Hybrid proton-photon inverse optimization with uniformity-regularized proton and photon target dose.

The goal of radiation therapy is to deliver tumoricidal dose to clinical target volume (CTV) while sparing organs-at-risk (OAR). We hypothesize that the joint use of proton and photon radiation therapy via appropriate hybrid proton-photon inverse planning method will be more favorable than proton or photon therapy alone, in terms of optimized combination of CTV coverage and OAR sparing. This work develops hybrid proton-photon inverse optimization method that simultaneously optimizes proton and photon variables. To account for delivery uncertainty, proton dose is targeted at CTV using robust optimization, and photon dose is targeted at either CTV using robust optimization or planning target volume (PTV) using the same setup shifts. The optimization objectives enforce OAR sparing and uniform CTV coverage for the total dose, while imposing uniform-dose regularization at targets for both the proton and photon component in order for both components to be individually deliverable. The hybrid problem with dose-volume-histogram (DVH) constraints is nonconvex and solved by iterative convex relaxations of DVH constraints and alternating direction method of multipliers (ADMM). Preliminary results suggest the hybrid proton-photon planning potentially improves proton or photon planning in terms of optimized combination of CTV coverage and OAR sparing.