iRECIST (Immune-related RECIST)

iRECIST extends RECIST 1.1 for checkpoint-inhibitor and other immunotherapy trials. Target and non-target rules mirror RECIST 1.1 (unidimensional longest diameters; nodes by short axis), but progressive disease is split into iUPD (unconfirmed) and iCPD (confirmed). Pseudoprogression - transient enlargement or new lesions from immune-cell infiltration - must not automatically end treatment. After iUPD, if the patient is clinically stable, imaging is repeated at 4-8 weeks. iCPD is assigned only if there is further growth in the category that drove iUPD (target sum increase >=5 mm from the iUPD time point, any further non-target increase, new-lesion sum increase >=5 mm for measurable new targets, any increase in non-target new lesions, or appearance of additional new lesions) or if a new RECIST-defined progression category appears. If tumor burden subsequently shrinks to meet iCR/iPR/iSD, the bar resets and a fresh iUPD is required before iCPD can be declared. New lesions are recorded as a separate new-lesion target sum (NL-T, up to 5 lesions, <=2 per organ) plus qualitative NL-NT; they are never folded into the original baseline target sum (unlike irRC/irRECIST).