3279 - Safety Profile of High-Dose Intensity Modulated Radiotherapy in Prostate Cancer: Preliminary Findings from the JROSG 17-5 Multicenter Prospective Observational Study in Japan

Y. Sekino¹, H. Ishikawa², K. Maruo³, M. Mizumoto¹, S. Itasaka⁴, T. Mizowaki⁵, Y. Ohkubo⁶, Y. Yoshioka⁷, H. Akamatsu⁸, T. Okumura⁹, K. Shirai¹⁰, S. Shimamoto¹¹, M. Kokubo¹², T. Sakamoto¹³, T. Maebayashi¹⁴, K. Nakamura¹⁵, Y. Muramoto¹⁶, N. Nishiyama¹⁷, T. Shimizuguchi¹⁸, and H. Sakurai¹; ¹Department of Radiation Oncology, Faculty of Medicine, University of Tsukuba, Tsukuba, Japan, ²QST Hospital, National Institutes for Quantum Science and Technology, Chiba, Japan, ³Department of Biostatistics, Institute of Medicine, University of Tsukuba, Tsukuba, Japan, ⁴Department of Radiation Oncology, Kurashiki Central Hospital, Kurashiki, Japan, ⁵Department of Radiation Oncology and Image-applied Therapy, Kyoto University Graduate School of Medicine, Kyoto, Japan, ⁶Department of Radiation Oncology, Saku Central Hospital Advanced Care Center, Saku, Japan, ⁷Radiation Oncology Department, Cancer Institute Hospital, Japanese Foundation for Cancer Research, Tokyo, Japan, ⁸Department of Radiology, Yamagata University Faculty of Medicine, Yamagata, Japan, ⁹Department of Radiation Oncology, Ibaraki Prefectural Central Hospital, Kasama, Japan, ¹⁰Department of Radiology, Saitama Medical Center, Jichi Medical University, Saitama, Japan, ¹¹Department of Radiation Oncology, Osaka General Medical Center, Osaka, Japan, ¹²Department of Radiation Oncology, Kobe City Medical Center General Hospital, Kobe, Japan, ¹³Department of Radiation Oncology, Kyoto Katsura Hospital, Kyoto, Japan, ¹⁴Department of Radiology, Nihon University School of Medicine, Tokyo, Japan, ¹⁵Department of Radiation Oncology, Hamamatsu University School of Medicine, Hamamatsu, Japan, ¹⁶Department of Radiation Oncology, Juntendo University Graduate School of Medicine, Tokyo, Japan, ¹⁷Department of Radiation Therapy, Hokkaido Cancer Center, Sapporo, Japan, ¹⁸Department of Radiation Oncology, Tokyo Metropolitan Cancer and Infectious Diseases Center, Komagome Hospital, Tokyo, Japan

Purpose/Objective(s): Intensity-modulated radiotherapy (IMRT) has been increasingly adopted for prostate cancer. However, the safety profile of high-dose IMRT in a Japanese population has not been thoroughly examined. We have conducted the JROSG 17-5 study, a multicenter nationwide study in Japan enrolling over 1,000 Japanese patients with localized prostate cancer to evaluate the safety of high-dose IMRT. Materials/

Methods: A multicenter, prospective observational study was conducted, enrolling men diagnosed with localized prostate cancer from April 2018 to October 2020 across 40 Japanese institutions. Eligibility criteria included pathologically confirmed prostate cancer, a clinical stage of T1c-T4N0M0, and a Karnofsky performance status exceeding 70. The primary endpoint is biochemical relapse-free survival, with secondary endpoints regarding overall survival and the incidence of grade 2 or higher toxicity as defined by the CTCAE scale. This preliminary analysis focuses on the incidence of grade 2 or higher late toxicity at 2-year. The study has been registered with the UMIN Clinical Trials Registry (UMIN000033172).

Results: A total of 1,044 patients were enrolled, with 1,012 undergoing analysis. The median follow-up period was 3.7 years, with a median age of 74 years (IQR 69-77). Distribution by NCC risk category was as follows: low risk 48 (4.7%), intermediate 421 (41%), and high risk 547 (54%). Comorbidities included diabetes mellitus in 183 patients (18%), anticoagulation therapy in 214 (21%), collagen disease in 12 (1.2%), and use of spaceOAR in 76 (7.5%). The median total dose was 74 Gy (range 54-80 Gy). Fraction sizes administered were 2 Gy per fraction in 720 patients (71%), 2.5 Gy per fraction in 179 (17.7%), and =3 Gy per fraction in 117 (11.6%). Across the overall population, genitourinary (GU) toxicities of grade 2 or higher were observed in 6.2%, and gastrointestinal (GI) events in 1.2%. In a subgroup analysis categorized by radiation dose per fraction, genitourinary (GU) toxicities were recorded at 6.0% for 2Gy, 7.4% for 2.5Gy, and 3.5% for doses =3Gy per fraction. Gastrointestinal (GI) complication rates were 1.6 % at 2Gy, 1.0% at 2.5Gy, and 0% for =3Gy per fraction.

Conclusion: High-dose IMRT is well-tolerated at the 2-year follow-up among Japanese patients, with long-term data on safety and efficacy eagerly anticipated.