The abscopal effect is a hypothesis in the treatment of

metastatic cancer Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then, ...

whereby shrinkage of untreated tumors occurs concurrently with shrinkage of tumors within the scope of the localized treatment. R.H. Mole proposed the term “abscopal” (‘ab’ - away from, ‘scopus’ - target) in 1953 to refer to effects of ionizing radiation “at a distance from the irradiated volume but within the same organism.”Mole, R. H. "Whole body irradiation—radiobiology or medicine? The British Journal of Radiology 26.305 (1953): 234-241. Initially associated with single-tumor, localized radiation therapy, the term “abscopal effect” has also come to encompass other types of localized treatments such as electroporation and intra-tumoral injection of therapeutics.Fend L, Yamazaki T, Remy C, et al. Immune Checkpoint Blockade, Immunogenic Chemotherapy or IFN-α Blockade Boost the Local and Abscopal Effects of Oncolytic Virotherapy. Cancer Res. 2017;77:4146-4157. However, the term should only be used when truly local treatments result in systemic effects. For instance, chemotherapeutics commonly circulate through the blood stream and therefore exclude the possibility of any abscopal response. The mediators of the abscopal effect of

radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radi ...

were unknown for decades. In 2004, it was postulated for the first time that the immune system might be responsible for these “off-target” anti-tumor effects.Demaria S, Ng B, Devitt ML, et al. Ionizing radiation inhibition of distant untreated tumors (abscopal effect) is immune mediated. Int J Radiat Oncol Biol Phys. 2004;58:862‐870. Various studies in animal models of melanoma,Twyman-Saint Victor C, Rech AJ, Maity A, et al. Radiation and dual checkpoint blockade activate non-redundant immune mechanisms in cancer. Nature. 2015;520:373-377.Rodriguez-Ruiz ME, Rodriguez I, Garasa S, et al. Abscopal effects of radiotherapy are enhanced by combined immunostimulatory mabs and are dependent on CD8 T cells and crosspriming. Cancer Res. 2016;76:5994-6005. mammary,Demaria S, Kawashima N, Yang AM, et al. “Immune-mediated inhibition of metastases after treatment with local radiation and CTLA-4 blockade in a mouse model of breast cancer. Clin Cancer Res. 2005;11:728‐734. and colorectal tumorsDovedi SJ, Cheadle EJ, Popple A, et al. “Fractionated radiation therapy stimulates anti-tumor immunity mediated by both resident and infiltrating polyclonal T-cell populations when combined with PD1 blockade. Clin Cancer Res. 2017; 23(18); 5514-26. have substantiated this hypothesis. Abscopal effects of Targeted intraoperative radiotherapy have been seen in clinical studies, including in randomized trials where women treated with lumpectomy for breast cancer combined with whole breast radiotherapy showed reduced mortality from non-breast-cancer causes when compared with whole breast radiotherapy. Furthermore, immune-mediated abscopal effects were also described in patients with metastatic cancer. Whereas these reports were extremely rare throughout the 20th century, the clinical use of immune checkpoint blocking antibodies such as

ipilimumab Ipilimumab, sold under the brand name Yervoy, is a monoclonal antibody medication that works to activate the immune system by targeting CTLA-4, a protein receptor that downregulates the immune system. Cytotoxic T lymphocytes (CTLs) can recogniz ...

pembrolizumab Pembrolizumab, sold under the brand name Keytruda, is a humanized antibody used in cancer immunotherapy that treats melanoma, lung cancer, head and neck cancer, Hodgkin lymphoma, stomach cancer, cervical cancer, and certain types of breast can ...

has greatly increased the number of abscopally responding patients in selected groups of patients such as those with metastatic melanoma.Hiniker SM, Reddy SA, Maecker HT, et al. A prospective clinical trial combining radiation therapy with systemic immunotherapy in metastatic melanoma. Int J Radiat Oncol Biol Phys. 2016;96:578-588.

Mechanisms

Similar to immune reactions against

antigens In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

from bacteria or viruses, the abscopal effect requires

priming Priming may refer to: * Priming (agriculture), a form of seed planting preparation, in which seeds are soaked before planting * Priming (immunology), a process occurring when a specific antigen is presented to naive lymphocytes causing them to ...

of immune cells against tumor antigens. Local irradiation of a tumor nodule may lead to immunogenic forms of tumor cell death and liberation of tumor cell-derived antigens. These antigens can be recognized and processed by

antigen-presenting cells An antigen-presenting cell (APC) or accessory cell is a cell that displays antigen bound by major histocompatibility complex (MHC) proteins on its surface; this process is known as antigen presentation. T cells may recognize these complexes usin ...

within the tumor (

dendritic cells Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. The ...

and

macrophages Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cell ...

Cytotoxic T cells A cytotoxic T cell (also known as TC, cytotoxic T lymphocyte, CTL, T-killer cell, cytolytic T cell, CD8+ T-cell or killer T cell) is a T lymphocyte (a type of white blood cell) that kills cancer cells, cells that are infected by intracellular pa ...

which recognize these tumor antigens may in turn be primed by the tumor antigen-presenting cells. In contrast to the local effect of irradiation on the tumor cells, these cytotoxic T cells circulate through the blood stream and are thus able to destroy remaining tumor cells in distant parts of the body which were not irradiated. Accordingly, increases in tumor-specific cytotoxic T cells were shown to correlate with abscopal anti-tumor responses in patients. Vice versa, the abscopal effect is abolished after experimental depletion of T cells in various animal models.Vanpouille-Box C, Diamond JM, Pilones KA, et al. TGFbeta is a master regulator of radiation therapy-induced antitumor immunity. Cancer Res. 2015;75:2232-2242. Abscopal effects of ionizing radiation are often blocked by the immunosuppressive microenvironment inside the irradiated tumor which prevents effective T cell priming. This explains why the effect is so rarely seen in patients receiving radiotherapy alone. In contrast, the combination of immunomodulatory drugs such as ipilimumab and pembrolizumab can partially reconstitute systemic anti-tumor immune reactions induced after local tumor radiotherapy. The optimal combination of radiation dose and fractionation with immunomodulatory drugs is currently under intensive investigation. In this context, it was proposed that radiation doses above 10 to 12 Gray might be ineffective in inducing immunogenic forms of cell death.Vanpouille-Box C, Alard A, Aryankalayil MJ, et al. DNA exonuclease Trex1 regulates radiotherapy-induced tumour immunogenicity. Nat Commun. 2017 Jun 9;8:15618. However, there is so far no consensus on the optimal radiation regimen needed to increase the chance of abscopal tumor regression.

References

{{Radiation oncology Cancer treatments Radiation therapy Immune system Medical treatments