Lung Oligometastases Treated with Stereotactic Ablative Radiation Therapy: Dosimetric Outcomes

Abstract

Stereotactic ablative radiotherapy (SABR) is frequently used in the treatment of oligometastases in the lungs. Treatment schedules remain variable, and information regarding the relationship between dosimetric parameters and local control (LC) is sparse. We aimed to assess these outcomes in a single-institution cohort. We retrospectively identified 113 patients with 224 lung metastases treated at a single institution from October 2006 to August 2017. Oligometastases was defined as 5 or fewer extracranial sites of disease. The most common SABR prescription was 50 Gy in 5 fractions, but dose-fractionation schedules ranged from 35 Gy in 5 fractions to 55 Gy in 5 to 10 fractions. Protracted fractionation was used at the discretion of the treating physician to decrease the risk of normal tissue complication or when third-party payers would not approve SABR (1-5 fractions). Biologically equivalent dose (BED) was calculated for tumors with a presumed alpha/beta ratio of 10. Follow-up was imaging-based for all but 9 lesions, for which no further imaging was available. Kaplan-Meier survival analyses and Cox proportional hazard modeling were used to describe local control (LC). Overall 1- and 2-year LC rates were 91% and 79%, respectively. The most common primary disease sites included head and neck (50 treated lesions), sarcoma (48), non-small cell lung cancer (38), colorectal (27), and genitourinary (22). The 2-year LC rates of the lung lesion treated with SABR differed significantly based on histology, with head and neck primaries demonstrating the lowest 2-year LC (57%; p < 0.0001). Tumors treated with a BED10 <100 Gy had very similar 2-year LC to those treated with BED10 ≥100 Gy (76% vs 79%; hazard ratio [HR] = 1.8; p = 0.09). Patients with smaller tumors, < 1 cm3, experienced improved 2-year LC (84% vs 76%; HR = 0.5; p = 0.07), though this was not statistically significant. LC did not differ significantly according to pulmonary lobe lesion location (p = 0.79), though non-apical metastases had notably higher 2-year LC rates than apical lesions (81% vs. 55%; HR = 0.5; 95% confidence interval [CI] = 0.2 – 1.0; p = 0.083). SABR resulted in excellent local control for lung oligometastases. Patients with lung metastases from head and neck primary malignancies appeared to experience worse LC than those with tumors of other primary sites. Those with smaller (<1 cm3) and non-apical tumors had slightly better LC relative to their counterparts. Protracted fractionation schemes with BED10 <100 Gy offered similar LC to more aggressive dose/fractionation schedules. Less aggressive dose/fractionation schedules are a reasonable option to improve normal tissue tolerance or when third-party payers do not approve SABR (1-5 fractions).