Colorectal tumours

Colorectal tumours are very common. They are the bread and butter of GI pathology. Non-tumour colon is dealt with in the colon article.

An introduction to gastrointestinal pathology is in the gastrointestinal pathology article. The precursor lesion of colorectal carcinoma (CRC) is, typical, an adenomatous polyp. Polyps are discussed in the intestinal polyps article.

Classification

• Colon & rectum, most common --by far-- is adenocarcinoma.

Other tumours - many (incomplete list):

• Mucinous carcinoma.
• Adenosquamous carcinoma.
• Signet-ring carcinoma.
• Squamous carcinoma.
• Neuroendocrine neoplasms (carcinoid tumours).
• Lipoma.
• Leiomyoma.
• Gastrointestinal stromal tumour (GIST) - dealt with in a separate article.
• Angiosarcoma.
• Lymphoma (Non-Hodgkin's lymphoma).

Grading

• "Adenocarcinoma in situ" and "high-grade dysplasia" is used interchangeably by many in the colon and rectum.
  • Splitting hairs - adenocarcinoma in situ is invasion into the lamina propria, high-grade dysplasia does not have lamina propria invasion. Ergo, the difference (in my opinion) amounts to seeing a desmoplastic stroma (adenocarcinoma) or not seeing one (dysplasia).

Grading of tumours:

• Tis - in situ (intramucosal).
• T1 - into submucosa (through mucularis mucosae).
  • This is different than elsewhere, e.g. in the small bowel tumour cells in the lamina propria is defined as T1. The rationale for the T1 definition in CRC is that no lymphatics are present in the mucosa, ergo no risk of distant spread.
• T2 - into muscularis propria.
• T3 - into fat beyond musclaris propria.
• T4 - into something else.

Nodes:

• N0 - no positive nodes.
• N1 - 1-3 positive nodes.
• N2 - 4+ positive nodes.

Staging of colorectal cancer

Simple version

Tumour/node grade for stage:

• Stage I - T1 or T2 N0 M0.
• Stage II - T3 or T4 N0 M0.
• Stage III - Tx N1 or N2 M0.
• Stage IV - Tx Nx M1.

Complex version

Detailed tumour/node grade for stage:

• Stage I - T1 or T2.
• Stage IIA - T3.
• Stage IIB - T4.
• Stage IIIA - T1 N1 or T2 N1.
• Stage IIIB - T3 N1 or T4 N1.
• Stage IIIC - Tx N2.
• Stage IV - Tx Nx M1.

Pathogenesis

Colorectal carcinoma is thought to arise from one of two pathways:

1. APC (adenomatous polyposis coli) gene mutation pathway, AKA classic adenoma-carcinoma pathway.
2. Serrated pathway, AKA mutator pathway, mismatch repair pathway.
   • Associated with microsatellite instability (MSI).
   • Common associated gene mutations:
     1. MLH1.
     2. MSH2.
     3. MSH6.

MSI cancers:

• Location: left-sided predominance.
• Prognosis: slightly better than other CRC without MSI.
• Treatment implication: different response to chemotherapy.
• Histomorphologic features:
  • Lymphocytic infiltrate.
  • Poorly differentiated mucinous or signet ring morphology.

MSI classification

MSI associated cancers can be classified into:

• MSI-H.
• MSI-L.

Syndromes

• Lynch syndrome AKA hereditary non-polyposis colorectal cancer syndrome (HNPCC).
• Familial polyposis coli (FPC).

See also

• Colon.
• Gastrointestinal pathology.