Local Control Analyses of Pulmonary Oligometastases Treated By Stereotactic Body Radiation Therapy (SBRT) from a Multi-institutional Survey in Japan

This study aimed to investigate local control (LC) rate and to identify affecting factors for LC of pulmonary oligometastases treated by SBRT using large data sets from a multi-institutional nationwide survey in Japan. A total of 1374 patients with 1545 targets from 68 institutions were collected. Eligibility was as follows: SBRT for pulmonary oligometastases was performed between 2004 January and 2015 June, biological effective dose (BED10) was 75 Gy or more. Local recurrence of thoracic primary tumor was excluded and all extrathoracic lesions were controlled at the time of SBRT. Cumulative LC rate was calculated using Kaplan-Meier method and the 95% confidence interval (95% CI) was calculated using Greenwood's formula. Cox proportional hazards model was applied for LC analyses and variables with a p-value of < 0.20 in univariate analyses were put in multivariate analysis (MVA). The median targeted tumor diameter was 1.5 cm (range, 0.3-6.5), and the median overall treatment time (OTT) of SBRT was 7 days (range, 3-81). Dose calculation algorithm was divided into type B (51.7%) which was equivalent of Analytical Anisotropic Algorithm, type C (9.3%) which was equivalent of Monte Carlo Algorithm and type A (34.8%) which was older generation algorithm than both. The median heterogeneity correction considered BED10 around isocenter was 126.9 Gy (range, 76.8-352.7). Primary sites were separated into lung (29.1%), colorectum (25.3%), genitourinary (13.5%), head and neck (8.6%), esophagus (8.5%) and others. Median follow-up period was 24.2 months (range, 0.1-143.7). Local recurrence occurred in 222 tumors and median time to local recurrence was 12.4 months (range, 2.9-98.7). Estimated 1-year, 3-year and 5-year LC were 92.0% (95% CI, 90.4-93.4%), 81.3% (95% CI, 76.7-83.6%) and 78.5% (95% CI, 75.6-81.2%), respectively. The result of MVA (n=785) showed that maximum diameter of targeted tumor (per 1 cm increase; Hazard ratio [HR], 1.320; p=0.023), dose calculation algorithm (type B vs. type A; HR, 0.541; p=0.012), OTT (per 10 days prolonged; HR, 0.564; p=0.022) and primary site of metastases were significantly associated with LC. In regard to primary sites, colorectum showed significantly lower LC rate compared to lung (HR, 2.837; p<0.001), genitourinary (HR, 6.473; p<0.001), head and neck (HR, 4.879; p<0.001) and others (HR, 2.478; p=0.005), genitourinary showed significantly better LC compared to colorectum (HR, 0.154; p<0.001) and esophagus (HR, 0.291; p=0.020) but the others showed no significant association. BED10 showed the tendency (per 10 Gy increase; HR, 0.924; p=0.102). To procure excellent LC rate, the use of type A algorithm should be avoided, the strategy of tumor diameter and primary sites should be considered and higher BED with longer OTT might help.