2223 - A Hybrid IMPT-VMAT Optimization Method for Clinically Deliverable Combined Proton-Photon Therapy

Purpose/Objective(s): Hybrid proton-photon therapy has been shown to provide superior robust target dose coverage and organ-at-risk (OAR) sparing compared to proton-only and photon-only therapy. This study addresses the clinical deliverability such as minimum monitor unit (MMU) constraints for protons and multi-leaf collimator (MLC) aperture constraints for photons, which are essential for practical clinical use.

Materials/Methods: The proposed hybrid IMPT-VMAT and fraction (HIV-F) optimization method minimizes the combined proton and photon dose distribution to achieve robust target coverage and optimal OAR sparing by jointly optimizing proton spots, MLC apertures, photon intensity, and fraction numbers. Optimization is initialized using pre-computed, sequenced photon fluences, while plan deliverability is ensured by enforcing MMU constraints and MLC leaf position bounds. By directly incorporating these deliverability factors, the method allows for automatic computation of leaf trajectories, improving both the clinical feasibility and practicality of hybrid treatment plans.

Results: HIV-F was validated in comparison with HIV (with fixed proton fraction number). Additionally, it was compared to the sequenced hybrid IMPT-IMRT and fraction optimization method (HII-F-S), which optimizes both proton and photon fluences and translates photon fluence into MLC apertures through leaf sequencing. Both HIV-F and HIV achieved superior conformality compared to proton-only and photon-only plans, with conformality index (CI) improvements of 0.75, 0.75, 0.66, and 0.65, respectively. Additionally, HIV-F reduced OAR toxicity, with the maximum lung dose decreasing to 49.4 Gy, 48.6 Gy, 51.7 Gy, and 52.9 Gy for each plan. Moreover, HIV-F achieved similar fraction to HII-F-S while offering a more conformal dose distribution.

Conclusion: We have developed a novel hybrid IMPT-VMAT and fraction optimization method, HIV-F, which simultaneously optimizes photon MLC apertures, proton scanning spot weights, and proton/photon fractionation, resulting in clinically deliverable combined proton-photon therapy.