TNM staging
In one sentence
TNM staging describes the primary tumor, regional lymph nodes and distant metastasis using a cancer-specific classification.
The intuition
A stage is a map of extent, while grade describes how cells look. The letters keep three locations separate: the original tumor, nearby nodes and distant sites. Their combinations and prefixes must be interpreted under the system used.
How it works
T describes size or local extension, N describes regional nodes, and M describes distant spread. Clinical c staging uses examination, imaging and biopsy before definitive treatment. Pathologic p staging uses surgical findings. The y prefix denotes assessment after treatment: ypT and ypN describe residual findings after neoadjuvant therapy.
For breast cancer, T3 describes a tumor larger than five centimeters. T4 concerns specified chest-wall or skin involvement; size alone does not make T4. N categories have clinical and pathologic definitions. Clinical N3b can apply when internal mammary and axillary nodes are clinically involved. It does not simply mean ten positive surgical nodes.
Anatomic stage groups differ from prognostic groups incorporating grade and receptor information. An overall group should come from the correct staging rules, rather than mechanically combining a pretreatment clinical T with post-treatment pathologic N.
Regional-node involvement is not the same as distant metastasis. A ypN-positive result can coexist with no known distant disease. Imaging suspicion, biopsy confirmation and pathologic findings are different evidence levels. Preserve those qualifiers when summarizing a record.
Why it matters in cancer
It keeps pretreatment extent, post-treatment pathology and distant disease separate when reading a cancer record.
Worked example
A breast tumor larger than five centimeters without T4 skin or chest-wall criteria is cT3. Suspicious internal mammary nodes can change the working N assumption when considered clinically involved. After treatment, ypT2 ypN1a is a new residual description; it does not erase the baseline map.
Common confusions
- Stage is not grade, RCB (residual cancer burden) class or a molecular subtype.
- Clinical and post-treatment pathologic prefixes are not interchangeable.
- Stage IIIB requires the relevant T4 criteria; cT3 is not enough.
Related concepts
RCB and pathology guide.
Sources and scope
General teaching, source-checked October 8, 2026. The examples are hypothetical unless a study is explicitly named. Expert and learner review remain pending.