Can reducing planning safety margins broaden the inclusion criteria for lung stereotactic ablative body radiotherapy?

Introduction Stereotactic ablative body radiotherapy (SABR) is currently indicated for inoperable, early-stage non-small cell lung carcinoma (NSCLC). Advancements in image-guidance technology continue to improve treatment precision and enable reductions in planning safety margins. We investigated the dosimetric benefits of margin reduction, its potential to extend SABR to more NSCLC patients and the factors influencing plan acceptability. Methods This retrospective analysis included 61 patients (stage IA-IIIA) treated with conventional radiotherapy. Patients were ineligible for SABR due to tumour size or proximity to organs at risk (OAR). Using Pinnacle auto-planning, three SABR plans were generated for each patient: a regular planning target volume margin plan, a reduced margin plan (gross tumour volume GTV+3 mm) and a non-margin plan. Targets were planned to 48Gy/4 or 50Gy/5 fractions depending on location. Plans were compared in terms of target coverage, OAR doses and dosimetric acceptability based on local guidelines. Predictors of acceptability were investigated using logistic regression analysis. Results Compared to regular margin plans, both reduced margin and non-margin plans resulted in significant reductions to almost all dose constraints. Dose conformity was significantly worse in non-margin plans (P < 0.05) and strongly correlated with targets' surface area/volume ratio (R-2 = 0.9, P < 0.05). 26% of reduced margin plans were acceptable, compared to 54% of non-margin plans. GTV overlap with OARs significantly affected plan acceptability (OR 0.008, 95% CI 0.001-0.073). Conclusion Margin reduction significantly reduced OAR doses enabling acceptable plans to be achieved for patients previously excluded from SABR. Indications for lung SABR may broaden as treatment accuracy continues to improve; further work is needed to identify patients most likely to benefit.