Immunotherapy orchestrates radiotherapy in composing abscopal effects: A strategic review in metastatic head and neck cancer

The treatment of metastatic head and neck squamous cell carcinoma (HNSCC) with a combination of radiotherapy (RT) and immunotherapy can augment treatment response and symptomatic relief. Combination therapy can also trigger a non-targeted tumor control event called the abscopal effect. This effect can be demonstrated by treatment with anti-programmed death 1/programmed death ligand 1 (PD-L1) and anti-cytotoxic T-lymphocyte-associated antigen 4 antibodies in combination with hypofractionated RT. Individual studies and clinical trials have revealed that combination radio-immunotherapy improves overall treatment response by successful initiation of the abscopal effect, which extends the treatment effects to non-targeted lesions. Growing attention to the abscopal effect may inspire innovations in current RT toward more effective and less toxic radiobiological treatment modalities for advanced HNSCC. We review the latest findings on the abscopal effect with emphases on therapeutic modalities and potential applications for treating metastatic HNSCC.