Proton therapy: clinical gains through current and future treatment programs

Proton beams can provide a substantial dosimetric advantage because of their unique depth-dose characteristics, which can be exploited to achieve significant reductions in normal tissue doses proximal and distal to the target volume. These may allow escalation of tumor doses, potentially improving local control and survival while at the same time reducing toxicity and improving quality of life. While many of the steps in proton and photon treatment planning processes are similar, there are also significant differences. Some of these arise from the unique physical characteristics of protons, while others are the result of their greater vulnerability to uncertainties, especially from inter- and intrafractional variations in anatomy. These factors must be considered in designing margins and field-shaping devices, as well as in designing treatment plans as a whole and in evaluating them. Ongoing research is aimed at better estimation of these uncertainties and their impact on proton therapy, and reducing these uncertainties through image guidance, adaptive radiotherapy and the development of novel imaging devices and dose computation algorithms. For proton therapy delivery, intensity modulation techniques are already in use, and will continue to be developed and utilized increasingly. The advantages include greater flexibility in dose shaping for improved target coverage and reduced normal tissue dose, potential improvement in plan robustness, and improvement in clinical efficiency. A spectrum of imaging techniques can now be used to assist our understanding of proton dosimetry in the patient, and PET imaging is the one that is furthest developed toward the goal of in vivo dose imaging. To decrease the cost of proton therapy and increase its availability, many technical improvements and practical delivery technologies are being developed, including compact proton machines that will soon become clinically available.