Multicenter End-to-End Evaluation of Single-Isocenter Multitarget Stereotactic Radiosurgery Approaches: Accuracy and Plan Quality

Purpose We conducted a multicenter study on single-isocenter multitarget stereotactic radiosurgery to dosimetrically assess end-to-end test results and identify approaches and techniques influencing spatial accuracy and treatment plan quality. Methods and Materials An anthropomorphic head phantom with radiochromic film and polymer gel inserts was used with a reference structure set of 5 brain metastases. End-to-end tests were performed on-site at 23 centers in Germany, Austria, and Switzerland, each following its own single-isocenter multitarget stereotactic radiosurgery protocol. Spatial accuracy was quantified by comparing planned and measured prescription isodose-volume centroids. Plan quality was assessed from treatment planning system calculations using the Paddick gradient index (GI) and Paddick conformity index. Statistical analyses, including a generalized linear model, correlated results with protocol parameters to identify favorable systems and techniques. Results The mean spatial offset between measured and calculated prescription isodose centroids across all centers and targets was 0 . 9 ± 0 . 4 (1 σ ) mm. Offsets above 1 mm were observed in 33% of centers. Better imaging-to-radiation isocenter consistency (ICC) yielded significantly higher accuracy ( P = . 002): d ICC < median = 0 . 6 ± 0 . 2 mm versus 1 . 1 ± 0 . 3 mm. Mean GI was 6 . 7 ± 3 . 3. Automated planning (AP) tools achieved significantly lower GI (4 . 9 ± 0 . 7) than conventional planning (8 . 1 ± 3 . 8, P = . 015). Mean Paddick conformity index was 0 . 75 ± 0 . 17, with AP significantly improving conformity (0 . 83 ± 0 . 07 vs 0 . 68 ± 0 . 19; P = . 028) and reducing variability in both indices. Target-to-isocenter distance had no significant influence on spatial accuracy, GI, or Paddick conformity index. Conclusions Spatial accuracy in a static phantom was primarily determined by ICC and less by specific delivery infrastructure or techniques, emphasizing the importance of a precise imaging isocenter calibration. AP tools significantly improved and standardized treatment plan quality across centers.