Systematic review of case reports on the abscopal effect

Radiation therapy is a highly effective local treatment for cancer. However, sporadic events of tumor regression in unirradiated fields, known as abscopal effect, have been observed for decades. This abscopal effect has more recently been postulated to be a result of antitumor immune response induced by radiation therapy. With the advent of modern immunotherapy, the potential for immune activation by radiation therapy defines a novel role for radiation therapy in systemic disease. In this context, we have searched documented cases abscopal effect of radiation therapy in literature. A total of 46 reported cases have been identified from 1969 to 2014 with median radiation dose of 31 Gy, median follow-up of 17.5 months, and median documented time to notice the abscopal effect was 2 months. This review systematically summarizes all clinical case reports of abscopal effect to gain insight into this uncommon but important phenomenon.