Clinical outcomes and factors involved in the local control of proton beam therapy for oligometastatic liver tumors in patients with colorectal cancer

Background and purpose There are no existing reports on proton beam therapy (PBT) for local control (LC) of liver metastasis of colorectal cancer (LMCRC). We calculated the LC rate of PBT for LMCRC and explored the influence of each factor on the LC rate. Materials and methods Cases in which PBT was performed at our center between 2009 and 2018 were retrospectively selected from the database. Patients with LMCRC without extrahepatic lesions and no more than three liver metastases were included. Effectiveness was assessed based on LC, overall survival (OS), and progression-free survival (PFS) rates. Adverse events (AEs) are described. Factors that may be related to LC were also investigated. Results This study included 23 men and 18 women, with a median age of 66 (range 24-87) years. A total of 63 lesions were included in the study. The most frequent dose was 72.6 Gy (relative biological effectiveness)/22 fractions. The median follow-up period was 27.6 months. The 3-year LC, OS, and PFS rates were 54.9%, 61.6%, and 16.7%, respectively. Our multivariate analysis identified the distance between the tumor and the gastrointestinal (GI) tract as a factor associated with LC (P = 0.02). No grade >= 3 AEs were observed. None of the patients experienced liver failure during the acute or late phase. Conclusion Care must be taken with tumors that have reduced planning target volume coverage owing to organs at risk restrictions, especially in tumors near the GI tract.