The Japanese nationwide cohort data of proton beam therapy for liver oligometastasis in breast cancer patients.

A nationwide multicenter cohort study on particle therapy was launched by the Japanese Society for Radiation Oncology in Japan in May 2016. We analyzed the outcome of proton beam therapy (PBT) for liver oligometastasis in breast cancers. Cases in which PBT was performed at all Japanese proton therapy facilities between May 2016 and February 2019 were enrolled. The patients were selected based on the following criteria: the primary cancer was controlled, liver recurrence without extrahepatic tumors and no more than three liver lesions. Fourteen females, with a median age of 57 years (range, 44-73) and 22 lesions, were included. The median lesion size, fraction (fr) size and biological effective dose were 44 (20-130) mm, 6.6 (2-8) gray (Gy) (relative biological effectiveness)/fr and 109.6 (52.7-115.2) Gy, respectively. The median follow-up period was 22.8 (4-54) months. The 1-, 2- and 3-year local control (LC) rates of liver metastasis from breast cancer were 100% for all. The 1-, 2- and 3-year overall survival rates were 85.7, 62.5 and 62.5%, respectively. The 1-, 2- and 3-year progression-free survival (PFS) rates were 50.0%, 33.3%, and 16.7%, respectively. The median PFS time was 16 months. Only one patient did not complete PBT due to current disease progression. One patient had Grade 3 radiation-induced dermatitis. None of the patients experienced radiation-induced liver failure during the acute or late phase. Owing to the low incidence of adverse events and the high LC rate, PBT appears to be a feasible option for liver oligometastasis in breast cancers.