| In recent years, immunotherapy has revolutionized the treatment of cancer. Immunotherapy has been used for decades (vaccines, interferon, high-dose interleukin 2), but only since the introduction of checkpoint inhibitors has it had a significant impact on the survival of patients with a variety of cancers. Currently, checkpoint inhibitors involving two specific targets are approved by the US Food and Drug Administration: cytotoxic T-cell lymphocyte-4–blocking antibodies (ipilimumab, approved by the US Food and Drug Administration for patients with melanoma) and programmed cell death protein 1/programmed death ligand 1–blocking antibodies (atezolizumab, avelumab, durvalumab, nivolumab, and pembrolizumab for patients with melanoma, renal cell carcinoma, non–small-cell lung cancer, head and neck cancer, bladder cancer, Merkel cell carcinoma, Hodgkin lymphoma, and others). As clinical trials with checkpoint inhibitors were ongoing, it was noted that occasionally atypical patterns of responses (a transient increase in the size of lesions or even appearance of new lesions) were seen, and therefore the most commonly used response criteria, namely Response Evaluation Criteria in Solid Tumors (RECIST) v1.1, may not capture all patients who received benefit from these therapies. |