Turning scans into a standardized response category
RECIST (Response Evaluation Criteria in Solid Tumors) gives oncologists a consistent way to call whether a tumor is shrinking, stable, or growing on imaging. A few measurable target lesions are selected at baseline and their diameters summed. At each follow-up scan the same lesions are remeasured, and the change defines the category: complete response (all target lesions gone), partial response (a substantial decrease), progressive disease (a substantial increase or a new lesion), or stable disease (in between).
Standardizing these thresholds matters most in clinical trials, where response rates are compared across drugs and centers. Without a shared rule, one radiologist's improvement could be another's stable disease.
Where the criteria fall short
Size-based criteria do not always capture how modern therapies work. Immunotherapy can cause a tumor to swell transiently with immune cells before shrinking, a pattern that simple size rules would misread as progression, which led to immune-specific adaptations of the criteria. RECIST is a practical convention, not a biological truth, and is one input among several an oncologist weighs.
Applying the RECIST thresholds
This is general educational information, not medical advice. Response assessment is performed by clinicians integrating imaging, symptoms, and other data, and the categories here are a simplified summary.