What it is and why it matters
When you have surgery, the segment of colon or rectum with the tumour and the lymph nodes is sent to the pathology department. There it is examined under the microscope and a report is written that sets the definitive stage (hence the letter "p", for pathological: pT, pN) and the factors that decide whether chemotherapy is needed. You are entitled to a copy; ask for it if you have not been given one.
The staging codes: pT, pN, pM
- pT (tumour): how deeply it has penetrated the wall. T1 submucosa, T2 muscle layer, T3 through the muscle into the surrounding fat, T4a reaching the surface (serosa/peritoneum), T4b invading a neighbouring organ.
- pN (nodes): N0 none involved; N1a 1 node; N1b 2-3; N1c tumour deposits without involved nodes; N2a 4-6; N2b 7 or more. It usually appears as a fraction: "2/18" means 2 involved out of 18 examined.
- pM: metastases confirmed in the specimen (rare in the colon surgery report; metastases are assessed with imaging).
T and N give the stage: I (T1-2 N0), II (T3-4 N0), III (any T with N1-2). You can read what each implies in our guides on stage II and stage III.
Number of nodes examined
At least 12 should be examined. If there are fewer, the stage is less reliable and it counts as a risk factor in stage II. It does not depend only on the surgeon: also on anatomy, age and whether there was treatment before surgery (in rectal cancer fewer are commonly found).
Grade of differentiation
Describes how much the tumour cells resemble normal cells. Well or moderately differentiated (G1-G2, "low grade") is the most common and better behaved; poorly differentiated or undifferentiated (G3-G4, "high grade") is more aggressive. Note: in MSI-H tumours high grade does not carry the same negative meaning.
Margins
The edges of the specimen. R0 = clear margins, the tumour was removed whole. R1 = tumour cells at the edge (microscopic); R2 = visible tumour left behind. In rectal cancer the circumferential resection margin (CRM) is key: it is considered involved if the tumour is 1 mm or less from it.
Other factors you will read
| Term | What it means | Why it matters |
|---|---|---|
| Lymphovascular invasion (LVI) | Tumour cells inside blood or lymph vessels | Risk factor for relapse |
| Perineural invasion (PNI) | Cells around nerves | Risk factor |
| Tumour budding | Small clusters of cells at the invasive front (low/intermediate/high) | High = more risk |
| Tumour deposits | Nodules of tumour in the fat without node structure | If no nodes are involved, they make it N1c (stage III) |
| Perforation / obstruction | The tumour broke through or blocked the wall | Risk factor |
| Tumour regression grade (TRG) | In rectal cancer, how much tumour remains after chemoradiotherapy (0 = complete response) | Prognosis |
The molecular study
Sometimes it is in the same report and sometimes in a separate "molecular biology" report:
- MMR / MSI: by immunohistochemistry it appears as "preserved expression of MLH1, MSH2, MSH6 and PMS2" (= pMMR, stable) or "loss of expression of…" (= dMMR, MSI-H). If MLH1 is lost, MLH1 methylation or BRAF is usually added to indicate whether it is sporadic or Lynch syndrome. Read more in MSI-H / dMMR.
- KRAS / NRAS / BRAF: "wild-type" or "mutated" with the specific change (p.G12D, p.V600E…). Requested mainly in metastatic disease. See KRAS and BRAF.
- HER2: by immunohistochemistry (0, 1+, 2+, 3+) and, if 2+, by hybridisation (FISH/ISH).
A worked example
"Adenocarcinoma of the sigmoid colon, moderately differentiated, pT3 pN1b (2/16), clear margins (R0), lymphovascular invasion present, perineural absent, low budding. Preserved expression of MMR proteins." Translation: a tumour through the wall but not reaching the surface, 2 nodes involved out of 16, fully removed, with one risk factor (LVI), stable (MSS) → stage IIIB; adjuvant chemotherapy will be recommended.
What to ask at your appointment
- Can I have a copy of the full report, including the molecular study?
- What is my exact stage and which risk factors appear?
- Are the margins clear? (In rectal cancer: how wide is the circumferential margin?)
- Has MMR/MSI been tested? And RAS/BRAF/HER2 if relevant?
- Is there anything in the report that changes the treatment you had proposed?
