A dosimetric analysis of a rotating gamma ray system and volumetric modulated arc therapy in retrospective replanning for primary liver cancer

Background This study is a retrospective replanning study. For primary liver cancer, radiotherapy is vital for inoperable patients; however, dosimetric data comparing the rotating gamma ray system (RGS) and volumetric modulated arc therapy (VMAT) remains scarce, complicating clinical technique selection. Notably, RGS refers to a body gamma ray platform designed for extracranial stereotactic body radiotherapy (SBRT) — this is distinct from the “Gamma Knife” system, which is specifically dedicated to intracranial radiosurgery. Purpose To compare the dosimetric differences between RGS and VMAT techniques in the radiotherapy of primary liver cancer. Methods 30 patients with primary liver cancer, underwent both RGS and VMAT planning. Analyzed planning target volume (PTV) parameters: coverage (Target Coverage Index, TCI), max/mean/min dose (D max /D mean /D min ), conformity (Conformity Index, CI), homogeneity (Homogeneity Index, HI), gradient index (Gradient Index, GI). Evaluated liver doses: D mean , V 20 , V 5 . Results In this dosimetric comparison of 30 primary liver cancer patients, RGS plans demonstrated significantly higher maximum tumor dose (D max : 10253 ± 196 cGy vs. VMAT: 6807 ± 315 cGy; P < 0.001), mean tumor dose (D mean median [IQR]: RGS: 7410 cGy [7222–7493] vs. VMAT: 5780 cGy [5675–5870]; P < 0.001), and target coverage (median [IQR]: RGS: 100% [99–100] vs. VMAT: 98% [98–98]; P < 0.001) compared to VMAT plans. RGS also exhibited a steeper dose fall‐off (GI: 3.32 ± 0.37 vs. VMAT: 5.84 ± 1.12; P < 0.001). Conversely, VMAT achieved superior conformity (CI: 0.83 ± 0.06 vs. RGS: 0.63 ± 0.08; P < 0.001), homogeneity (HI: 0.23 ± 0.05 vs. RGS: 0.57 ± 0.05; P < 0.001), and significantly reduced low‐dose liver irradiation (V 5 : 23.7% ± 10.2% vs. RGS: 34.4% ± 12.8%; P < 0.001). The mean liver dose was also significantly lower with VMAT (616 ± 253 cGy vs. RGS: 677 ± 314 cGy; P = 0.039). No significant differences were observed in minimum tumor dose (D min ) or liver V 20 . Conclusions In radiotherapy for primary liver cancer, the rotating gamma ray system offers the characteristic of highly focused central dose with rapid peripheral dose fall‐off, ensuring high tumor dose while better protecting adjacent liver tissues. VMAT provides superior conformity and homogeneity, with better control of the mean liver dose and low‐dose irradiation volumes.