Single isocenter stereotactic radiosurgery for patients with multiple brain metastases: dosimetric comparison of VMAT and a dedicated DCAT planning tool

BACKGROUND: In this dosimetric study, a dedicated planning tool for single isocenter stereotactic radiosurgery for multiple brain metastases using dynamic conformal arc therapy (DCAT) was compared to standard volumetric modulated arc therapy (VMAT). METHODS: Twenty patients with a total of 66 lesions who were treated with the DCAT tool were included in this study. Single fraction doses of 15–20 Gy were prescribed to each lesion. Patients were re-planned using non-coplanar VMAT. Number of monitor units as well as V(4Gy), V(5Gy) and V(8Gy) were extracted for every plan. Using a density-based clustering algorithm, V(10Gy) and V(12Gy) and the volume receiving half of the prescribed dose were extracted for every lesion. Gradient indices and conformity indices were calculated. The correlation of the target sphericity, a measure of how closely the shape of the target PTV resembles a sphere, to the difference in V(10Gy) and V(12Gy) between the two techniques was assessed using Spearman’s correlation coefficient. RESULTS: The automated DCAT planning tool performed significantly better in terms of all investigated metrics (p < 0.05), in particular healthy brain sparing (V(10Gy): median 3.2 cm(3) vs. 4.9 cm(3)), gradient indices (median 5.99 vs. 7.17) and number of monitor units (median 4569 vs. 5840 MU). Differences in conformity indices were minimal (median 0.75 vs. 0.73) but still significant (p < 0.05). A moderate correlation between PTV sphericity and the difference of V(10Gy) and V(12Gy) between the two techniques was found (Spearman’s rho = 0.27 and 0.30 for V(10Gy) and V(12Gy), respectively, p < 0.05). CONCLUSIONS: The dedicated DCAT planning tool performed better than VMAT in terms of healthy brain sparing and treatment efficiency, in particular for nearly spherical lesions. In contrast, VMAT can be superior in cases with irregularly shaped lesions.