Institutional experience with SRS VMAT planning for multiple cranial metastases.

Background and PurposeThis study summarizes the cranial stereotactic radiosurgery (SRS) volumetric modulated arc therapy (VMAT) procedure at our institution. Materials and MethodsVolumetric modulated arc therapy plans were generated for 40 patients with 188 lesions (range 2-8, median 5) in Eclipse and treated on a TrueBeam STx. Limitations of the custom beam model outside the central 2.5mm leaves necessitated more than one isocenter pending the spatial distribution of lesions. Two to nine arcs were used per isocenter. Conformity index (CI), gradient index (GI) and target dose heterogeneity index (HI) were determined for each lesion. Dose to critical structures and treatment times are reported. ResultsLesion size ranged 0.05-17.74cm(3) (median 0.77cm(3)), and total tumor volume per case ranged 1.09-26.95cm(3) (median 7.11cm(3)). For each lesion, HI ranged 1.2-1.5 (median 1.3), CI ranged 1.0-2.9 (median 1.2), and GI ranged 2.5-8.4 (median 4.4). By correlating GI to PTV volume a predicted GI=4/PTV0.2 was determined and implemented in a script in Eclipse and used for plan evaluation. Brain volume receiving 7Gy (V-7Gy) ranged 10-136cm(3) (median 42cm(3)). Total treatment time ranged 24-138min (median 61min). ConclusionsVolumetric modulated arc therapy provide plans with steep dose gradients around the targets and low dose to critical structures, and VMAT treatment is delivered in a shorter time than conventional methods using one isocenter per lesion. To further improve VMAT planning for multiple cranial metastases, better tools to shorten planning time are needed. The most significant improvement would come from better dose modeling in Eclipse, possibly by allowing for customizing the dynamic leaf gap (DLG) for a special SRS model and not limit to one DLG per energy per treatment machine and thereby remove the limitation on the Y-jaw and allow planning with a single isocenter.