Purpose or Case Report: The Response Evaluation Criteria in Solid Tumors (RECIST, version 1.1) have gained widespread use in oncology clinical trials. RECIST 1.1 provides a standardized set of rules for response assessment in solid tumors using tumor shrinkage, based on standard imaging modalities. Standardized tumor response criteria are critically important in comparing results among clinical trials.

1) Review the steps in using the RECIST 1.1 guidelines
2) Present representative case examples

Baseline study: First step is identifying target (measurable) and non-target (non-measurable) lesions. Non-nodal target lesions need to be > 10 mm in long axis in axial plane only, while lymph nodes need to >15 mm in short axis. Target lesions include up to 2 measurable lesions per organ and 5 total lesions. Intravenous contrast is mandatory. CT is preferred imaging study but MRI can be substituted. All other lesions are categorized as non-target lesions. Finally, sum of the diameters for all target lesion is recorded.
Of note, bone lesions are not measurable, although an associated soft tissue mass is measurable. Sclerotic and lytic bone lesions are included as non-measurable disease. Nodes < 10 mm in short axis, pleural effusion, ascites and simple cysts are totally excluded from imaging response assessment.

Follow up studies: Sum of the diameters of all unequivocal target lesions is recorded and percent change from baseline or nadir is calculated. Non-target lesion status is recorded as absent, present or unequivocal progression. New lesions are also reported.
Overall response for target and nontarget lesions with or without new lesions is evaluated. RECIST 1.1 assigns four categories of response: complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD).

Future directions: Although increase in tumor size remains an important parameter for evaluating disease response, response may occur after an initial increase in tumor burden and regression of initial lesions may occur despite development of new lesions, especially with use of immunotherapy. This has led to new immunotherapy response criteria: Immune-related response criteria (irRC) and Immune-response RECIST (irRECIST).

Image-based outcome measures are commonly used to assess treatment response in clinical trials and have played a role in the regulatory drug approval of oncologic therapies. Therefore, it is imperative that radiologists understand the application of image-based response criteria.
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