Rules for selecting target lesions
For accurate assessment, lesions must be properly selected prior to the start of therapy. The main rule is that lesions must be clearly measurable.
- Solid organs: Lesions are selected whose largest dimension exceeds 1 cm.
- Lymph nodes: Measurement is performed strictly along the short axis (shortest dimension). A node may be included in the target list only if its short axis exceeds 1.5 cm.
- Quantitative limits: A total of no more than 5 target lesions can be selected per patient. Furthermore, a maximum of 2 lesions should be attributed to a single organ.
All other manifestations of the disease (if present) are recorded as non-target. They are also evaluated dynamically, but are not summed in the mathematical calculation.
Categories of response to therapy
After completing treatment cycles, a follow-up study is performed, and the new dimensions are compared with the baseline. There are four main response categories:
- Complete Response (CR, Complete Response): Complete disappearance of all target lesions.
- Partial Response (PR, Partial Response): The sum of the longest diameters of the target lesions decreases by at least 30% from the baseline value. Example: in renal cell carcinoma, the lesion size decreased from 85 mm to 53 mm (by 37%) — this is classified as PR.
- Progressive Disease (PD, Progressive Disease): An increase in the sum of the longest diameters by at least 20%, or the appearance of entirely new tumor nodules.
- Stable Disease (SD, Stable Disease): An intermediate state where changes in size do not meet the criteria for either a partial response or progression.
This objective categorization allows for direct comparison of measurement results across different patients and clinics.
Limitations of standard criteria
Despite their widespread use, classical RECIST criteria have a number of significant limitations that the radiologist must keep in mind:
- Subjectivity (observer variability). The selection of specific lesions for measurement remains the responsibility of the investigator. Different specialists may choose different nodes, which is why clinical trials involve an independent review committee.
- Ignoring structure. The standard algorithm does not account for qualitative changes within the tumor. For example, massive necrosis or cyst formation may begin during treatment — the tumor is dying, but its physical size remains unchanged for some time.
- Residual masses (post-treatment residual masses). Following successful treatment of solid tumors (e.g., in lung cancer or lymphomas), pathologically altered tissue often remains on imaging. Conventional imaging is unreliable for accurately predicting the pathomorphological nature of a residual mass: it is impossible to distinguish a fibrous scar from viable tumor based on size alone.
Modified evaluation systems
Due to the aforementioned limitations, the standard approach is regularly adapted for specific clinical tasks and new types of therapy:
- Choi criteria (Choi criteria). Developed for gastrointestinal stromal tumors (GISTs) and modified for renal cell carcinoma. They take into account not only linear size, but also the intensity of contrast enhancement.
- Immune-response criteria (immune-response criteria). The introduction of immunotherapy required a change in the rules. These criteria account for the phenomenon of delay in response and pseudoprogression, where the tumor temporarily increases in size due to infiltration by immune cells.
- The role of 18F-FDG PET/CT. Molecular imaging methods are often used to detect residual disease amidst post-therapeutic changes. PET/CT has high sensitivity in assessing metabolic activity (especially in lymphomas and head and neck cancer), helping to select patients for repeat biopsy or adjuvant radiotherapy.
Briefly: answers to topical questions
How is the dynamics of non-target lesions evaluated according to RECIST criteria?
The dynamics of non-target (additional) lesions are evaluated based on the achievement of complete response or disease progression.
- Complete Response (Complete Response - CR) — disappearance of all additional lesions, normalization of tumor marker levels, and reduction of all lymph nodes to a short-axis diameter of less than 10 mm.
- Progressive Disease (Progressive Disease - PD) — unequivocal progression of additional lesions or the appearance of one or more new lesions.
What are the requirements for CT slice thickness for accurate measurement of lesions according to RECIST 1.1?
For accurate lesion measurement on CT data, slice thickness should be no more than 5 mm. With this slice thickness, the minimum lesion size is 10 mm. Lesions must be precisely measured in at least one dimension; the largest diameter in the measurement plane is recorded.
Remember the abbreviations: CR (Complete) = 100% disappearance, PR (Partial) = minus 30%, PD (Progressive) = plus 20% or new lesions. Everything in between is SD (Stable).
Class and exam questions
Why are lymph nodes measured along the short axis rather than the long axis?
Reactive (benign) node enlargement often occurs along the long axis. Neoplastic involvement makes the node more spherical, specifically increasing the short axis. Therefore, a short-axis diameter greater than 1.5 cm is important for targeted nodes.
Can a lesion be considered progressive (PD) if its size has not increased?
Yes. Disease progression status is established not only when the sum of diameters of existing lesions increases by 20%, but also upon the appearance of any new metastatic lesions.
How to distinguish residual tumor from a post-therapeutic scar?
Conventional CT is unreliable here. 18F-FDG PET/CT is often used to assess the metabolic activity of the tissue. However, a repeat biopsy is frequently required for definitive diagnosis.
What else to study on this topic
- Choi criteria for gastrointestinal stromal tumors
- Pseudoprogression in immunotherapy: imaging signs
- The role of 18F-FDG PET/CT in assessing residual masses
- Radiation pneumonitis and organizing pneumonia
- Specific pulmonary toxicity of chemotherapeutic agents (bleomycin, methotrexate)
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