Adaptive radiotherapy as a strategy for universal dosimetric accuracy across tumor sizes and locations in lung SBRT for NSCLC.

e20102 Background: Inter-fractional anatomical changes in lung SBRT raise concerns about dosimetric accuracy and potential target under-dosage. Adaptive radiotherapy (ART) is a proposed strategy to correct these variations. This study aims to quantify the dosimetric degradation from these changes and assess the necessity of ART to restore plan quality. Methods: This retrospective planning study retrospectively included 33 lung SBRT patients, each prescribed with 50Gy/5Fx radiation therapy on an emulator. For each patient, three plans were generated: 1) the original approved Reference plan (REF); 2) the non-adaptive Scheduled plan (SCH), created by recalculating the REF plan on the daily anatomy; and 3) the re-optimized Adaptive plan (ADP) on the same daily anatomy. Dosimetric parameters for planning target volume (PTV) coverage (PTV D99%, PTV D95%) and organs at risk (OARs) were compared using the Wilcoxon signed-rank test. The ART benefit (ADP-SCH improvement) was correlated with tumor diameter and lobe location using the Wilcoxon rank-sum test. Results: Compared to the REF plan, the SCH plan showed significant target coverage degradation. The median PTV D99 dropped from 48.0 Gy (REF) to 43.1 Gy (SCH) (p < 0.001) . The ADP plan significantly recovered this loss, restoring the median PTV D99 to 47.5 Gy (p < 0.001 vs SCH), a value nearly identical to the original REF plan. In terms of PTV D95, nine patients showed dose degradation over 5.0 Gy in SCH plans with a maximum of 14.5 Gy. Crucially, ADP plans corrected a “false sparing” effect seen in SCH plans for serial OARs; for example, median spinal cord Dmax increased from 8.3 Gy (SCH) to 9.15 Gy (ADP) (p = 0.022), aligning with the REF plan's intent. This dosimetric benefit was necessary regardless of tumor characteristics. There was no significant correlation between the magnitude of PTV D99 improvement and tumor diameter (p = 0.72) or lobe location (e.g., Upper vs. Mid/Lower, p = 0.13, Upper/Mid vs. Lower, p = 0.82). Conclusions: Inter-fractional anatomical variations in lung SBRT lead to clinically significant target under-dosage when a standard IGRT workflow (SCH) is used. ART (ADP) effectively and accurately restores target coverage to the intended (REF) level. The need for ART is ubiquitous and cannot be predicted by baseline tumor size or location. These findings suggest that a verification-based ART strategy is dosimetrically necessary to ensure treatment quality and safety in lung SBRT.