A Practical Approach to Transmission and Dosimetric Leaf Gap Selectionfor Two Identical Linacs for Stereotactic Radiosurgery Treatments
H Xie1, E Elder1,2, B B Ghavidel1,2, S Charyyev1, R Liu1, Y Liu1,2, J Zhang1,2, K J Luca1,2, K D Godette1,2, X Yang1,2, T Liu1,2, J Roper1,2*, (1) Department of Radiation Oncology and (2) Winship Cancer Institute, Emory University, Atlanta, GA
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PO-GePV-M-299 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Linac-based stereotactic radiosurgery (SRS) has demonstrated its feasibility and is widely adopted in clinical practices. However, treatment planning systems (TPS) commissioned for conventional IMRT/VMAT may not be suitable for calculating small fields for SRS as the latter has a strong dependence on MLC transmission and dosimetric leaf gap (DLG) values. The purpose of this work is to present a practical approach for the selection of these MLC parameters for SRS treatments.
Methods: A retrospective random selection of SRS plans, including multi-metastases (MM) brain and single small target cases, treated with 6xFFF on an EDGE linac (Varian, Palo Alto, CA) were anonymized, re-optimized and re-calculated for delivery on two TrueBeam linacs. The plans were measured with EPID portal dosimetry (PD). The MM cases involved MLC blocking large portions of the EPID image for 100% of the treatment. These low dose values were compared to predicted dose distributions. The TPS transmission value was iterated until agreement was obtained between predicted and measured low dose values. In a similar approach the single lesion plans were used to adjust the DLG at the high dose and penumbra regions. Gamma analysis, with 3% dose difference and 1 mm distance to agreement for dose levels larger than 10% of the global maximum, was performed with PD and SRS MapCheck (Sun Nuclear, Melbourne, FL).
Results: The gamma passing rates for single-small target and multi-metastases plans were 98.4±1.9% (80.4±5.8% previously) and 98.1±1.4% (94.4±8.4% previously), respectively for Linac 1 and 98.2±1.2% (93.3±5.3% previously) and 98.6±1.0% (78.6±15.1% previously) for Linac 2. Gamma passing rates of the SBRT cases were retained at levels ≥98.2% on both machines. The MapCheck results were consistent with PD results.
Conclusion: This work demonstrated a practical approach to transmission and DLG selection for SRS treatments with a TPS previously commissioned for conventional IMRT/VMAT.
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