Planning Tumor Volume and Local Control Following Stereotactic Body Radiation Therapy of Early-Stage NSCLC

Purpose/Objective(s) Stereotactic body radiation therapy (SBRT) is often used to treat primary early-stage non-small cell lung carcinoma (NSCLC) as an alternative to surgical management. The planning methods used for delivering SBRT differs across institutions with various planning target volume (PTV) margins utilized. The purpose of this study is to evaluate the impact of PTV margin and other factors on local control following SBRT. Materials/Methods A total of 283 patients with single lesion early-stage NSCLC treated with SBRT at our institution between 2017-2019 were included in this retrospective analysis. The median age was 75 years, with 57% female gender. Of the treated tumors, 68.6% were peripheral while 31.4% were central. Adenocarcinoma was the most common histology at 35.3% followed by 27.2% squamous cell carcinoma with other/unknown histology in the remaining 37.5% of patients. Mean gross tumor volume (GTV) was 9.7 cc (0.3-92.1) and mean PTV was 33.5 cc (5.3-170). The most common fractionation scheme used was 54 Gy in 3 fractions followed by 48 Gy in 4 fractions with mean biologically effective dose at α/β=10 (BED10) of 149.5 Gy. Local recurrence was based on recurrence of the treated lesion on follow-up imaging, defined as either significant enlargement not consistent with post-treatment radiation fibrosis or significant residual FDG-avidity if a PET/CT scan was obtained. Univariate analysis was performed via Kaplan-Meier analysis to analyze the association of local control with GTV volume, ratio of PTV to GTV volume, BED10, histology, and tumor location. Results The median follow-up was 1.6 years. Overall, 30 of the 283 patients (10.6%) developed a local recurrence. Kaplan-Meier analysis of local control and patients with a PTV to GTV ratio greater than the median of 4.74 showed statistically significant better local control at 3-years of 90.9% versus 67.7% (p=0.004). Local control was also significantly associated with GTV volume with 3-year local control of 91.9% for tumors <5 cc versus 67.7% for those >5 cc (p=0.007). The BED10 was significantly associated with local control with a 3-year local control of 82.2% for >150 Gy BED10 versus 78.3% for 100-150 Gy BED10 (p=0.016). Local control was not associated with histology (p=0.146) or central versus peripheral location (p=0.25). Conclusion Larger PTV to GTV ratio, smaller tumor volume, and a BED10 of greater than 150 Gy were significantly associated with better local control in patients treated with SBRT for early-stage NSCLC. These factors should be considered when treatment planning for SBRT in these patients.