Abscopal effect in recurrent colorectal cancer treated with carbon-ion radiation therapy: 2 case reports

The abscopal effect involves shrinkage and/or remission of distant, unirradiated disease following local irradiation. It is rare and mechanistically remains under study, although recent case reports have seen incidence of the effect increasing with administration of immunomodulatory agents, such as the CTLA-4 inhibitor ipilimumab. The abscopal effect appears to occur in response to radiation therapy and is immune system mediated, although research has predominantly focused on the use of conventional photon irradiation. Particle irradiation offers unique dose distributive benefits, concentrating dose in target tissue while sparing healthy tissue in a manner theoretically superior to photon. Meanwhile, heavy-ion irradiation, such as with carbon ions, offers additional biological effects owing to a superior linear energy transfer delivered on a per-particle basis. Carbon-ion radiation therapy (CIRT) has been used successfully in the treatment of recurrent rectal cancer.