Room: Karl Dean Ballroom A1
Purpose: Existing Volumetric Modulated Arc Therapy (VMAT) using coplanar arcs is highly efficient but usually dosimetrically inferior to intensity modulated radiation therapy (IMRT) with optimized non-coplanar beams. In this study, we proposed a novel integrated optimization framework that incorporates simultaneous gantry/couch rotation during the arc delivery (4Ï€VMAT) to improve VMAT plan quality without compromising delivery efficiency.
Methods: The 4Ï€VMAT optimization framework includes a Direct Aperture Optimization (DAO) module and a beam trajectory selection (BTS) module. DAO was achieved by utilizing a least square dose fidelity objective, along with an anisotropic total variation term to encourage the fluence smoothness, a single segment term for imposing simple apertures, and a group sparsity term for selecting optimal beam angles. Optimal gantry/couch trajectories were selected using the Dijkstra's algorithm on the graph, where the edge costs and the node costs were determined by the gantry rotation speed and the estimated fluence map at the current iteration, respectively. Beams leading to collision were excluded. The optimization framework was solved by alternating between the DAO module and the BTS. The feasibility of 4Ï€VMAT using one full-arc or two full-arcs was tested on a brain, a lung, and a prostate cancer patient. The plan was compared against a coplanar VMAT (2Ï€VMAT) plan using one additional arc and collimator rotation.
Results: Compared to 2Ï€VMAT, 4Ï€VMAT reduced the average maximum and mean organs-at-risk (OARs) dose by 7.68% and 2.86% of the prescription dose with the same target coverage. R50 was reduced by 19.7%. Doses to the brainstem and the proximal bronchus, which are the dose limiting organs for the GBM and lung patients, respectively, were reduced by 3.2 Gy (66.7%) and 16.2 Gy (41.5%).
Conclusion: The novel 4Ï€VMAT approach affords efficient delivery of non-coplanar arc trajectories that lead to dosimetric improvements compared with coplanar VMAT using more arcs.
Funding Support, Disclosures, and Conflict of Interest: This research is supported by NIH U19AI067769, DE-SC0017687, NIH R21CA228160, DE-SC0017057, NIH R44CA183390, NIH R43CA183390, and NIH R01CA188300.