Using three-dimensional equieffective dose mapping to audit a methodology for calculating permitted doses for head...

Reirradiation is increasingly common, but accounting for prior dose to the spinal cord and brainstem is challenging. This study compares a simple isodose-based method of previous dose compensation with voxel-wise equivalent dose in 2 Gy fractions (EQD_2/2) dose mapping to assess accuracy and safety.

Ten head and neck reirradiation cases were retrospectively reviewed. During planning, original dose distributions were mapped onto the reirradiation computed tomography scan. Isodoses within the cord and brainstem were used to segment dose-level substructures, with cumulative doses kept within defined tolerances. Following 3D EQD_2/2 mapping, the same cases were audited by recalculating cumulative maximum doses. Cumulative doses from both methods were compared.

Retrospective EQD_2/2 analysis confirmed all cumulative doses were within tolerance, with up to 9.7 Gy difference between the two methods. In two cases, cord and brainstem doses approached tolerance.

A simple, isodose-based approach to spinal cord and brainstem reirradiation tolerance calculation has been shown to be safe when retrospectively compared with voxel-wise EQD_2/2 mapping. This method can be implemented in any planning system capable of generating contours from isodose lines, offering a practical alternative where advanced EQD_2/2 dose accumulation software is unavailable.