Effects of Differences in the Increment Parameter Controlling Sector Numbers on Volumetric-Modulated Arc-Based...

Purpose A volumetric-modulated arcs (VMAs) technique, including non-coplanar arcs, enables efficient generation and implementation of suitable dose distribution in linac-based stereotactic radiosurgery (SRS) using a multileaf collimator (MLC) for brain metastases (BMs). In a Monaco^® treatment planning system (TPS) (Elekta AB, Stockholm, Sweden), an increment (Inc) parameter controls the number of sectors for each arc in VMA optimization. However, the optimal Inc parameter setting has remained to be determined. This study, therefore, aimed to investigate the impacts of differences in the Inc parameter on VMA-based SRS planning for BMs. Materials and methods This planning study targeted 30 clinical BMs with a gross tumor volume (GTV) of 0.08-48.09 cc (median 9.81 cc), which were previously analyzed. The treatment platform included a 5 mm leaf-width MLC Agility^® (Elekta AB, Stockholm, Sweden) and Monaco^® TPS (Elekta AB). The prescribed dose was uniformly assigned to the GTV near-minimum dose (DV-0.01 cc), the minimum dose of a GTV minus 0.01 cc, i.e., D>95% for GTV >0.20 cc and D95% for GTV ≤0.20 cc, to minimize the uncovered GTV to the equivalent of a 3 mm diameter lesion. The VMA planning was optimized to prioritize GTV dose conformity and the steepness of the dose gradient outside the GTV, without dose constraints within the GTV boundary, according to previously established methods. The Inc parameter settings of 10º, 20º, and 30º (Inc 10, Inc 20, and Inc 30) were compared, and all other parameters were unified. Results The Inc 30 was significantly inferior in GTV dose conformity and the steepness of dose gradients both outside and inside the GTV boundary, including the gradual dose attenuation margin outside the GTV surface. The Inc 10 had the most inhomogeneous GTV dose with the highest dose increase (2-4 mm) inside the GTV boundary and the longest total calculation time (tCT). The Inc 20 had the shortest tCT and showed a tendency to be superior in the concentric lamellarity of dose gradients 2 mm outside and 2-4 mm inside the GTV boundary. There was no significant difference between the Inc 10 and Inc 20, except for the superiority of the Inc 20 in the tCT and the concentric lamellarity of dose gradients. Conclusions The Inc parameter setting has significant impacts on the process and quality of treatment planning for VMA-based SRS for BMs. The Inc parameter of 20º per arc is recommended for templating in terms of the overall plan quality and the reasonable tCT. The Inc 10º can be an option if it is necessary to further enhance the steepness of dose increase inside the GTV boundary, although the tCT increases considerably.