A method for optimizing LINAC treatment geometry for volumetric modulated arc therapy of multiple brain metastases

Purpose: Volumetric modulated arc therapy (VMAT) is a rotational delivery technique in which MLC shapes, dose rate, and gantry rotation speed are optimized to produce conformal dose distributions. The aim of this work is to develop a beam projection method for deriving the optimal table and collimator angles for multilesion treatment planning. Methods: The method consists of four steps. The first step is to define the vector of beam-eye-view (BEV)- Y -axis in the treatment planning CT coordinates. The second step is to project each target onto the BEV- Y -axis vector. In the third step, the best table and collimator angle are found with a brute-force optimization technique that minimizes MLC leaf sharing between lesions. The fourth step is to generate an optimized VMAT plan with appropriate table/collimator angles and evaluate the plan quality. Results: The authors tested the method on three example cases with targets of various locations in the brain and sizes ranging from 1.18 to 17.86 cm³. Applying the optimized geometric parameter to generate VMAT plan, a reduction of the 12 Gy volume was more than 6.1% for all cases; the plan homogeneity (D2% - D95%) was improved from 5.88±1.21 to 5.21±0.93 Gy vs a VMAT plan with the manufacturer recommended table and collimator angles. Conclusions: The authors conclude that the use of the projection method minimizes the sharing of MLC leaves between lesions and improves the plan quality for multilesion VMAT delivery.