Histological Type

Adenocarcinoma, no specific type (NST)

— diagnostic criteria are: (a) malignant epithelial changes, in (b) a desmoplastic stroma, with (c) invasion beneath the muscularis mucosae. In practice a combination of (a) and (b) is the most useful.

Mucinous carcinoma

— tumour area >50% mucinous component.

— worse prognosis (5-year survival decreased by 10-15%) compared with an equivalent-stage adenocarcinoma, NST, although this remains controversial.

Signet ring cell adenocarcinoma

— poor prognosis in the rectosigmoid of young or elderly people with a linitis plastica pattern of annular thickening and stenosis.

— distinguish from secondary carcinoma, e.g. gastric signet ring cell carcinoma in young females, prostate carcinoma [prostate-specific antigen (PSA) positive] in older males.

Others

— neuroendocrine carcinoma:

carcinoid/large cell/small cell (right colon, prognosis poor)

— adenocarcinoid:

composite adenocarcinoma and carcinoid

— adenosquamous carcinoma: caecum

— squamous cell carcinoma:

rectal: can be seen in ulcerative colitis/schistosomiasis/amoebiasis. Exclude spread from an anal carcinoma or cervical carcinoma. Need intercellular bridges ± keratinization with no gland/mucin formation

— undifferentiated carcinoma:

(a) good prognosis—medullary carcinoma. Circumscribed and expansile margin with solid sheets of tumour cells, intra- and peritumoral lymphocytes. Sporadic or in HNPCC and strongly associated with MSI-H (see below)

(b) poor prognosis—pleomorphic and diffusely infiltrative

— mixed differentiation:

e.g. adenocarcinoma (NST) with small cell carcinoma.

— metastatic carcinoma:

transcoelomic spread: stomach, ovary, endometrium, gut, pancreas direct spread: prostate, anus, cervix, kidney distant spread: breast (infiltrating lobular), malignant melanoma, lung Metastatic disease can infiltrate bowel wall and protrude into the mucosa mimicking endoscopically and macroscopically a primary lesion—a relevant previous history is crucial to diagnosis.

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