Background and Purpose: Radiation therapy is one of the methods for the treatment of brain tumors. Besides using Co-60-based radiation, LINAC-based stereotactic radiosurgery (SRS) has been widely used in the treatment of intracranial tumors. This study aims to compare the dosimetric outcomes of different SRS methods for typical brain tumors of various sizes, as dictated by beam energy, MLC leaf width, and prescribed isodose level (IDL) using the dynamic conformal arc. Methods: Hypothetical target lesions of 4 different sizes were contoured on the brain MRI images of an index patient. The PTVs were constructed to fall within four respective categories (<0.5, 0.5 - 1.0, 1.0 - 4.0, 4.0 - 10.0 cc), in inverse proportion to the prescription dose range of 16 - 22.5 Gy. For each lesion, SRS plans were analyzed independently between: 1) two photon beam energies with different dose rates (6X-FFF with 1400 MU/min vs. 10X-FFF with 2400 MU/min); 2) two MLC leaf widths (0.5 cm vs. 0.25 cm); and 3) two different prescribed isodose lines (90% IDL vs. 80% IDL). The degree of normal tissue sparing was evaluated by the amount of brain volume receiving more than 12 Gy (V12) and the dose Gradient Index (GI), defined as the ratio between the volume receiving more than 50% of the prescribed dose (V50%) and the PTV volume. Tumor dose inhomogeneity was analyzed in relation to the maximum dose (Dmax) deposited, while conformity was assessed using the Conformity Index (CI), defined as the ratio of the V100% volume to the PTV volume. Treatment beam-on time was also compared. All three independent variables (beam energy, MLC leaf width, and prescribed IDL) were held constant, except for the one being analyzed, with mean relative differences in the dosimetric outcomes determined for all four lesions of respective volumes. Results: The 4 PTV volumes contoured were: 0.35, 0.68, 1.76, and 5.60 cc, respectively. Dmax was affected only by the prescribed IDL, while the tumor CI did not exhibit any significant difference for all 3 variables analyzed. Compared to a beam energy of 6X FFF, the 10X FFF plans were found to have a GI and V12 that were both increased by about 13% and 14%, respectively. However, the higher beam energy resulted in a shorter beam-on time by approximately 49%. Compared to 0.25 cm MLC leaf-width, the 0.5 cm plans have higher GI by 22.12% ± 8.64%, higher V12 by 22.20% ± 11.12%, and shorter beam-on time by 2.63% ± 1.07%. Compared to 80% IDL, the 90% IDL
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