(Tuesday, 7/16/2019) 1:45 PM - 3:45 PM
Room: Stars at Night Ballroom 1
Purpose: LINAC radiosurgery is often performed using dynamic conformal arcs or VMAT. For multifocal disease, conformal arc techniques can struggle to deliver the prescription dose and high conformity for irregular targets. VMAT can provide greater coverage and conformity, but can also result in highly modulated, non-intuitive MLC trajectories which can struggle to block in-between targets and may leave MLC gaps. Our purpose is to develop a hybrid technique for multifocal disease that combines the intuitive MLC trajectories of conformal arc techniques with the flexibility of VMAT.
Methods: A Conformal Arc Informed VMAT (CAVMAT) planning technique was developed in which conformal arcs are assigned subgroups of targets to maximize blocking between targets. Arc weights are optimized to achieve the desired dose per target, while minimizing MU variation per arc. The optimized conformal arc plan serves as the starting point in a VMAT inverse optimization to improve dose and conformity to each target. Nineteen multifocal VMAT cases were re-planned using CAVMAT to a prescription dose of 20Gy to each target. The following metrics of plan quality were used to compare the VMAT and CAVMAT plans: volume of healthy brain receiving 6Gy, 12Gy, and 16Gy, conformity index, and the total number of MU.
Results: For the CAVMAT plans, the average decrease in V6Gy and V12Gy was 9% (9cc ±19.0) ±16% and 3% (1cc ±1.6) ±14% respectively. On average V16Gy slightly increased by 4% (0.7cc ±0.71 ) ±16%. Total MU increased 4 ±26% but varied by plan (42% lower to 66% higher) compared to VMAT plans. The average conformity index was 1.47 ±0.29 for CAVMAT and 1.47 ±0.35 for VMAT.
Conclusion: The CAVMAT technique provides reduced low-dose spill to normal brain tissue compared to VMAT, with comparable conformity and MU. Further work will focus on automation and the consequences of target size.
Keywords
Not Applicable / None Entered.
Taxonomy
TH- External beam- photons: intracranial stereotactic/SBRT
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