Response Evaluation Criteria in Solid Tumors (RECIST) is a set of standardized rules used to determine how effectively a cancer treatment is working against solid tumors. This system provides a common language for doctors and researchers worldwide to objectively measure changes in tumor size over time. Establishing a consistent, measurable method for assessing tumor burden is necessary for accurately comparing the effectiveness of different therapies across various clinical trials and institutions. RECIST focuses solely on changes in tumor dimensions, offering a clear, objective assessment that informs treatment decisions and drug development.
Why Standardization is Essential in Cancer Treatment
Before standardized response criteria, hospitals and research groups used varying methods to measure tumors. This lack of uniformity meant researchers might use different measurements or select varying numbers of lesions for monitoring. Consequently, it was nearly impossible to draw reliable conclusions about a new drug’s efficacy, slowing cancer research and complicating regulatory approval.
The introduction of the initial RECIST guidelines in 2000, and the subsequent revision to RECIST 1.1, created a single, internationally recognized framework. RECIST 1.1 ensures that data collected from patients globally can be accurately compared, allowing for robust meta-analyses and the rapid adoption of successful treatments. The strict rules governing lesion measurement provide a foundation of trust in oncology clinical trial results. This standardization is particularly important because clinical trial outcomes, such as objective response rate and progression-free survival, often serve as surrogate markers for overall survival in drug approval decisions.
The Mechanics of Tumor Measurement
RECIST 1.1 requires the selection of specific tumors to be tracked throughout treatment, categorizing them as “target lesions” or “non-target lesions.” Target lesions must be measurable, defined as having a longest diameter of at least 10 millimeters on a CT or MRI scan. Pathological lymph nodes must have a short axis of at least 15 millimeters. To represent the entire disease burden, a maximum of five target lesions can be selected in total, with no more than two lesions coming from any single organ.
These selected target lesions are measured at baseline, and their longest diameters are summed to create the “Sum of the Longest Diameters” (SLD). The SLD serves as the reference point for all future comparisons. All other sites of cancer, including smaller lesions or those with irregular shapes like ascites or pleural effusions, are classified as non-target lesions. While non-target lesions are not quantitatively measured, they are qualitatively assessed for presence, disappearance, or unequivocal progression, which influences the final overall response determination. The overall tumor response is primarily calculated based on the percentage change in the SLD compared to the original baseline measurement.
Interpreting the Response Categories
The RECIST system classifies a patient’s overall response to therapy based on the change in the SLD and the status of non-target lesions.
Complete Response (CR)
A Complete Response is the most favorable outcome, requiring the total disappearance of all target lesions and all non-target lesions. Any pathological lymph nodes must shrink to a normal size (less than 10 millimeters in short axis). This status indicates no radiologic evidence of cancer remaining.
Partial Response (PR)
Partial Response is achieved when the sum of the longest diameters decreases by at least 30 percent compared to the baseline SLD. This category demonstrates significant tumor shrinkage, often signaling that the current treatment regimen is effective. There must also be no evidence of progressive disease elsewhere. Both CR and PR are often grouped together to calculate the Objective Response Rate in clinical trials.
Stable Disease (SD)
Stable Disease is categorized when the tumor burden does not meet the criteria for a partial response or progressive disease. This classification means the SLD change is less than a 30 percent decrease but has not reached the threshold for progression. Stable disease can be a desirable outcome, particularly for slow-growing tumors, as it suggests the treatment is preventing the cancer from advancing.
Progressive Disease (PD)
Progressive Disease is declared if the SLD increases by at least 20 percent from the smallest sum recorded during the study, known as the nadir. This increase must also represent an absolute increase of at least 5 millimeters. The appearance of one or more new lesions automatically classifies the patient as having progressive disease, regardless of changes in the target lesions.
When RECIST Isn’t the Whole Story
RECIST 1.1 is effective for chemotherapy and targeted therapies that cause immediate tumor shrinkage, but its size-based measurement has limitations, particularly with modern immunotherapies. Treatments like immune checkpoint inhibitors activate the immune system, which can cause immune cells to infiltrate the tumor. This infiltration leads to temporary tumor swelling or inflammation before shrinkage occurs. RECIST may incorrectly categorize this initial swelling, known as “pseudo-progression,” as Progressive Disease, potentially leading to premature discontinuation of an effective treatment.
To address these complexities, alternative evaluation methods have been developed, such as the Immune-related Response Criteria (irRC). These criteria allow for continued treatment past an initial increase in tumor size if the patient is clinically stable. Furthermore, RECIST only measures size and cannot account for changes in tumor activity or cell death within a lesion. Supplementary imaging techniques like Positron Emission Tomography (PET) scans and the corresponding PERCIST criteria are sometimes used to assess metabolic activity. The overall assessment of a patient’s cancer status must always integrate the objective size measurements from RECIST with the patient’s clinical well-being and the specific mechanism of the treatment being used.