Cancer immunotherapy (also known as immuno-oncology), a promising anti-cancer strategy by harnessing
the body’s own immune system against cancer, has emerged as the “fifth therapeutic pilla” in the field of
cancer treatment since surgery, chemotherapy, radiation and targeted therapy. Clinical efficacy of several immunotherapies
has been demonstrated in clinical settings, however, only a small subset of patients exhibit dramatic
or durable responses, with the highest reported frequency about 10-40% from single-agent PD-L1/PD-1 inhibitors,
suggesting the urgent need of consistent objective response biomarkers for monitoring therapeutic response
accurately, predicting therapeutic efficacy and selecting responders. Key elements of therapeutic responses to
cancer immunotherapies contain the cancer cell response and the alternation of inherent immunological characteristics.
Here, we document the literature regarding imaging the key elements of therapeutic responses to cancer immunotherapies
using PET. We discussed PET imaging approaches according to different response mechanisms underlying
diverse immune-therapeutic categories, and also highlight the ongoing efforts to identify novel immunotherapeutic
PET imaging biomarkers. In this article, we show that PET imaging of the key elements of therapeutic
responses to cancer immunotherapies using PET can allow for more precise prediction, earlier therapy response
monitoring, and improved management. However, all of these strategies need more preclinical study and clinical
validation before further development as imaging indicators of the immune response.