Choose up to five measurable target lesions (two per organ) at baseline, measure the same lesions the same way at every time point — longest diameter for lesions, short axis for nodes — and sum them; new lesions mean progression whatever the sum.
Orient first
- Measurable: ≥ 10 mm longest diameter on CT (slice ≤ 5 mm); nodes ≥ 15 mm short axis. Nodes 10–15 mm are non-target; < 10 mm is normal.
- Response: CR (disappearance, nodes < 10 mm), PR (≥ 30% decrease from baseline sum), PD (≥ 20% and ≥ 5 mm increase from the nadir, or new lesions), SD otherwise.
- Immunotherapy uses iRECIST (unconfirmed progression) — state the criteria set.
Acquire the study
- The same phase and slice thickness (≤ 5 mm) at every time point; portal venous phase CT chest, abdomen and pelvis as a rule.
The manoeuvre
- Baseline: choose reproducible, well-defined target lesions — not cavitating, not previously irradiated, not bone-only.
- Measure each target in the axial plane: longest diameter in mm for lesions, short axis in mm for nodes.
- Sum of diameters; at follow-up, compare with baseline for response and with the nadir for progression.
- Non-target lesions: present, absent, or unequivocal progression.
- New lesions: unequivocal — equivocal ones are followed, not scored.
What confirms it
- A response category with the baseline and nadir sums and the percentage change stated.
What licenses you to exclude it
- Not applicable — RECIST is a measurement framework; state when disease is non-measurable.
The classic misread
- Measuring a different lesion or axis at follow-up.
- Calling a new lesion on a different phase or slice thickness.
- Adding a node's longest diameter instead of its short axis.