Room: Track 6
Purpose: To determine appropriate CTV-PTV setup margins for different spine SBRT fractionation schedules that would achieve 90% CTV coverage with the prescription dose for 90% of patients and not exceed spinal cord dose tolerances.
Methods: The SDE2 algorithm was used to model all sources of interfraction and intrafraction uncertainty. Components of systematic and random errors were directly measured for use in the margin. Residual setup error, the remaining difference in position between the CTV before treatment and the planned position, was quantified for 20 patients. End-to-end system accuracy was quantified from the measured geometric shift between the planned and delivered dose distributions on a diode dosimeter array for 14 dose planes. Intrafraction motion was quantified using the C-Rad Catalyst system to measure patient surface motion for 26 patients. Penumbral width, defined as the distance between 90% and 50% isodose lines in a spinal SBRT plan, was also measured. Using the measured inputs, the corresponding CTV-to-PTV setup margins were then generated using the SDE2 algorithm for 1 to 5 fraction treatments. To validate the model, a five-fraction spinal SBRT plan was created using the calculated margin. The isocenter was shifted by random offsets for 140 simulated treatments, and the doses to the CTV and the spinal cord were quantified.
Results: A maximum systematic error of 0.611 mm, maximum random error of 1.31 mm, minimum penumbral width of 3.57 mm, and CTV coverage and population proportion constraints of 90% were used as input for the SDE2 algorithm. A conservative margin width of 2.42 mm in the superior/inferior direction was calculated, resulting in 90% CTV coverage for 96% of patients.
Conclusion: This work determined a margin for spine SBRT which meets treatment goals. The margin includes all known sources of error associated with radiotherapy and was verified on a clinical spine SBRT plan.
Funding Support, Disclosures, and Conflict of Interest: Work was partially supported by a research grant from Elekta, Ltd.