4–6 This is especially pertinent with the growing literature and use of immune checkpoint inhibitors (ICI), including anti-CTLA-4, Programmed-Death 1 (PD-1) and Programmed-Death Ligand 1 (PD-L1) antibodies in pre-clinical models and early clinical trials of primary and metastatic brain tumors, and the proposed immune-mediated synergistic effect of radiation and checkpoint inhibitor therapy. This phenomenon is of particular importance to the neurosurgical and neuro-oncology community as there have been multiple reports of the abscopal effect occurring following radiation treatment for metastatic disease to the brain. 1–3 While not completely understood, the abscopal effect is likely an immune-mediated mechanism where the tumor microenvironment is disrupted by local radiation, allowing improved antigen presentation with subsequent anti-tumor activity at distant sites. This effect was first described in the 1950s and has since been reported in numerous case studies. The abscopal effect is an oncologic phenomenon in which radiation to one tumor site causes tumor regression in a distant, nonirradiated location. Four papers investigated incidence of abscopal effects in patients with metastatic melanoma to the brain who received immune checkpoint inhibitor therapy ( n = 160) two papers found an abscopal effect in 35% and 52% of patients ( n = 16, 21 respectively), and two papers found no evidence of abscopal effects ( n = 61, 62).Ībscopal effect, brain metastasis, local radiation with distant anti-tumor effect, neuro-oncology, spine metastasis

Median progression-free survival was 13 months following radiation. Ten case studies (57%) showed complete resolution of extra-CNS tumor burden. Most common sites of extracranial tumor reduction were lung and lymph nodes.

One patient had radiation to the body and an intracranial abscopal effect was observed.

Patients undergoing whole brain radiotherapy (6) had an average dose of 33.6 Gy over 8–15 fractions, and those undergoing stereotactic radiosurgery (5) had an average dose of 21.5 Gy over 1–5 fractions. Eleven patients had irradiation to the brain and 2 to the spine. Case reports of the abscopal effect were in relation to the following cancers: melanoma (6 patients), breast cancer (3), lung adenocarcinoma (2), non-small-cell lung cancer (2), hepatocellular carcinoma (1), and renal cell carcinoma (1).

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