SU-E-T-613: Physics Considerations for Single-Isocenter Volumetric Modulated Arc Radiosurgery for Treatment of Multiple Intracranial Targets




To address physics challenges associated with single isocenter radiosurgery for multiple intracranial metastases (SIRMIT). Because the Varian High Definition MLC has thicker leaves >4cm from the isocenter, we predict inferior dose falloff and plan quality for small targets located >4cm from isocenter. To address this concern, we evaluate various isocenter placement strategies including one that places the isocenter closer to smaller targets. We also evaluate the significance of arc geometry avoidance angles to limit lens dose.


11 SIRMIT plans with isocenter at centroid were retrospectively analyzed to determine the relationship between relevant dosimetric indices and distance from isocenter (e.g. relative tumor volume). We investigated three isocenter placements for four SIRMIT patients whose lesions had larger variations in target volume; placements included: centroid, Eclipse's built-in method, and an “inverse center of mass” (ICM) method that weights the isocenter towards smaller lesions. Three VMAT SIRMIT plans were prepared according to institutional and Clark et al.'s guidelines to investigate the effect of arc geometry on lens dose.


Dose conformity and gradient fall-off tended to be better for larger tumors (>1cc) close to isocenter. For the four largest tumors (per plan), centroid CI, GI, and HI values were 9±9%, 12±4%, and 3.8±4.9% better than the next best isocenter placement strategy. None of the methods yielded significantly higher conformity for small tumors, however ICM GI and HI values were better for three of the small tumors by 6±12% and 1.8±2.2% respectively. Lens dose range decreased from [3.3%, 11.4%] to [0.1%, 0.5%] by avoiding arc geometries that enter/exit through the eyes.


CI and GI values were poorer for small distal targets, especially >6cm. Large tumors benefited from centroid placement more than small tumors did from ICM. It is necessary to consider arc geometry avoidance angles to adequately reduce lens dose.