TITLE:
Optimizing LINAC-Based Stereotactic Radiosurgery Dosimetric Outcomes as Dictated by Beam Energy, Multileaf Collimator Size, and Prescribed Isodose Level
AUTHORS:
Sophia Zhang, Suhong Yu, Chengyu Shi
KEYWORDS:
SRS, DCA, MLC
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.14 No.4,
November
25,
2025
ABSTRACT: 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 (12) 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 plans have higher GI by 25.73% ± 9.09%, higher V12 by 25.27% ± 10.78%, lower Dmax by 11.02% ± 0.19%, and shorter beam-on time by 12.80% ± 0.78%. This study showed that: 1) lower beam energy plans may provide better brain sparing but incur much longer beam-on time; 2) thinner MLC leaf width may provide better brain sparing at the expense of slightly longer treatment time; and 3) prescribing dose at lower IDL may provide better brain sparing but result in higher tumor dose inhomogeneity and longer beam-on time. Conclusion: Beam energy, MLC leaf width, and prescribed IDL may each affect tumor dose coverage, normal brain tissue sparing, or treatment duration. LINAC-based SRS practitioners can select a customized treatment strategy in order to achieve plan goals.