Roadmap: proton therapy physics and biology

The treatment of cancer with proton radiation therapy was first suggested in 1946 followed by the first treatments in the 1950s. As of 2020, almost 200,000 patients have been treated with proton beams worldwide and the number of operating proton therapy facilities will soon reach one hundred. Proton therapy has long moved from research institutions into hospital-based facilities that are increasingly being utilized with workflows similar to conventional radiation therapy. While proton therapy has become mainstream and has established itself as a treatment option for many cancers, it is still an area of active research for various reasons: the advanced dose shaping capabilities of proton therapy cause susceptibility to uncertainties, the high degrees of freedom in dose delivery offer room for further improvements, the limited experience and understanding of optimizing pencil beam scanning, and the biological effects differ from photon radiation. In addition to these challenges and opportunities currently being investigated, there is an economic aspect because proton therapy treatments are, on average, still more expensive compared to conventional photon based treatment options. This roadmap highlights the current state and future direction in proton therapy categorized into four different themes, improving efficiency, improving planning and delivery, improving imaging, and improving patient selection.