iRECIST: A Case Based Users Guide for Radiologists

Immunotherapy with checkpoint inhibitors (ICI) has revolutionized oncology by stimulating the immune system to target cancer cells. While effective in treating various malignancies, ICI presents unique challenges in radiological response assessment. Traditional criteria, such as RECIST 1.1, were designed for cytotoxic chemotherapy and fail to account for pseudo-progression-an immune-related phenomenon where tumour size transiently increases due to immune cell infiltration before eventual shrinkage. This occurs in a minority of patients and can lead to misclassification of treatment response. To address this, new assessment criteria have been developed. The immune-related response criteria (irRC) introduced a delayed assessment of new lesions, followed by immune-related RECIST (irRECIST), which sought to align with RECIST 1.1. However, inconsistencies in its application led to the development of iRECIST in 2016, a standardized framework integrating RECIST 1.1 with immunotherapy-specific modifications. Despite its potential to become the gold standard, iRECIST is complex and challenging to implement consistently. This review outlines key differences between RECIST 1.1 and iRECIST, explains their necessity, and provides comprehensive flowcharts and graphical representations to aid interpretation. By addressing common clinical scenarios and frequently asked questions, this article aims to enhance understanding and application of iRECIST in clinical practice.