Abstract:
The purpose of this study was to evaluate the dosimetric effects between single isocenter (SI) and double isocenters (DI) VMAT SRT of multiple brain metastases. There are two parts of study, which were experimental for energy selection, technique comparison and clinical part to use appropriate technique. As the first part, three lesions with 3 cm distance apart and 1 cm size for all lesions were created as standard plan by varying energy. Twelve VMAT SRT plans by varying energy with 6 MV, 10 MV, 6 FFF and 10 FFF were planned on simulated three lesions of CT image. 18 VMAT SRT plans with varying lesions size, number and distance were simulated on patient CT image using Eclipse treatment planning system version 15.0. The plan consisted of 3 techniques in: 2 coplanar arcs SI (2 Arcs SI), 1 coplanar combine with 2 non-coplanar arcs SI (3 Arcs SI) and 1 coplanar and 2 non-coplanar arc DI (6 Arcs DI). The VMAT plans were generated with 21 Gy prescription dose to all lesions in 3 fractions. The plans were evaluated in terms of Conformity index (CIPaddick), Homogeneity index (HIICRU), and Gradient index (GIPaddick) for PTV and V12Gy and V6Gy for normal brain. The same dose constraints were used to optimize for all cases. The results showed that 6FFF was suitable energy to apply the technique comparison because it provides the best conformity and gradient parameters when compared to 6MV, 10MV and 10FFF energy. For the technique comparison when changing the size, number and distance between the lesions using energy 6FFF, it was found that 3 arcs SI and DI were improvement in average GI (14.79±5.83, 13.70±4.72) than the 2 arcs SI (17.56±6.15) while HI and CI values were comparable for all techniques. GIPADDICK and CIPADDICK of two techniques; 3 Arcs SI and 6 Arcs DI, were not significantly showed in results with p value while HI is slightly better in 6 Arcs DI (HIICRU p value= 0.01). For normal brain, V12Gy for 2 Arcs and 3 Arcs SI plans were comparable with DI and the volumes of normal brain receiving 6 Gy in 3 arcs SI and DI (77.40 ± 34.30 cm3, 68.94 ± 30.50 cm3) were better than 2 arcs SI (108.10 ± 57.20cm3). Moreover, the number of arcs and treatment time were increased by nearly 2-fold and inconvenience in practice in DI. In conclusion, 3 arcs non-coplanar SI VMAT technique was present the best in dosimetric evaluation in 2-5 lesions metastases SRT.
