Response evaluation criteria in solid tumors

Response evaluation criteria in solid tumours or RECIST refers to a set of published rules used to assess tumour 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 tumours treated with immunotherapy, various rule sets have been proposed including the Immune Response Evaluation Criteria in Solid Tumours (iRECIST), the Immune-modified Response Evaluation Criteria in Solid Tumours (imRECIST), the Immune-related Response Evaluation Criteria in Solid Tumours (irRECIST); and the Immune-related Response Criteria (irRC) ¹. Additionally, the Immunotherapy Response Assessment in Neuro-Oncology (iRANO) can be used for brain tumours treated with immunotherapy ¹.

The RECIST criteria can be used with CT, MRI or conventional radiography (in some instances) ^3,8.

Terminology

RECIST terminology characterises 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.

Measurement

Measurable vs non-measurable 

• measurable lesions ⁸: 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). Exception are lymph nodes when used as target lesions, which must be ≥15 in short axis.

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

  • ≥20 mm at chest radiography

• non-measurable lesions ⁸:

  • small lesions (<10 mm)

  • leptomeningeal disease

  • ascites

  • pleural effusions

  • pericardial effusion

  • inflammatory breast disease

  • lymphangitis cutis or pulmonis

  • abdominal masses that are not confirmed and followed up with imaging techniques

• special considerations ⁸:

  • bone metastases

    • bone scintigraphy, FDG-PET and plain films are not suitable for accurate measurement

    • purely sclerotic metastases are non-measurable

    • lytic or mixed lesions containing a soft tissue component meeting measurable criteria may be used

  • cystic lesions

    • simple cysts should not be considered malignant (measurable or non-measurable)

    • cystic lesions thought to represent metastases, meeting measurable criteria may be used, but non-cystic lesions are preferred as target lesions

  • lesions treated with prior local therapy

    • not usually considered measurable, unless there has been unequivocal progression

Measurement methods

• the same imaging modality should be used to characterise and compare lesions identified at baseline and follow-up

• CT is the preferred method due to its availability and reproducibility

• MRI may be used with discretion due to variable acquisition methods impacting image quality, slice thickness and lesion conspicuity

• chest x-ray lesions may be considered measurable if they are clearly defined and surrounded by aerated lungs, although CT is preferred

• ultrasound is not considered adequately reproducible or reliable

Target vs non-target

• target lesions

  • measurablechosen at baseline and always remain as target lesions. They should be reported on all subsequent imaging, even if they become non-measurable

  • up to a maximum of 5 measurable lesions perchosen, with no more than 2 of the same organ and 10(e.g. if a patient has two organs involved, then a maximum of 4 target lesions total,can be chosen). The chosen lesions should be representative of all the 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

  • record the longest axial diameter of the lesion, unless the target is a lymph node, in which the short axis is used

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

  • sum of diameters (SLD) of all target lesions should be reported, which will be used to assess disease response (see classification of disease below)

• non-target lesions

  • can include both non-measurable or measurable disease, although measurements are not required and non-measurable lesionscan be reported as "present", "absent", or "unequivocal progression"

  • measurableall other lesions that exceed the maximum acceptable number(or sites of targetdisease), including pathological lymph nodes should ideally be recorded

  • multiple non-target lesions (i.e. five inof 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 tocan be measured; the choices for non-target lesions at follow-up timepoints are total disappearance (or resolution to normal size, for nodesrecorded together (e.g. "multiple liver metastases"), continued presence, and unequivocal progression

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

Classification of disease

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 tumours 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 tumour 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

See also

• WHO criteria in tumour response

• RECIST 1.1: comparison with RECIST 1.0

• Choi Response Criteria

• Lugano classification: response evaluation criteria for CT

• Lugano classification: response evaluation criteria for PET-CT

• PERCIST 1.0