Feasibility of Single Non-Coplanar Models for Stereotactic Radiosurgery of Brain Metastases.

Purpose/Objective(s) To compare the dosimetry differences between coplanar and non-coplanar modes and analyze the feasibility of single non-coplanar mode in the stereotactic radiosurgery of brain metastases. Materials/Methods Retrospective analysis were made on the 14 patients with brain metastases and all the plans take the VMAT and FFF technology with the prescription of 25 Gy. The distinction between plans which used coplanar and single non-coplanar and multiple non-coplanar were compared in the study, including the dose distribute of targets, Conformal index (CI), Gradient index (GI50, GI25), brain tissue exposure, beam-on time and execution efficiency. Results There was no significant difference in beam-on time and conformal index (CI) of target between coplanar and non-coplanar plan (P>0.05). All plans met clinically requirements, but differences were observed in the dosimetric parameters. The value of GI50 and GI25 in coplanar plan (GI50 = 4.84 ± 0.52, GI25 = 16.13 ± 2.73) was much higher than that in single non-coplanar (GI50 = 3.66 ± 0.44, GI25 = 9.94 ± 1.41) and multiple non-coplanar plan (GI50 = 3.73 ± 0.42, GI25 = 9.32 ± 1.23). In the meantime, the dose of brain tissue (V10 = 11.46 ± 3.47, V5 = 35.25 ± 11.79) in coplanar plan exceed the dose of brain tissue in single non-coplanar (V10 = 10.44 ± 3.16, V5 = 28.55 ± 9.62) and multiple non-coplanar plan (V10 = 10.61 ± 3.40, V5 = 25.37 ± 9.04). There was no obvious difference in beam-on time, conformal index (CI) of target and GI50 between Single non-coplanar and multiple non-coplanar plan (P > 0.05). However, multiple non-coplanar plans could achieve drop dose in low dose area. The value of V5 in brain tissue of multiple non-coplanar plans was much lower than that in single non-coplanar plan (P = 0). Conclusion Compared with non-coplanar plans, coplanar plans which have the shortest time and the highest execution efficiency in the whole process can achieve the similar prescription dose distribution without increasing the beam-on time. However, the non-coplanar plans can achieve faster out-of-target dose drop and protect the brain tissue better. The quality of single non-coplanar plans was similar to that of multiple non-coplanar plans. Multiple non-coplanar plans have significant advantages at low doses at the cost of prolong treatment time and negate delivery efficiency. The use of single-group non-coplanar models may be preferred in clinical practice. Author Disclosure M. Liu: None. J. Wu: None. X. Yao: None. K. Yuan: None. D. Zhang: None. B. Tang: None. J. Yin: None.