Staging and Response Assessment
Staging systems
TNM describes anatomical extent: T for primary tumour size and invasion, N for regional nodes, M for distant metastasis. Prefixes carry meaning: c (clinical), p (pathological), y (after neoadjuvant therapy) and r (recurrent). Stage group is derived from TNM and carries prognostic meaning. Some diseases use non-TNM systems: Ann Arbor/Lugano for lymphoma, FIGO for gynaecological cancer, BCLC for hepatocellular carcinoma, ISS/R-ISS for myeloma.
RECIST 1.1 essentials
Measurable non-nodal lesions need a longest diameter of at least 10 mm on CT; lymph nodes are measurable when the short axis is at least 15 mm. Select up to two target lesions per organ and five in total; the baseline sum of diameters is the reference.
Complete response: disappearance of all target lesions with all nodes under 10 mm short axis. Partial response: at least 30% decrease from baseline sum. Progressive disease: at least 20% increase from the smallest sum on study AND an absolute increase of at least 5 mm, or any new lesion. Stable disease lies between partial response and progressive disease.
Worked example: baseline sum 100 mm, follow-up 68 mm gives a 32% decrease, so partial response. If baseline is 100 mm, nadir 60 mm and current 75 mm, the increase from nadir is 25% and 15 mm, so progressive disease. A rise from 12 mm to 15 mm is not progressive disease because the 5 mm absolute rule is not met.
Alternatives
iRECIST (unconfirmed progression), PERCIST (metabolic), RANO (brain), Lugano with the Deauville score (lymphoma), Choi criteria (GIST).
Traps
Progression is measured from the nadir, never from baseline, and any new lesion is progressive disease under RECIST 1.1.