[Comparison of RECIST 1.1 and iRECIST for Response Evaluation in Solid Tumours]

Immunotherapy is a relatively new and developing modality in oncological treatment, which may significantly improve treatment results for some patients with malignant tumors. With the increasing number of clinical trials, the demand for a suitable tool to assess and compare treatment responses is growing. Currently, the most common response assessment system for solid tumors is RECIST (response Evaluation Criteria in Solid Tumors) version 1.1. However, in immuno-oncology, a small percentage of patients manifest a new response pattern termed pseudoprogression, in which, after the initial increase in tumor burden or after the discovery of new lesions, a response or at least a prolonged stabilization of the disease can occur. This patient group would be included in the progression category when using RECIST 1.1 and effective treatment would be discontinued. Therefore, iRECIST criteria were established to capture the phenomenon of pseudoprogression, the need for PD confirmation (according to RECIST 1.1) was introduced, and changes were made in the evaluation of new lesions.

The present work introduces criteria for the evaluation of oncological responses in solid tumors using RECIST version 1.1 and iRECIST immunotherapy variant (including a brief overview of previous immune criteria). These criteria are compared in an immuno-oncological context and their potential pitfalls are discussed.

iRECIST criteria were established by expert consensus; however, sufficient data for final validation has not yet been collected. As a result, RECIST 1.1 should be the primary assessment system in immuno-oncology. The use of iRECIST should be reserved for research purposes (testing and validation). Distinguishing pseudoprogression from true PD in patients treated with immunotherapy remains a major challenge in oncological imaging.Key words: RECIST - response criteria - immunotherapy - measurement - tumor burden The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study. The Editorial Board declares that the manuscript met the ICMJE recommendation for biomedical papers.Submitted: 24. 9. 2017Accepted: 3. 10. 2017.