Determining Simple and Effective Cost Functions for an Efficient Volumetric-Modulated Arcs-Based Stereotactic Radiosurgery for Single Brain Metastases Using Monaco® Planning System

Introduction Volumetric-modulated arcs (VMA) can produce dose distributions suitable for stereotactic radiosurgery (SRS) with a multi-leaf collimator (MLC) for brain metastases (BMs). The treatment planning and verification for VMA are more complicated than for dynamic conformal arcs. The longer the preparation time from image acquisition to the start of irradiation, the higher the risk of tumor growth and/or displacement. This planning study aimed to exploit the simple and effective cost function (CF) for establishing semi-automatic efficient VMA optimization for SRS of single BMs. Materials and methods The study population included 30 clinical BMs with a gross tumor volume (GTV) of 0.72-44.30 cc (median 9.81 cc) and a depth of 20-79 mm (median 41 mm). The treatment platform included a 5-mm leaf-width MLC Agility^® (Elekta AB, Stockholm, Sweden) and a planning system Monaco^® (Elekta AB). Among various physical and biological CFs available, three combinations consisting of just two or three physical CFs were compared. The Target Penalty CF was uniformly used for ensuring the GTV dose. Three different CF combinations were applied for reducing the surrounding tissue doses: (1) the Conformality alone with the 4-cm margin around target (MAT) that optimizes the limited voxels around the GTV (_woQO); (2) the Conformality with the 4-cm MAT and the Quadratic Overdose (_wQO_4 cm); and (3) the Conformality with the 8-cm MAT that optimizes the overall voxels around the GTV and the Quadratic Overdose (_wQO_8 cm). The prescribed dose was uniformly assigned to each GTV <i>D</i> _{V-0.01 cc}, the minimum dose of GTV minus 0.01 cc. Results Adding the Quadratic Overdose (_wQO_4 cm and _wQO_8 cm) significantly improved the overall dose distribution in comparison to the _woQO, while no significant difference was observed between the _wQO_4 cm and _wQO_8 cm overall. However, for the GTVs of ≥14 cc, the GTV dose conformity and dose gradient outside the GTV boundary, including the dose attenuation margin, were significantly superior in the _wQO_8 cm than _wQO_4 cm. In addition, for the GTV depth of ≥41 mm, the GTV dose conformity and the dose concentric lamellarity at 2 mm outside the GTV were significantly superior in the _wQO_8 cm than _wQO_4 cm. Meanwhile, for the GTVs of ≥10 cc, the GTV dose was significantly more inhomogeneous in the _wQO_4 cm than the _wQO_8 cm. In addition, for the GTVs of <10 cc and the depth of ≤40 mm, the dose concentric lamellarity at 4 mm inside the GTV surface was significantly higher in the _wQO_4 cm than the _wQO_8 cm. Conclusions Applying at least three physical CFs to a GTV and the head surface contour is recommended as an effective and efficient optimization method using Monaco for VMA-based SRS of single BMs. In addition, optimizing the overall voxels around the GTV is suitable for reducing the surrounding tissue dose, especially for large and deeply located lesions. Templating the combination of the three CFs with the detailed settings allows for semi-automated and rapid treatment planning, facilitating the prompt start of irradiation after image acquisition.