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

INTRODUCTION

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.

MATERIALS AND

METHODS

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 PTVeval. First, we compared the epidural space dose that overlapped with the PTVeval 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.

RESULTS

The dose to the epidural space overlapping with the PTVeval 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 PTVeval 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.

CONCLUSIONS

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.