Evaluation of response to immune checkpoint inhibitors: Is there a role for positron emission tomography?

Table 1 Key features of positron emission tomography Response Criteria in Solid Tumors, European Organization for Research and Treatment of Cancer 1999, Response Evaluation Criteria in Solid Tumors 1.1 and immune related Response Criteria

Category	PERCIST	EORTC 1999	RECIST 1.1	irRC
Target lesions	The hottest single tumor lesion (SUL peak) at baseline 18F-FDG PET	The most 18F-FDG-avid lesions (SUV BSA). Number of lesions not specified	Maximum, 5	Maximum, 15 lesions
New lesion	Results in progressive disease at first appearance	Results in progressive disease at first appearance	Results in progressive disease at first appearance	Up to 10 new visceral and 5 cutaneous lesions may be added to the sum of the products of the two largest perpendicular diameters of all index lesions at any time point
Complete response	CMR: Complete resolution of 18F-FDG uptake within the target lesion (< mean liver activity and indistinguishable from background/blood pool and no new 18F-FDG-avid lesions)	CMR: Complete absence of 18F-FDG uptake	Disappearance of all target and nontarget lesions Nodes must regress to < 10 mm short axis No new lesions Confirmation required
Partial response	PMR: A reduction of a minimum of 30% in the target tumor 18F- FDG SUL peak	PMR: A decrease in SUV > 25%	≥ 30% decrease in tumor burden compared to baseline Confirmation required	≥ 50% decrease in tumor burden compared with baseline1 Confirmation required
Progressive disease	PMD: A 30% increase in 18F-FDG SUL peak or advent of new 18F-FDG-avid lesions	PMD: An increase in SUV > 25% or appearance of new lesions	≥ 20% + 5 mm absolute increase in tumor burden compared with nadir Appearance of new lesions or progression of nontarget lesions	≥ 25% increase in tumor burden compared with baseline, nadir or reset baseline1 New lesions added to tumor burden Confirmation required
Stable disease	SMD: Disease other than CMR, PMR or PMD	SMD: Increase in SUV by < 25% or decrease in SUV by < 15%	Neither partial response nor progressive disease

¹If an increase in tumor burden is observed at the first scheduled assessment, the baseline is reset to the value observed at the first assessment. PERCIST: PET Response Criteria in Solid Tumors; EORTC: European Organization for Research and Treatment of Cancer; RECIST: Response Evaluation Criteria in Solid Tumors; irRC: Immune related Response Criteria; CMR: Complete metabolic response; PMR: Partial metabolic response; PMD: Progressive metabolic disease; SMD: Stable metabolic disease; SUL: SUV normalized to lean body mass; SUV BSA: SUV normalized for body surface area; SUV: Standardized uptake value.