Objectives: Stereotactic radiosurgery (SRS) of multiple brain metastases delivered via a single isocenter VMAT plan is increasingly popular. This technique delivers SRS much more efficiently compared with multiple isocenter techniques, while sparing uninvolved brain tissue, resulting in improved quality of life. This technique is primarily limited to linear accelerators equipped with high-definition MLC characterized by 2.5mm wide central leaves. Here we explore whether this technique's clinical applicability can be broadened to include standard definition (5mm) MLC leaf width. While the benefits of small MLC leaf widths (2.5mm) are well-documented for conformal radiosurgery techniques, this has not been extensively studied for VMAT. Therefore, we examined the effect of leaf width on dosimetric quality of single-isocenter, multiple target VMAT radiosurgery plans considering several methods of mitigating any dosimetric plan quality degradation caused by a wider leaf width. Methods: 20 patients with 3-10 intracranial brain metastases originally treated with 2.5 mm MLCs were re-planned using standard 5mm MLCs. The same treatment geometry was used with 3-5 VMAT arcs with flattening filter free 6 MV photons and dose 18-20Gy in single fraction (n=16), or 5 fractions of 5-5.5Gy (n=4). Conformity index, low and moderate isodose spill (V30% and V50%) were selected for analysis and V12Gy was also analyzed for single fraction cases. Standard MLC plans were modified in several ways to mitigate the degradations of dose quality values and get similar dose quality values as in original HDMLC plans. First, VMAT arcs of the standard MLC plans were duplicated and collimator angles were shifted by 10°, 15°, and 90°. Second, one and two more VMAT arcs were added to standard MLC plans and were equally spaced. Results: When plans were re-calculated using standard definition 5mm MLCs, the MU changed from 5826±2334 to 5572±2220 (p=0.99), CI increased by 2.2%±0.04% (p = 0.98), V30% and V50% increased by 27.7%±0.15% and 20.2%±0.12% (p < 0.01 for both) respectively, and V12Gy increased by 17%±0.11% (p < 0.01). The V12Gy change did not cause sufficient concern of radionecrosis
in any single fraction case to warrant fractioning treatment. Adding duplicated VMAT arcs with modified collimator angles did not improve the plan quality compared to standard MLC plans. For the standard MLC plans with one more VMAT arc, almost all dosimetric quality values improved, with the largest remaining difference being in V30%. When two more VMAT arcs were added, the percent differences further improved for all dosimetric quality values. Conclusions: Using 5mm MLCs for single isocenter multitarget VMAT leads to minor increase in conformity index and moderate increases in low and moderate isodose spill. Among the tested methods to mitigate changes in dosimetric quality, adding two more VMAT arcs to standard MLC plans was able to give close dosimetric values as in original 2.5mm high defintion MLC plans.