Response evaluation criteria in solid tumors

Last revised by Henry Knipe on 7 Nov 2023

Response evaluation criteria in solid tumors or RECIST refers to a set of published rules used to assess tumor burden in order to provide an objective assessment of response to therapy. They were initially introduced in 2000 with revision in 2009 (RECIST 1.1).

For the evaluation of tumors treated with immunotherapy, various rule sets have been proposed including the Immune Response Evaluation Criteria in Solid Tumors (iRECIST), the Immune-modified Response Evaluation Criteria in Solid Tumors (imRECIST), the Immune-related Response Evaluation Criteria in Solid Tumors (irRECIST); and the Immune-related Response Criteria (irRC) 1. Additionally, the Immunotherapy Response Assessment in Neuro-Oncology (iRANO) can be used for brain tumors treated with immunotherapy 1.

The RECIST criteria can be used with CT, MRI or conventional radiography (in some instances) 3.

RECIST terminology characterizes lesions as measurable vs non-measurable and target vs non-target. Measurable lesions are the ones that can be assessed quantitatively (see rules below). From among the measurable lesions, target lesions are selected (those followed through the patient's treatment course). Once a lesion is target, it is always target, even if it falls below the size limits for what is considered measurable at baseline.

Re-evaluation of a prescribed number of thus defined lesions over subsequent scans classifies the patient's disease burden as improving, stable or progressive.

  • measurable lesions: defined as lesions not under the non-measurable list below and having a longest diameter of:

    • ≥10 mm at spiral CT with 5 mm reconstructed section thickness in the axial plane (not the sagittal or coronal planes at CT or MR imaging)

    • ≥20 mm at non-spiral CT with 10 mm section thickness

    • ≥20 mm at chest radiography

  • non-measurable lesions 3 :

  • target lesions

    • measurable lesions, up to a maximum of 5 lesions per organ and 10 lesions total, representative of all involved organs, should be selected and identified as target lesions

    • should be selected on the basis of size (those with the longest diameter) and suitability for accurate repeated measurements

    • lesions in mobile organs (e.g. gastrointestinal tract, ovaries) might be inappropriate 

  • non-target lesions

    • can include both measurable and non-measurable lesions

    • measurable lesions that exceed the maximum acceptable number of target lesions (i.e. five in the same organ or 10 in the body) are thus included in the group referred to as non-target lesions

    • non-target lesions do not need to be measured; the choices for non-target lesions at follow-up timepoints are total disappearance (or resolution to normal size, for nodes), continued presence, and unequivocal progression

    • ideally, should include all lesions not chosen as target lesions

The assessment criteria are then used to essentially classify the disease into 3 categories:

  • response: improving disease

    • complete response: disappearance of all lesions

    • partial response: >30% decrease in sum of all target lesions in longest axis measurement

  • stable disease

  • progressive disease

The key changes in modifications in the revised criteria are 5:

  • a reduction in the maximum number of target lesions from ten to five, with a:

    • maximum of two per organ

    • longest diameter >10 mm

  • in lymph nodes (LNs) the short axis rather than the long axis should be measured, with:

    • normal: <10 mm

    • non-target LN: ≥10 mm but <15 mm and 

    • target LN : ≥15 mm

  • osteolytic lesions with a soft tissue component and cystic tumors may serve as target lesions; an additional requirement for progressive disease (PD) of target lesions is not only a ≥20% increase in the sum of the longest diameter (SLD) from the nadir but also a ≥5 mm absolute increase in the SLD (the other response categories of target lesion are unchanged)

  • progressive disease non-target lesions can only be applied if the increase in non-target lesions is representative of change in overall tumor burden; moreover PD on the basis of non-target lesions should only be assigned if the worsening of non-target lesions is sufficiently dramatic that it does not matter what the target lesions look like

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