PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s): Local recurrence for spinal stereotactic body radiation therapy (SBRT) often occurs in the epidural space immediately adjacent to the planning target volume (PTV). It is unknown if a clinically significant reduction in local recurrence could be achieved safely by requiring a minimum dose in the epidural space near the PTV. We hypothesize that it is possible to impose a minimum dose of 1000 cGy to 95% of the epidural space near the PTV for single fraction cases without compromising the dose objectives placed on the spinal cord and PTV.
Materials/Methods: 19 spinal SBRT plans were retrospectively reviewed. For standardization, the plans were renormalized to deliver 1800 cGy in one fraction. For each plan, the region of the epidural space of interest (RESI) was defined as the overlap between a 3mm shell around the spinal cord and a 4mm expansion of the PTV. These cases were re-planned using three coplanar volumetric modulated arc therapy (VMAT) arcs centered on the PTV with collimator angles of 0, 45, and 315 degrees. Prescriptions for the new plans were normalized to 1800 cGy to the PTV in one fraction, with the D95 and D5 PTV constraints being 1750 cGy and 1950 cGy, respectively; the D10 and Dmax spinal cord constraints were 1000 cGy and 1400 cGy, respectively. Assuming that these constraints were met, dose to the RESI was maximized through optimization in a treatment planning system. The dose delivered to the RESI was computed from the newly-produced plans, along with the D95 and D5 of the PTV and the D10 and Dmax of the spinal cord. Data were analyzed using descriptive statistics.
Results: Among the 13 cases that were capable of meeting the dose-volume constraints imposed on the spinal cord and PTV, the mean and standard deviation of the RESI D95 was 1074 cGy and 79 cGy, respectively, with 11 out of 13 of the reproduced plans achieving an RESI D95 above 1000 cGy. For the 6 cases that could not meet the constraints on the spinal cord and PTV regardless of RESI prioritization, the mean and standard deviation of the RESI D95 was 1137 cGy and 104 cGy, respectively, with the lowest value being 1056 cGy.
Conclusion: We conclude that 1000 cGy to 95% of the RESI volume is a feasible dose constraint to impose during single-fraction spinal SBRT treatment planning. In cases where the PTV is very near or overlaps with the spinal cord, this dose constraint tends to have little impact on planning optimization, since dose to the RESI is pushed above this constraint without prioritization. A prospective study is needed to determine the clinical impact of this additional dose constraint.
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