1575. Feasibility Analysis of an Epidural Dose Constraint in SBRT

Authors: Elizabeth Howell; Patrick Jensen, BS; John Kirkpatrick, MD, PhD; Scott Floyd, MD, PhD; Jordon Torok, MD; Qiuwen Wu, PhD; C. Rory Goodwin, MD, PhD (Durham, NC)

Introduction: The epidural space represents a common site of local recurrence following spinal stereotactic body radiation therapy (SBRT). It remains to be established whether a reduction in recurrence could be achieved by requiring a minimum dose in this space. In this study, we investigated the feasibility of a minimum dose constraint of 1000 cGy to 95% of the epidural space near the planning tumor volume (PTV) without compromising the dose objectives placed on the spinal cord for single fraction cases. Methods: 19 spinal SBRT plans were retrospectively reviewed. Cases were re-planned using three coplanar volumetric modulated arc therapy (VMAT) arcs centered on the PTV. Plan prescriptions were normalized to 1800 cGy to the PTV in one fraction, with D95 and D5 PTV constraints being 1750 cGy and 1950 cGy, respectively. D10 and Dmax spinal cord constraints were 1000 cGy and 1400 cGy, respectively. A region of the epidural space of interest (RESI) was defined as the overlap of a 4mm expansion of the PTV and 3mm around the spinal cord. Dose to the RESI was then optimized in a treatment planning system. The D95 and D5 of the PTV, the D10 and Dmax of the spinal cord, and dose delivered to the RESI were computed and analyzed using descriptive statistics. Results: 13 cases were ultimately capable of meeting dose-volume constraints. Mean RESI D95 for these cases was 1074 cGy (SD = 79 cGy). 11 reproduced plans achieved an RESI D95 above 1000 cGy. For the 6 cases that failed constraints, mean RESI D95 was 1137 cGy (SD = 104 cGy), with the lowest value being 1056 cGy. Conclusion: 1000 cGy to 95% of the RESI volume seems a feasible dose constraint during single-fraction spinal SBRT treatment planning. Prospective studies are warranted to determine the clinical impact of this constraint.