Room: AAPM ePoster Library
Purpose: To investigate the effect of number of targets over the plan quality metrics for single-isocenter clinical plans using a newly developed auto planning module to treat multiple brain metastases with a single isocenter.
Methods: A retrospective investigation of the plan quality of 60 SRS clinical plans treated with HyperArc was performed. The clinical plans investigated were delivered in a single fraction with prescriptions ranging from 19 Gy to 24 Gy and equivalent sphere diameters of less than 2 cm. Plans were divided in three groups according with the number of treated metastases per isocenter: one (28 patients, 28 lesions), two to three (17 patients, 37 lesions) and more than four (15 patients, 72 lesions). The plan quality metrics investigated were the PTV_Mean, PTV_Max, RTOG CI, Paddick CI, GI and ICRU HI. The overall plan quality was scored according with the RTOG 95-08 criteria.
Results: Mean and median values for PTV_Mean and PTV_Max did not show a significant difference depending on the number of treated lesions, however, the standard deviation increased. PTV_Mean was 114.9±2.1%, 111.4±3.9 and 112±5.8% for the three groups, and PTV_Max values were 129.7±4.9%, 124.5±8.0% and 127±10.2%. Significant differences between the third group and the first two were observed for the conformity metrics. The RTOG CI values were 1.18±0.15, 1.20±0.11 and 1.54±0.52, while the Paddick CI values were 0.75±0.07, 0.75±0.06 and 0.62±0.16. Significant differences among all groups were observed for the GI values: 3.8±0.6, 5.1±1.49 11.9±25. The values of the ICRU HI did not show any significant differences among the groups: 0.23±0.04, 0.21±0.06 and 0.23±0.07.
Conclusion: RTOG 95-08 per protocol score can be achieved for SRS plans treating from one up to three lesions with single isocenter. Acceptable variations are observed in some of the plans treating more than four lesions.