Purpose: Brainlab’s Elements Cranial VMAT (CVMAT) v3.0 introduces a 4Pi optimization that includes automated gantry and couch positioning alongside collimator optimization. In contrast, v1.5 uses a fixed collimator angle and predefined templates for gantry and couch angles that are manually modified by the planner. This study evaluates the 4Pi generated plans for clinical acceptability in terms of plan quality and delivery accuracy on a VersaHD. Methods: We chose a recent sample of 9 clinical plans that represent our catalog of target locations, fractionation schemes, and target sizes. Cases were replanned in CVMAT v3.0 using 4Pi optimization. These plans were evaluated for target coverage using Inverse Paddick Conformity Index (ICI), normal tissue sparing using Gradient Index (GI), and maximum dose to the predefined "most important organ at risk.” Plans were dosimetrically evaluated using a microdiamond detector and SRS Mapcheck in a StereoPHAN to evaluate deliverability. Results: All 4Pi generated plans were able to produce target dose metrics within clinical acceptable tolerances (ICIs < 1.3 and GIs < 4.5), while maintaining organ at risk doses within AAPM Task Group 101 tolerances. All fields had >95% gamma pass rates using 3%/1mm, 10% threshold, absolute dose. MicroDiamond measured doses were within 5% of predicted for all cases with an average absolute difference of 1.73%+/-1.69%. The largest dose discrepancy occurred as a microDiamond measured underdosing for the smallest target (<0.36cc, <11.0mm diameter). Conclusion: 4Pi automated couch, gantry, and collimator angle selection in CVMAT v3.0 simplifies beam setup while producing clinically acceptable plans. Initial deliverability of 4Pi plans has been verified. Work is ongoing to elucidate the source of dose discrepancies for small targets.
Loughery B, Knill C, Sandhu R, Lin L, Seymour Z. Evaluation of 4Pi arc positioning for cranial VMAT SRS in elements. Med Phys. 2022 Jun;49(6):E956-E957.