Physics controversies in proton therapy.

The physical characteristics of proton beams are appealing for cancer therapy. The rapid increase in operational and planned proton therapy facilities may suggest that this technology is a "plug-and-play" valuable addition to the arsenal of the radiation oncologist and medical physicist. In reality, the technology is still evolving, so planning and delivery of proton therapy in patients face many practical challenges. This review article discusses the current status of proton therapy treatment planning and delivery techniques, indicates current limitations in dealing with range uncertainties, and proposes possible developments for proton therapy and supplementary technology to try to realize the actual potential of proton therapy.