The Abscopal Effect: Could a Phenomenon Described Decades Ago Become Key to Enhancing the Response to Immune Therapies in Breast Cancer?

Background: The term “abscopal effect” was defined in 1953. In oncology the term is used to describe systemic antitumor effects triggered by local irradiation (nontarget effect). Although the mechanism of the abscopal effect is not completely understood yet, it has been demonstrated that in situ tumor vaccination, and the resulting antitumor immune response, is one of the key factors. Summary: The development of immune therapies has recently led to concepts combining local radiotherapy and immune therapy with the aim of enhancing the response to immune therapy by the immunological mechanisms summarized in the term abscopal effect. This concept has also been investigated in less immunogenic tumors such as breast cancer. Initial data are promising but the hypothesis that the combination of checkpoint inhibitors and local radiotherapy could be an effective combination in breast cancer has to be proven by ongoing trials. Substitution of local radiotherapy by local hyperthermia could be an option in selected cases. Key Messages: Combination of checkpoint inhibitors with local radiation or hyperthermia in breast cancer is a promising approach and could enhance the response rates generated by immune therapy alone through the antitumor immune response initiated by the abscopal effect.