Impact of Concurrent Coincident Use of Metformin During Lung Stereotactic Body Radiation Therapy

Introduction Recent data suggest synergy of chemoradiotherapy and metformin in locally-advanced non-small cell lung cancer (NSCLC). It remains unclear if similar synergy exists with stereotactic lung body radiation therapy (SBRT) and metformin. We analyzed the role of metformin on progression-free survival (PFS) and toxicity in the setting of lung SBRT. Methods We identified 31 patients on metformin-treated with SBRT for early-stage NSCLC. Eighty-nine similarly treated patients were chosen as controls. Kaplan-Meier method was used to estimate cumulative PFS probabilities. Results Median follow-up was 30.7 months. Forty-two patients had diabetes, 31 (74%) of which were taking metformin concurrent with SBRT. Median PFS for metformin-users vs. metformin non-users was 36.4 months vs 48.9 months, respectively (p = 0.29). Among diabetic patients, median PFS for metformin users was 36.4 months and was unobserved for non-users (p= 0.40). On univariable analysis, male sex (p = 0.03) and tumor size (p = 0.01) were associated with the risk of progression or death; use of metformin was not significant (p = 0.34). There was no difference in grade ≥2 radiation pneumonitis between metformin users vs non-users (p = 0.51) Conclusion In this retrospective sample of lung SBRT patients, we did not detect a meaningful effect of concurrent metformin use on PFS. Since SBRT and conventional RT may have different cell kill mechanisms, the previously described beneficial effects of metformin may not apply in a hypofractionated setting. These results should be validated in an independent dataset, and we await the results of ongoing clinical trials.