1cc (53.3% vs. 13.2%, p=.002); a Rectal V3440cGy <2cc (60.0% vs. 7.9%, p<.001); and a Rectal V3000cGy <4cc’s (53.3% vs. 7.9%, p<.001). There was no difference in Rectal max dose. There was no statistical difference in Bladder or CTV dosimetry between those who had hydrogel spacer placement and those who did not.


Conclusions: Patients with hydrogel spacer placement have a significant decrease in both high and moderate rectal doses when treated via ongoing RTOG protocols. While previous series characterizing the effect of hydrogel on IMRT plans have suggested extra-rectal improvements, this series found dosimetric parity for bladder and CTV endpoints. This data should be correlated with clinical toxicity outcomes as they evolve.

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