Semester of Graduation

Summer 2020


Master of Science (MS)


Physics and Astronomy

Document Type



Purpose: No consensus currently exists in the radiotherapy community about the correct margin size to use for spinal SBRT. Margins have been proposed to account for various errors individually, but not with all errors combined to result in a single margin value. The purpose of this work was to determine a setup margin for spinal SBRT based on known and measurable errors during radiotherapy to achieve at least 90% coverage of the clinical target volume (CTV) with the prescription dose for at least 90% of patients and not exceed a 30 Gy point dose or 23 Gy to 10% of the spinal cord subvolume. Methods: The random and systematic error components of intrafraction motion, residual setup error, and end-to-end system accuracy and the penumbral width of a spinal SBRT plan were measured. The patient’s surface displacement was measured to quantify intrafraction motion, the residual setup error was quantified by re-registering accepted daily cone beam computed tomography (CBCT) setup images, and the measurement of the displacement between measured and planned dose profiles in a phantom quantified the end-to-end system accuracy. These errors and parameters were used to identify the minimum acceptable margin size. The margin recommendation was validated by assessing dose delivery across 140 simulated patient plans suffering from various random shifts representative of the measured errors. Results: The errors were quantified in three dimensions and the analytical margin generated was 2.4 mm. With this margin applied in the superior/inferior direction only, at least 90% of the CTV was covered with the prescription dose for 96% of the 140 patients simulated. With this margin applied, there was minimal negative effect on the spinal cord dose levels. Conclusions: The findings of this work support that a 2.4 mm margin applied in the superior/inferior direction can achieve at least 90% coverage of the CTV for at least 90% of viii dual-arc volumetric modulated arc therapy (VMAT) spinal SBRT patients in the presence of errors when immobilized with vacuum bags and treated at Mary Bird Perkins Cancer Center (MBPCC).

Committee Chair

Fontenot, Jonas