Response Evaluation Criteria in Solid Tumors (RECIST) are a set of published rules used to provide an objective measurement of tumor burden in response to conventional systemic therapy. They were introduced in 2000 (RECIST 1.0), with the latest revision in 2009 (RECIST 1.1).
Imaging findings of tumors treated with immunotherapy are distinct from those treated with conventional systemic therapy and various rule sets have been published, see 1,9-12:
Immune-related Response Criteria (irRC): introduced in 2009
Immune-related Response Evaluation Criteria in Solid Tumors (irRECIST): introduced in 2013
Immune Response Evaluation Criteria in Solid Tumors (iRECIST): introduced in 2017
Immune-modified Response Evaluation Criteria in Solid Tumors (imRECIST): introduced in 2018
Immunotherapy Response Assessment in Neuro-Oncology (iRANO): for assessment of brain tumors treated with immunotherapy, introduced in 2015
The RECIST criteria can be used with CT, MRI or conventional radiography (in some instances) 3,8.
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Terminology
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.
Measurement
Measurable vs non-measurable
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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
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non-measurable lesions 8
small lesions (<10 mm)
inflammatory breast disease
lymphangitis cutis or pulmonis
abdominal masses that are not confirmed and followed up with imaging techniques
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special considerations 8
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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
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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
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lesions treated with prior local therapy
not usually considered measurable, unless there has been unequivocal progression
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Measurement methods
the same imaging modality should be used to characterize 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
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target lesions
chosen 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, with no more than 2 of the same organ (e.g. if a patient has two organs involved, then a maximum of 4 target lesions can be chosen). The chosen lesions should be representative of all the involved organs.
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 (SOD) of all target lesions should be reported on the initial and all follow-up studies. The initial SOD is recorded as the baseline sum diameters (BSD), which is compared to on subsequent studies to determine response criteria (see below).
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non-target lesions
can include both non-measurable or measurable disease, although measurements are not required and can be reported as "present", "absent", or "unequivocal progression"
all other lesions (or sites of disease), including pathological lymph nodes should ideally be recorded
multiple non-target lesions of the same organ can be recorded together (e.g. "multiple liver metastases")
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special considerations
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target lesions becoming "non-measurable":
target lesions must be recorded at each follow-up regardless of size
if too small to measure, and the reporting radiologist thinks the lesion has disappeared, then 0 mm should be recorded
if too small to measure, but the lesion is faintly seen, then 5 mm should be recorded
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lesions that split or coalesce:
a target lesion that has fragmented into smaller lesions on follow-up should have the longest diameter of each lesion measured and summed together
target lesions that have coalesced should be measured at the longest diameter
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Response criteria
Based on assessment of target and non-target lesions, the disease can be classified as 8:
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complete response (CR):
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requires all of:
disappearance of all target and non-target lesions
pathological lymph nodes must have reduced to <10 mm in short axis
no new lesions
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partial response (PR):
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requires all of:
at least 30% decrease in SOD of target lesions compared to baseline sum diameters (BSD)
non-progressive disease of non-target lesions
no new lesions
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progressive disease (PD):
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either one of:
any new lesions
at least 20% relative and 5 mm absolute increase of SOD of target lesions compared to smallest SOD ever recorded for the patient
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stable disease (SD):
not meeting criteria for PD or PR
For detailed changes on the most recent criteria, see: RECIST 1.1: comparison with RECIST 1.0.