The elusive abscopal effect after radiotherapy: A systematic review and meta-analysis of the published literature

Background The abscopal effect is a phenomenon where a tumor that is not the intended site of treatment also shrinks in response to therapy; and is one of the most coveted and rare effects of cancer therapies. Previous attempts to summarize literature on the abscopal effect have focused on specific disease sites or been done on a limited basis. We performed a comprehensive review of every publication and case report regarding the abscopal effect induced by radiotherapy from the 1950s to February 2023 to identify potential trends and patterns in its induction based on specific disease/ treatment site, radiation dose, and other factors. Methods Literature that described the abscopal effect was identified through search of online databases: Pubmed, Medline, and Google Scholar for all published articles from 1953 leading to February of 2023 that included the terms “Abscopal Effect” and “Radiation” or “Radiotherapy.” Demographics (patient and tumor characteristics), radiation and abscopal effect analysis, outcome and treatment analysis, and risk of bias were reported and summarized. Results A final total of 92 papers corresponding to 99 separate lesions or cases were included in the meta-analysis. The most common site of induction of the abscopal effect and of the target of the abscopal effect was the lung. One manuscript reported a nonstandard treatment protocol, the remainder of cases received radiation within reasonable treatment guidelines. The majority of manuscripts included most important information (cancer site, number of prior treatments, histopathology, outcome information, and dose). Conclusion Additional research is urgently needed to understand how and why some treatment regimes are able to induce the abscopal effect. For the time being, this remains a rare phenomenon.