Comparison of planning techniques for single-isocenter multiple-target (SIMT) stereotactic radiosurgery

Since 2010, Duke University Medical Center has used the single-isocenter technique to treat patients with multiple brain metastases. The purpose of this project is to compare treatment planning techniques for these patients who received treatment. First, we want to determine if volumetric modulated arc therapy (VMAT) or dynamic conformal arc therapy (DCAT) is the better method for treatment for incoming patients. Next, we want to know if using U-frame or frameless masks provide better plan quality. Lastly, we want to test the use of a stationary couch to simulate imaging while treating with the moving gantry. DCAT plans were created for each of the 40 single-isocenter patients who received VMAT at Duke University Medical Center from 2016 to 2018. These patients were randomly selected based only on the number of metastases, from 2 to 14. We created the DCAT plans using 5 couch positions, 2 collimator angles, and 100° arcs on BrainLab Elements. We modeled U-frame and frameless masks using 100° and 180° arcs, respectively. To simulate imaging, we kept the couch at 0° while using only 180° arcs. The clinical VMAT plans delivered to the 40 patients had an average conformity index of 1.47 and average gradient index of 8.57. Average whole-brain V3 Gy and V5 Gy were 14.07% and 5.80%, respectively. In comparison, using DCAT the conformity index was 1.75 and the gradient index was 6.87. Whole-brain V3 Gy and V5 Gy were 11.25% and 5.59%, respectively. The frameless mask plans had conformity and gradient indexes of 1.68 and 6.39 and V3 Gy and V5 Gy of 11.39% and 5.09%, respectively. Using VMAT for the imaging cases, we found conformity and gradient indexes of 1.59 and 11.99 and V3 Gy and V5 Gy of 18.04% and 8.41%. Using DCAT for the imaging cases had conformity and gradient indexes of 2.02 and 9.86 and V3 Gy and V5 Gy of 14.21% and 7.25%, respectively. Overall, VMAT plans had higher conformity index with lower gradient index at the cost of healthy brain protection compared to DCAT. Frameless masks also increased the conformity index and decreased the gradient index with no significant impact on low doses to the brain. The use of imaging while treating should be considered with the benefit when imaging on a case-by-case basis.