Combined proton-photon treatments - A new approach to proton therapy without a gantry

Purpose: Although the number of proton therapy centres is growing worldwide, proton therapy is still a limited resource. The primary reasons are gantry size and cost. Therefore, we investigate the potential of a new design for proton therapy, which may facilitate proton treatments in conventional bunkers and allow the widespread use of protons. Materials and methods: The treatment room consists of a standard Linac for IMRT, a motorized couch for treatments in lying position, and a horizontal proton beamline equipped with pencil beam scanning. As proton beams are limited to a coronal plane, treatment plans may be suboptimal for many tumour sites. However, high-quality plans may be realized by combining protons and photons. Treatment planning is performed by simultaneously optimizing IMRT and IMPT plans based on their cumulative physical dose. We demonstrate this concept for three head&neck cancer cases. Results: Optimal combinations use photons to improve dose conformity while protons reduce the integral dose to normal tissues. In fact, combined treatments improve on single-modality IMRT and fixed beamline IMPT plans for quality-of-life-limiting OARs and retain most of the integral dose reduction in the healthy tissues of the pure IMPT plans. The lower doses that can be obtained with multi-modality treatments reduce the risk for side effects compared to single-modality IMRT plans. Conclusion: Combined proton-photon treatments may play a role in developing a new solution for proton therapy without a gantry. Optimal combinations improve on IMRT plans and reduce the risk of side effects while making protons available to more patients. (C) 2019 Elsevier B.V. All rights reserved.