Target Lesions
title: “Target Lesions” type: descriptor category: recist/descriptor tags: [recist, target-lesions, measurement, sum-of-diameters] sources:
- file: “recist_eisenhauer_2009.pdf” page: “232-235” date_created: 2025-04-15 date_modified: 2025-04-15 confidence: high anki_deck: “Radiology::RECIST” weak_area: false
Definition
Target lesions are a subset of measurable lesions selected to represent overall tumor burden. They are tracked quantitatively — their diameters are summed at every timepoint to assess response. The sum of diameters is the primary numerical metric in RECIST.
Selection Rules
Maximum Number
- 5 target lesions total (reduced from 10 in RECIST 1.0)
- Maximum 2 per organ (reduced from 5 per organ in RECIST 1.0)
- If a patient has measurable disease in >5 sites, prioritize the largest and most reproducible
Selection Criteria
Target lesions should be selected based on:
- Largest size — prioritize the biggest measurable lesions
- Suitability for accurate repeated measurements — lesions that can be reliably measured on serial scans
Two Lesions Per Organ Rule
For paired organs (lungs, kidneys, adrenals, breasts, ovaries), the limit is 2 per organ, not 2 total. “Organ” means the anatomic organ, not the paired structure. All mediastinal nodes count as one organ; all cervical nodes count as another.
Consistency Requirement
The same target lesions must be measured at every follow-up timepoint. You cannot swap in a different lesion if the original one grows or shrinks beyond usefulness.
What Gets Measured
Extra-nodal (Solid) Lesions
- Measure the longest diameter (LD) in the plane of measurement
- Record the single largest diameter — not an average or perpendicular measurement
Nodal Lesions
- Measure the short axis diameter (SAD) — the largest diameter perpendicular to the long axis
- SAD is used because it is the most reproducible nodal measurement (see Measurable Disease)
The Sum of Diameters
The sum of diameters (SoD) = sum of LD of all extra-nodal target lesions + sum of SAD of all nodal target lesions.
This sum is the surrogate for overall tumor burden and is tracked longitudinally.
| Lesion Type | What to Measure |
|---|---|
| Solid tumor (lung, liver, soft tissue) | Longest diameter |
| Lymph node (any station) | Short axis diameter |
| Mixed solid + nodal | Sum both using correct measurement per type |
Lesion Disappearance
If a target lesion disappears completely:
- Assign value of 0mm (not removed from the sum)
- The 0mm is included in all subsequent sums
- This matters: a lesion that vanishes then reappears has sum contribution of 0 → then its measurement re-enters the sum
Lesion Coalescence
When two target lesions fuse or coalesce:
- If a plane of separation exists between them, measure each lesion separately
- If truly fused with no separable plane, measure the longest diameter of the coalesced mass
- The coalesced measurement may underestimate true combined tumor burden — this is accepted per RECIST rules
Coalescing Node Pitfall
Lymph nodes that coalesce into a conglomerate can appear to enlarge on serial CT — potentially triggering false PD. Know this is expected behavior per RECIST. The coalesced measurement is used.
Lesion Splitting
If a target lesion splits into multiple discrete components:
- Measure each component separately
- The sum of the components replaces the original single measurement
- This ensures the sum continues to represent true tumor burden
Response Assessment — Target Lesions Only
| Response | Criterion |
|---|---|
| CR | All target lesions = 0mm (disappeared); all nodes <10mm SAD |
| PR | ≥30% decrease in sum vs. baseline |
| PD | ≥20% increase in sum vs. nadir AND ≥5mm absolute increase |
| SD | Neither PR nor PD |
NADIR vs. Baseline
PR is calculated against baseline sum. PD is calculated against nadir (smallest sum on study). These are different references. See Nadir Concept.
What Happens to Measurable Lesions NOT Selected as Targets?
They become non-target lesions — followed qualitatively (present/absent/unequivocal progression), not measured. See Non-target Lesions.
Post-Progression Considerations
After PD is declared (by target growth, non-target progression, or new lesion):
- Most protocols stop recording target lesion measurements
- If re-treatment with a new agent is planned, a new baseline and new target lesion selection is required
- The new target lesion set can be different from the original set