Other Malignancy

Gastrointestinal mesenchymal and stromal tumours

— Smooth muscle tumours and GISTs (rare). See also Chapters 2 and 4.

Colon

— benign appearing lesions are rare.

— usually aggressive with metastases and death related to: mitoses >6/50 hpfs, infiltrative growth pattern into the muscularis propria, mucosal invasion, cellularity, coagulative necrosis.

Anorectal

— lesions arising from the muscularis mucosae (i.e. submucosal leiomy-omatous polyp) are usually treatable by local excision or polypectomy.

— lesions arising from the muscularis mucosae are considered malignant if: cellular, >5 cm diameter, infiltrative into the muscularis propria. However, if originating in the muscularis propria even bland lesions (sparse cellularity, 0-1 mitoses/50hpfs) need long-term follow-up, as there is a tendency for local recurrence and even potential metastases.

Carcinoid tumour

— (a) chronic ulcerative colitis ^ enteroendocrine cell hyperplasia ^ microcarcinoids. (b) carcinoid polyp <1cm diameter. (a) and (b) are benign and managed by endoscopic surveillance and biopsy. (c) ulcerated tumour: malignancy relates to: size >2 cm diameter, invasion beyond submucosa, angioinvasion—necessitates resection.

— lesions 1-2 cm diameter are also potentially malignant and require wide local excision.

— right colonic carcinoids tend to be large and ulcerated with adverse prognosis but the commoner rectal carcinoid is usually solitary and <1 cm in diameter. They show variable expression of neuroendocrine markers (chromogranin negative, synaptophysin positive) and many are prostatic acid phosphatase positive which can cause diagnostic confusion with the differential diagnosis of secondary prostatic ade-nocarcinoma (PSA positive).

Neuroendocrine differentiation can be present in up to 50% of usual type colorectal adenocarcinomas and is not prognostically significant.

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