A Monte Carlo tool for combined photon and proton treatment planning verification

Photons and protons are usually used independently to treat cancer. However, at MGH patients can be treated with both photons and protons since both modalities are available on site. A combined therapy can be advantageous in cancer therapy due to the skin sparing ability of photons and the sharp Bragg peak fall-off for protons beyond the tumor. In the present work, we demonstrate how to implement a combined 3D MC toolkit for photon and proton (ph-pr) therapy, which can be used for verification of the treatment plan. The commissioning of a MC system for combined ph-pr involves initially the development of a MC model of both the photon and proton treatment heads. The MC dose tool was evaluated on a head and neck patient treated with both combined photon and proton beams. The combined ph-pr dose agreed with measurements in solid water phantom to within 3%mm. Comparison with commercial planning system pencil beam prediction agrees within 3% (except for air cavities and bone regions).