Review of clinical results of charged-particle therapy for esophageal cancer.

Esophageal cancer is one of the common causes of cancer-related death. The treatment for esophageal cancer, particularly unresectable cases, is a difficult problem. Reports about charged-particle therapy including proton beam therapy and carbon-ion radiotherapy for esophageal cancer have increased. The objective of this study was to review the clinical results of charged-particle therapy for esophageal cancer. Charged-particle therapy was used with an expectation of increasing overall survival with reducing toxicities because charged-particle therapy can reduce the irradiated dose for normal tissues around the target tumor due to its characteristics, hence the name Bragg peak. Proton beam therapy showed a superior distribution of irradiation dose over X-ray therapy including intensity-modulated radiotherapy in silico, but clinical results were not the same. Some reports suggested that proton beam therapy may reduce acute and late toxicities, particularly in the heart and lung, during and after treatment, although it cannot lead to a higher overall survival than that in X-ray therapy. On the other hand, there are a few reports about carbon-ion radiotherapy for esophageal cancer. The special feature of carbon-ion radiotherapy is that hypofractionated radiotherapy is possible as compared to that in X-ray therapy or proton beam therapy. However, the true clinical impact of proton beam therapy or carbon-ion radiotherapy remains unclear because there are no prospective clinical trials comparing charged-particle therapy to X-ray therapy. In view of charged-particle therapy may become one of the treatment choices for esophageal cancer, further studies are needed.