Proton beam reirradiation

Abstract

Proton therapy is a modality of radiation therapy with unique physical properties relative to photon (X-ray) therapy. Each proton beam is modulated to deposit the maximum radiation dose in the target, with essentially no radiation to tissues beyond the target. Compared to photon treatments, highly conformal treatment plans can typically be developed with fewer proton beams, significantly reducing the overall exposure of nontarget tissues to radiation. Given the narrow therapeutic window of reirradiation, proton therapy is of great interest as a mechanism to potentially avoid or reduce toxicities of reirradiation by limiting the volume of nontarget tissues receiving additional radiation dose. In some diseases, proton reirradiation may improve outcomes by facilitating safer radiation dose escalation to recurrent and potentially radioresistant tumors or providing better target coverage while respecting constraints to critical normal structures. In uncommon cases, proton therapy may permit reirradiation when the dosimetry achieved with other modalities is felt to preclude safe reirradiation. Clinical experience with proton reirradiation is currently limited to relatively small patient series and is highly heterogeneous. To better understand the value of proton therapy in reirradiation relative to other radiation modalities, prospective evaluation with more homogenous patient populations is needed to evaluate predefined end points based on rational clinical hypotheses. In this chapter, the rationale and published clinical results of proton therapy for reirradiation are reviewed for a variety of disease sites, with case examples provided.