Proton stereotactic body radiation therapy for liver metastases-results of 5-year experience for 81 hepatic lesions.

Background: To report on our institutional experience using Proton stereotactic body radiation therapy (SBRT) for patients with liver metastases.

Methods: All patients with liver metastases treated with Proton SBRT between September 2012 and December 2017 were retrospectively analyzed. Local control (LC) and overall survival (OS) were estimated using the Kaplan-Meier method calculated from the time of completion of Proton SBRT. LC was defined according to Response Evaluation Criteria in Solid Tumors (RECIST) guidelines (version 1.1). Toxicity was graded according to Common Terminology Criteria for Adverse Events (CTCAE) version 4.0.

Results: Forty-six patients with 81 lesions were treated with Proton SBRT. The median age was 65.5 years old (range, 33-86 years) and the median follow up was 15 months (range, 1-54 months). The median size of the gross tumor volume (GTV) was 2.5 cm (range, 0.7-8.9 cm). Two or more lesions were treated in 56.5% of patients, with one patient receiving treatment to a total of five lesions. There were 37 lesions treated with a biologically effective dose (BED) ≤60, 9 lesions with a BED of 61-80, 22 lesions with a BED of 81-100, and 13 lesions with a BED >100. The 1-year and 2-year LC for all lesions was 92.5% (95% CI, 82.7% to 96.8%). The grade 1 and grade 2 toxicity rates were 37% and 6.5%, respectively. There were no grade 3 or higher toxicities and no cases of radiation-induced liver disease (RILD).

Conclusions: Proton SBRT for the treatment of liver metastases has promising LC rates with the ability to safely treat multiple liver metastases. Accrual continues for our phase II trial treating liver metastases with Proton SBRT to 60 GyE (Gray equivalent) in 3 fractions.