Dosimetric effects of dose calculation grid size on the epidural space dose

The epidural disease progression is the most common pattern of failure after spine stereotactic body radiotherapy. The aim of this study was to clarify the effect of the dose calculation grid size (CGS) during volumetric modulated arc therapy planning on the dose to the epidural space target.

In the planning, the volume obtained by subtracting the planning organ at risk volume (PRV) of the spinal cord and/or cauda equina from the planning target volume (PTV) was defined as the PTV_eval. First, we compared the epidural space dose that overlapped with the PTV_eval at dose CGSs of 1 mm and 2 mm. Next, we compared the dose that can be given, according to the isotropic distance from the PRV of the spinal cord and/ or cauda equina at dose CGSs of 1 mm and 2 mm.

The dose to the epidural space overlapping with the PTV_eval was significantly larger at the dose CGS of 1 mm (60 to 80 cGy, 3% of the prescription dose) than at the dose CGS of 2 mm (p < 0.01). In addition, compared with the dose CGS of 2 mm, the dose CGS of 1 mm provided a larger dose to 95% of the volume in the regions where the PTV_eval overlapped at isotropic distances of 0 to < 1, 1 to 2, 2 to 3, 3 to 4, and 4 to 5 mm from the PRV of the spinal cord and/or cauda equina.

During spine stereotactic body radiotherapy by volumetric modulated arc therapy, the dose CGS of 1 mm improved the dose calculation accuracy and increased the dose to the epidural space target compared with the dose CGS of 2 mm.