Interpretation of PlanIQ-derived feasibility dose-volume histogram f-values in lung SBRT with heterogeneous dose distributions.

Feasibility dose-volume histogram (FDVH) analysis estimates patient-specific organ-at-risk dose-reduction feasibility, but the interpretation of FDVH-derived f-values in lung stereotactic body radiotherapy (SBRT) with heterogeneous target dosing remains unclear. This study evaluated FDVH-derived f-values in 25 peripheral lung cancer cases planned with VMAT-based SBRT. Plans were prescribed to 50 Gy in 4 fractions and normalized so that planning target volume (PTV) D95% corresponded to the prescription dose, while allowing target heterogeneity with PTV D2% of approximately 135% to 145% of the prescription dose. FDVH analysis was performed using PlanIQ under 2 conditions: Condition A used the clinical prescription dose, and Condition B used adjusted target reference doses derived from the mean minimum doses to the gross tumor volume (GTV) and PTV excluding the GTV (PTV-GTV). FDVH-derived f-values were evaluated for the ipsilateral normal lung and optimization Ring ROIs, and cases classified as the Impossible region (f = 0) were analyzed separately. For the ipsilateral normal lung, 6, 17, and 9 cases were classified as the Impossible region under Condition A for V10 Gy, V20 Gy, and mean lung dose, respectively; the corresponding numbers under Condition B were 5, 14, and 9. For the Ring ROIs, f-values were obtained for all cases under Condition B, with the highest median f-value observed for Ring 2 under both conditions. These findings suggest that FDVH estimates may not fully correspond to clinical dose distributions in lung SBRT with heterogeneous target dosing. FDVH-derived f-values should be interpreted with consideration of target dose heterogeneity and dose fall-off.