Dosimetric Parameters Predicting Local Failure after Stereotactic Body Radiation Therapy Using the Robotic Radiosurgery System for Oligometastatic Lesions in the Lung and Liver

Abstract

Stereotactic body radiation therapy (SBRT) is an option for local therapy for lung or liver oligometastases. The purpose of this study was to investigate safety and efficacy of SBRT using the robotic radiosurgery system for lung and liver oligometastases and evaluate which dosimetric parameters predict local failure (LF). In total, 76 patients with 114 lesions (lung: 70, liver: 44) were treated with SBRT using the robotic radiosurgery system between May 2013 and July 2016 at our institute. The study cohort included patients with 1–3 synchronous lung/liver metastases and a controlled primary lesion. Patients with less than 6 months of follow-up or Eastern Cooperative Oncology Group performance status (PS) >2 were excluded. Total radiation dose range was 48–64 Gy in 3, 4, or 8 equal fractions. The radiation dose delivered to the planning target volume (PTV) was prescribed to the 75–85% isodose line, covering ≥95% of the PTV. Median follow-up and age were 19 months (range, 7–43) and 69 years (range, 37–91), respectively. Forty-four patients (57.9%) were male and 32 patients (42.1%) were female. Four patients (5.3%) had a PS of 2. Primary tumors were observed in colorectum (n = 73 lesions; 64.0%), gastric region (n = 6 lesions; 5.3%), ovary (n = 6 lesions; 5.3%), esophagus (n = 5 lesions; 4.4%), and others (n = 24 lesions; 21.0%). Eighty-three lesions (72.8%) were treated with chemotherapy before SBRT. At SBRT consultation, 46 patients (60.5%) had one, 22 (29.0%) patients had two, and 8 (10.5%) patients had three metastatic lesions. The median maximum tumor diameter (MTD) was 19 mm (range, 5–57) and median gross tumor volume (GTV) was 3.2 mL (range, 0.2–106.8). The median PTV biological effective dose (BED10), mean GTV BED10, and minimum GTV BED10 were 122.0 Gy (range, 43.3–181.0), 178.0 Gy (range, 92.6–239.0), and 144.5 Gy (range, 36.6–208.5), respectively. Nine lesions (7.9%) were treated with 3 fractions, 80 lesions (70.2%) with 4 fractions, and 25 lesions (21.9%) with 8 fractions. Thirty lesions (26.3%) were treated with chemotherapy after SBRT. The 1- and 2-year LC rates were 88.8% and 78.0%, respectively. Absence of post SBRT chemotherapy (p=0.006), MTD >20 mm (p=0.016) and minimum GTV BED10 <112.5 Gy (p=0.038) were significant predictors of LF. Histology of colorectal cancer (p=0.542) was not a significant predictive factor of LF. The 2- and 3-year overall survival rates were 76.0% and 51.2%, respectively. Grade 3, grade 2, and grade 1 adverse events were observed in 1, 2 and 53 patients, respectively. SBRT with the robotic radiosurgery system is a safe and effective treatment for lung or liver oligometastases. Among dosimetric parameters, minimum GTV BED10 <112.5 Gy was a significant predictive factor for LF. A larger patient cohort or a prospective study is needed to validate our results.