The phenomenon of pseudoprogression in cancer immunotherapy: is everything so unambiguous?

When evaluating the effect of therapy for malignant neoplasms with inhibitors of CTLA-4, PD-1 and PD-L1, the phenomenon of pseudoprogression may occur. Pseudoprogression is an increase in the volume of tumor tissue due to immunocompetent cells (lymphocytes, macrophages) mobilized into the tumor focus under the action of immunotherapy. As the antitumor effect of lymphocytes and macrophages is realized, the tumor decreases or disappears over time. Pseudoprogression occurs with varying frequency in various types of cancer. It may also matter which immune checkpoint inhibitors is used to treat a solid tumor or lymphoproliferative disease. Currently, several immune-related response-evaluation criteria have been developed, which can help diagnose the phenomenon of pseudoprogression. But, unfortunately, none of these criteria clearly distinguish pseudoprogression from true tumor progression. In the case of an erroneous judgment about the effect of treatment, immunotherapy ends, and the patient may not get a chance for long-term remission. Using two clinical examples (immunotherapy for metastatic kidney cancer and recurrent Hodgkin lymphoma), the authors discuss the pitfalls of evaluating the effectiveness of treatment with checkpoint inhibitors.