A study of single-isocenter for three intracranial lesions with VMAT-stereotactic radiosurgery: Treatment planning techniques and plan quality determination

Abstract

Objective: To compare a modified single-isocenter technique between (1) 6 MV and 6FFF and (2) fixed collimator angles and adjusted collimator angles for three intracranial lesions by using VMAT-SRS. Materials and methods: Twenty patterns of three intracranial lesions varying in size and location were generated. The VMAT plans using Eclipse version 13.6 were initially generated according to the University of Alabama, Birmingham’s (UAB’s) guidelines. Planning parameters including 6 MV, 6FFF, and collimator angles were further modified. All plans were normalized to achieve a 99% dose coverage with 20 and 24 Gy to 5 mm and 10 mm lesions, respectively. Dosimetric parameters, including CIRTOG, CIPaddick, GI, HI, mean dose to the normal brain, and V5Gy and V12Gy, were analyzed using Wilcoxon or paired t-test. Results: The 6 MV plans with adjusted collimator angle provided better CIRTOG (1.217 vs. 1.266, p = 0.007) and CIPaddick (8.30 vs. 8.13, p = 0.007), while the 6FFF plans were not statistically different. For both energies, the adjusted collimator angles were less than V5Gy (p < 0.01), V12Gy (p < 0.01) and GI (p < 0.001) compared to the fixed collimator angles of UAB protocol, while the HI index was similar. The plans with 6FFF offered superior plan quality than 6 MV for target coverage (CIRTOG 1.222 vs. 1.266, p = 0.005 and CIPaddick 0.832 vs. 0.813, p = 0.002), dose fall off (GI 7.246 vs. 8.264, p < 0.001) and normal brain sparing (V12Gy 3.802 vs. 4.224, p < 0.001 and V5Gy 22.092 vs. 24.966, p < 0.001). Conclusion: The optimization of collimator angles show an improvement in dose fall-off and normal brain sparing relative to the fixed collimator angles. Plans with 6 FFF provide a better plan quality than 6 MV.