3306 - Risk Factor Analysis of Acute and Late Toxicity after Stereotactic Body Radiotherapy Using Prostate Cancer Patient Data from a Hydrogel Spacer Phase 2 Study

Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) has become a popular technique for treating prostate cancer. A hydrogel spacer is widely used in combination with SBRT to reduce toxicity. However, the risk factors for adverse events following SBRT remain unclear. We hypothesize that there are specific risk factors related to toxicity. Therefore, the aim of this study is to identify the risk factors associated with acute and late toxicity using patient data from our Phase 2 study of SBRT with a hydrogel spacer.

Materials/Methods: Our Phase 2 study is a prospective, single-center, single-arm trial. Localized prostate cancer patients received a hydrogel spacer and underwent SBRT with a total dose of 36.25 Gy delivered in 5 fractions using flattening filter-free volumetric modulated arc therapy. Acute and late physician-assessed gastrointestinal (GI) and genitourinary (GU) toxicities were recorded and classified according to the Common Terminology Criteria for Adverse Events v4. For comparison, we utilized a historical cohort of prostate cancer patients who underwent SBRT without a hydrogel spacer during the same period. Propensity score matching was conducted to balance covariates. Univariate and multivariate binary logistic regression analyses were performed to determine the variables associated with acute and late GI and GU toxicities.

Results: From February 2017 to July 2018, 40 patients were enrolled in the study, with a median follow-up duration of 63 months. Thirty-nine patients from the Phase 2 study were matched 1:1 with the retrospective cohort. In the spacer group, 69% and 18% of patients experienced grade 1 and 2 acute GI toxicity, respectively, while 56% and 26% in the no spacer group experienced the same grades. Grade 2 and 3 late GI toxicity occurred in 10% and 0% of the spacer group, compared to 13% and 5% in the no spacer group. Late GI and GU toxicity two years after SBRT was significantly lower in the spacer group. No variables correlated with acute GI toxicity were identified. The International Prostate Symptom Score (IPSS) at the initiation of SBRT was associated with acute GU toxicity in both univariate and multivariate analyses. Hydrogel spacer use, rectum V100%-50%, rectum maximal dose, and mean rectum dose were correlated with late GI toxicity, while hydrogel spacer use was also correlated with lower late GU toxicity two years after SBRT in univariate analysis. In multivariate analysis, hydrogel spacer use was associated with reduced late GI and GU toxicity.

Conclusion: Our analysis indicates that the IPSS score at the initiation of SBRT is associated with acute GU toxicity, and the use of a hydrogel spacer correlates with lower late GI and GU toxicity two years after SBRT for prostate cancer.