J Xue*, J Teruel, N Bice, P Galavis, M Malin, A Rea, D Barbee, NYU Langone Medical Center, New York, NY

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  J Xue

PO-GePV-T-368 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Implementing cranial single isocenter multi-mets (SIMM) requires accurate beam modelling and MLC characterization. This work describes commissioning a beam model for SIMM using SRS MapCheck for model optimization and validation.

Methods: Ten non-clinical SIMM plans were generated by HyperArc in Eclipse 15.6 using 10X-FFF and Millenium120 MLC, varying the: quantity, sizes, and locations of targets. An existing beam model optimized for non-SIMM IMRT and VMAT was used as a starting model for SIMM modeling. SRS MapCheck (Sun Nuclear) was used for validation in conjunction with StereoPhan and Gafchromic film. Each verification plan was calculated for a selected target on the particular measurement plane, and delivered with planned gantry rotation and couch positions.

Results: Validation tests with the SRS MapCheck showed a median 96.4% (91.1%, 98.9%) passing rate using a global Gamma criteria of 2%/1mm for all SIMM plans. Passing rates increased with decreasing MLC dosimetric leaf gap (DLG) or transmission factor (TF) values from the initial model’s clinical values (DLG/TF = 0.12/0.0156). Optimal DLG appeared between 0.06 and 0.08 with TF kept at 0.0156. The overall gamma passing rates also increased at lower TF values tested from 0.008 to 0.014. For non-SIMM plans, however, lower Gamma passing rates were observed with decreasing DLG/TF values. It’s reported that the SRS MapCheck can systematically be under response by 1-2%. When scaling SRS MapCheck measured doses up by approximately 1.5%, the optimal DLG would have increased close to that in the original beam model.

Conclusion: Using SRS MapCheck alone could potentially lead to reduced DLG/TF parameters less than their ideal clinical values for SIMM. Commissioning a beam model with the SRS MapCheck should be done with supplementary measurement including films.

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