RECIST is a set of standardized guidelines used in oncology to determine how well a solid tumor cancer patient is responding to a specific treatment. The acronym stands for Response Evaluation Criteria in Solid Tumors, providing a uniform language for doctors and researchers. This framework allows for the objective evaluation of whether a tumor is shrinking, remaining stable, or growing larger over time. This standardization is primarily used in clinical trials to ensure that the results of new drug therapies are accurate and comparable across different studies and institutions.
The Purpose and Scope of RECIST
The purpose of the Response Evaluation Criteria in Solid Tumors is to bring consistency and objectivity to the assessment of treatment efficacy in cancer research. Before RECIST was widely adopted, different clinical centers often used their own methods for measuring tumor response, making it difficult to compare results across studies. This inconsistency challenged the determination of whether a new drug was truly effective.
RECIST addresses this by providing a universally accepted set of rules for measuring changes in tumor size on medical imaging. Its application focuses specifically on solid tumors, such as carcinomas or sarcomas. The criteria are invaluable for pharmaceutical companies, regulatory bodies, and academic researchers running clinical trials. Using these uniform guidelines helps to ensure that data supporting a new treatment’s effectiveness is robust and based on clear, verifiable measurements.
Identifying and Measuring Disease
The application of RECIST begins with selecting specific tumors to monitor, relying heavily on medical imaging like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). The lesions chosen for numerical tracking are called “Target Lesions.” These are typically the largest, most clearly measurable tumors, limited to five lesions total and no more than two per organ under RECIST 1.1.
For each Target Lesion, the longest diameter is measured. To establish the baseline, the longest diameters of all selected lesions are summed to create the “Sum of the Longest Diameters” (SLD). This single number represents the patient’s overall measurable tumor burden at the start of therapy. Subsequent imaging scans track how this total sum changes during treatment.
Areas of disease that cannot be reliably measured are classified as “Non-Target Lesions.” These are not tracked numerically but are monitored for their presence, absence, or “unequivocal progression.” This dual-tracking system ensures that while the bulk of the response is objectively quantified, the overall spread of the disease is also qualitatively accounted for.
Defining Treatment Outcomes
The core function of RECIST is to categorize the patient’s response into one of four distinct outcomes based on the change in the Sum of the Longest Diameters (SLD). These four categories are Complete Response (CR), Partial Response (PR), Stable Disease (SD), and Progressive Disease (PD). These objective definitions allow physicians and researchers to communicate the effect of a treatment with precision.
A patient achieves a Complete Response (CR) when all Target Lesions disappear entirely. Non-Target Lesions must also vanish, and pathological lymph nodes must shrink to a size considered normal, defined as having a short axis diameter of less than 10 millimeters. This indicates no detectable evidence of the disease remains in the tracked areas.
A Partial Response (PR) is defined as a significant reduction in the measurable tumor burden, requiring at least a 30% decrease in the SLD compared to the baseline measurement. For a PR to be officially declared, the reduction must be confirmed with a follow-up scan taken at a later date, typically at least four weeks after the initial measurement.
Progressive Disease (PD) is declared when the tumor burden increases. This requires a minimum of a 20% increase in the SLD, calculated using the smallest sum recorded since treatment began (the nadir). The absolute size increase must also be at least 5 millimeters, which prevents minor measurement fluctuations from incorrectly triggering a progression diagnosis. The appearance of any new lesion is also grounds for declaring Progressive Disease.
If the measured change does not meet the criteria for either a Partial Response or Progressive Disease, the patient’s result is categorized as Stable Disease (SD). This means the tumors have neither shrunk enough nor grown enough to change category. A period of stable disease can still be a clinically meaningful outcome, especially in the context of chronic or slow-growing cancers.
Evolution and Standardization
The original RECIST guidelines were introduced in 2000, but a major revision, RECIST 1.1, was published in 2009 to address ambiguities and improve the criteria’s performance. This update was based on extensive data analysis to refine the rules for better accuracy and reproducibility.
RECIST 1.1 reduced the maximum number of Target Lesions from ten down to five total, with a limit of two per organ. This adjustment was made because tracking fewer, well-chosen lesions provided a reliable representation of the overall tumor burden.
The 1.1 version also provided clearer rules for the assessment of lymph nodes. Nodes must now have a short axis of at least 15 millimeters to be considered measurable Target Lesions. Lymph nodes between 10 and 15 millimeters are considered pathological but are tracked as Non-Target Lesions. The updated criteria also clarified the definition of disease progression, requiring an absolute 5-millimeter increase in the sum of diameters in addition to the 20% relative increase.