[iRECIST: assessment of immunotherapy response].

[BACKGROUND] Immune checkpoint inhibitors (ICI) have revolutionized the treatment of numerous types of cancer but have also led to atypical response patterns such as pseudoprogression, hyperprogression, and mixed responses. These phenomena pose methodological challenges to the conventional RECIST 1.1 criteria (Response Evaluation Criteria in Solid Tumors), which were developed for cytotoxic therapies. In particular, pseudoprogression-defined as initial tumor enlargement followed by regression-may lead to misinterpretation and premature discontinuation of therapy.

[OBJECTIVES] To describe the rationale, structure, and clinical relevance of the iRECIST criteria introduced in 2017.

[METHODS] The clinical background necessitating adaptation of established response criteria is outlined, followed by a detailed explanation of response assessment using iRECIST. Finally, an overview of the current evidence on the utility of iRECIST for evaluating treatment response under immune checkpoint inhibition is provided.

[RESULTS] iRECIST is based on RECIST 1.1 principles but introduces the category of "unconfirmed progressive disease" (iUPD). In cases of new lesions or apparent progression, a follow-up scan after 4-8 weeks is recommended to distinguish between true progression (iCPD) and pseudoprogression. While one meta-analysis reported only minor differences in objective response rate and progression-free survival, other studies demonstrated marked differences across several outcome parameters, improved interreader reliability, and better detection of pseudoprogression when applying iRECIST.

[CONCLUSION] iRECIST provides adapted criteria for assessing treatment response under immune checkpoint inhibitor therapy. It enables differentiation between true progression and pseudoprogression, thereby preventing unwarranted treatment discontinuations. Prospective validation in larger cohorts remains desirable.