Dose-Range-Dependent Voxel-Based Biological Dose Conversion From Thoracic Ultra-Hypofractionation to Conventional Fractionation: A Pilot Evaluation and Framework for Future Multicenter Studies

The growing use of hypofractionated radiotherapy and reirradiation has increased the need for voxel-based conversion of hypofractionated dose distributions to conventional-fractionation equivalents for organ-atrisk (OAR) dose-constraint evaluation.Some voxel-based approaches apply a uniform equivalent dose in 2 Gy fractions (EQD2) to all voxels, which may not fully reproduce the dose characteristics of conventional fractionated radiotherapy, where dose per fraction decreases below 2 Gy with increasing distance from the target.Moreover, use of voxel-based conversion approaches requires validation against established OAR dose constraints.Twelve thoracic ultra-hypofractionated treatment plans prescribed in five fractions were converted using To2GyConstraints version 1.0 (Healthy Innovations, Rachiine, Zgharta, Lebanon; www.healthy-innovations.com), a cloud-based software tool that performs dose-range-dependent voxelbased biological dose conversion from hypofractionated to conventional fractionation.It applies doserange-dependent models: linear-quadratic-linear (LQ-L)-based EQD2 for high-dose voxels, linear-quadratic (LQ)-based EQD2 for intermediate-dose voxels, and equivalent dose in x Gy fractions (EQDx) for low-dose voxels.Original and converted dose distributions were evaluated using corresponding OAR constraints for their respective fractionation regimens.Constraint-violation status was compared using the original ultrahypofractionated assessment as the reference.An updated standardized data-collection template incorporating thoracic OAR constraints was developed for future multicenter evaluation.The pilot evaluation demonstrated feasibility and showed promising agreement in OAR constraint-violation assessment between original and converted dose distributions.The standardized workflow provides a framework for multicenter evaluation.Dose-range-dependent voxel-based biological dose conversion using To2GyConstraints is feasible for thoracic ultra-hypofractionated radiotherapy.By accounting for radiobiological behavior across different dose ranges, this approach may improve the representation of conventional-fractionation equivalents for OAR assessment.Larger multicenter evaluation is warranted.