M. Ogita1, H. Yamashita1, Y. Miki1, S. Sawayanagi2, and Y. Nozawa1; 1Department of Radiology, The University of Tokyo Hospital, Tokyo, Japan, 2Department of Radiology, the University of Tokyo Hospital, Tokyo, Japan
Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) delivers a larger daily dose in small fractions and has become a popular technique for prostate cancer treatment. A hydrogel spacer is widely used in prostate radiotherapy to reduce rectal toxicity by separating the rectum from the prostate. Because the clinical data of SBRT in combination with hydrogel spacer are limited, we conducted a phase 2 study to evaluate the safety and efficacy of SBRT with hydrogel spacer. We previously showed that the use of hydrogel spacer improved patient-reported acute bowel toxicity. Here, we report 5-year physician-assessed late toxicity and oncological outcomes.Materials/
Methods: This trial is a prospective single-center, single-arm phase 2 study. Localized prostate cancer patients were eligible for the study. All patients inserted a hydrogel spacer and received SBRT of 36.25 Gy in 5 fractions with flattening filter-free volumetric modulated arc therapy. The primary endpoint was physician-assessed acute gastrointestinal (GI) toxicity within three months assessed by the Common Terminology Criteria for Adverse Events v4. (reported previously). The secondary endpoints were physician-assessed late GI and genitourinary (GU) toxicities, patient-reported outcomes, and biochemical progression-free survival (bPFS). Propensity score-matched analysis compared patients with the hydrogel spacer with those without the spacer (control). The control data were obtained from our institutions retrospective cohort, which received SBRT without a hydrogel spacer. Results: Forty prostate cancer patients were enrolled between February 2017 and July 2018. The median follow-up duration was 63 months (6-77). Three, 25, 6, and 6 patients had low-, intermediate-, high-, and very high-risk prostate cancer. The grade 2 late GI and GU toxicities were observed in four (10%) and 16 (40%) patients, respectively. No grade 3 or worse late GI and GU toxicities were observed. The 5-year bPFS was 100%. Thirty-nine patients (spacer group) were matched with the retrospective cohort (control group) at a matching ratio of 1:1. After matching, grade 2 and 3 late GI toxicities were seen in 4 (10%) and 0 (0%) patients in the spacer group versus in 5 (13%) and 2 (5%) patients in the control group. Late GI toxicity after three years was significantly lower in the spacer group (P < 0.01). The 5-year bPFS was 100% in the spacer group and 89% in the control group (P = 0.03). Conclusion: SBRT with a hydrogel spacer showed excellent bPFS without late severe adverse events. A hydrogel spacer may contribute to reducing late GI toxicity after three years.
