Abscopal Effect of Radiation Therapy and Signal Transduction

The abscopal effect is a potentially important phenomenon as a topic for basic research on tumor control and clinical oncology. Although it has been described in various malignancies, it is a rarely recognized clinical event. This phenomenon may arise mainly as a result of an activated immune system mediated through cytokines. Until recently, the abscopal effect referred to distant effects observed after local radiation therapy. However, some investigators argue that the term should now be used interchangeably with the “distant bystander effect.” From several aspects, including the distant bystander effects of other local therapies, we discuss the poorly researched but potentially intriguing abscopal effect that follows radiation therapy. Recently, experimentally-induced abscopal effects have been reported. Those reports show favorable inhibition of tumor growth not only at the irradiated site, but also at areas distant from the irradiated site using experimental protocols designed to induce reproducible abscopal effects. If consistent induction of the abscopal effect could be potentiated by intravenous administration of an immunostimulant, oncologists will seize upon the application of this effect for clinical use. Though the abscopal effect is still extremely controversial in view of the data now available, it can be hoped that translational research may offer a new concept for cancer therapy, namely, chemokine administration following local irradiation, leading to development of novel therapies for the treatment of advanced or metastatic cancer.