The abscopal effect is a phenomenon in the treatment of metastatic cancer where localized treatment of a tumor causes not only a shrinking of the treated tumor, but also a shrinking of tumors outside the scope of the localized treatment. R.H. Mole proposed the term “abscopal” (‘ab’ - away from, ‘scopus’ - target) in 1953 to refer to radiation’s effects “at a distance from the irradiated volume but within the same organism.” Initially associated with single-tumor, localized radiation therapy, the term has also come to encompass other types of localized treatments such as electroporation and intra-tumoral injection of therapeutics. While this phenomenon is extremely rare, its effect on the cancer can be stunning, leading to the disappearance of malignant growths throughout the entire body. Such success has been described for a variety of cancers, including melanoma, cutaneous lymphomas, and kidney cancer.
Scientists are not certain how the abscopal effect works to eliminate cancer in patients. Studies in mice suggest that the effect may depend upon activation of the immune system. In a case study reported at Memorial Sloan-Kettering Cancer Center in New York City, changes in a metastatic melanoma patient’s immune system were measured over the course of treatment. The team observed changes in tumor-directed antibody levels and immune cell populations that occurred at the time of the abscopal effect. These findings support the idea that a localized treatment may broadly stimulate the immune system to fight cancer. At this time, various immune system cells, including T-cells and dendritic cells, are believed to play a primary role.
Effects in tissues adjacent to the irradiated area are bystander effects and are not necessarily mediated by the same mechanisms as abscopal effects.