Response Evaluation Criteria In Solid tumors (RECIST) Criteria is a standard method to assess a patient’s tumor response to treatment by observing whether solid tumors reduce in size, remain unchanged, or increase. To apply RECIST, at least one tumor lesion must be detectable and measurable using radiography (i.e., X-rays, CT scans, MRI scans). Patients can have a complete response (CR), a partial response (PR), progressive disease (PD), and stable disease (SD). These data are used in clinical trials to understand the efficacy of drugs.