Joint Optimization of Radiotherapy Treatments Involving Multiple Radiation Modalities

Most radiotherapy treatments are performed with high-energy photons. Particles have both advantages and disadvantages compared to photons regarding their dosimetric characteristics, availability, and radiobiological properties. In this article, we investigate how to optimally combine photons and particles. We introduce treatment plan optimization methodology to simultaneously optimize multiple radiation modalities for three applications: 1) electron–photon combinations are illustrated for a superficial tumor, where electrons may reduce integral normal tissue dose. Joint optimization is based on cumulative physical dose as both modalities are delivered in each fraction; 2) proton–photon combinations are demonstrated for a liver stereotactic body radiotherapy treatment in which 1 fraction is delivered with protons and 4 with photons, showing that protons are best exploited by delivering higher doses per fraction to parts of the tumor. Such combinations can be optimized based on cumulative biologically effective dose to incorporate fractionation; and 3) carbon–photon combinations, optimized based on the cumulative biological effect, are demonstrated for glioblastoma where carbon ions can deliver a high dose to the radioresistant gross tumor volume while photons are superior for protecting normal brain within the clinical target volume through fractionation. In conclusion, through simultaneous optimization of multimodality treatments, the distinct characteristics of different radiation types can be exploited to achieve improvements over single-modality treatments.