— requires destructive invasion through the muscularis mucosae by malignant glands (sometimes but often not with a desmoplastic reaction), or the presence of epithelium in extra-appendiceal mucus (cytokeratins and CEA can be useful in demonstrating this). The latter can be difficult to distinguish from a LAMN with mucin dissection and peritoneal spillage (see below).

— identified as primary by a mucosal adenomatous lesion.

— histologically of usual colorectal type, often well differentiated mucinous in character.

— rarely signet ring cell carcinoma: distinguish from goblet cell carcinoid (chromogranin positive) and metastatic gastric/breast carcinoma (infiltrating lobular type). In this respect it is necessary to know of previous operations to the stomach and breast and these sites may have to be investigated. Breast carcinoma may also be ER/PR positive and cytokeratin 7 positive/20 negative, whereas gut cancers are usually CEA positive and cytokeratin 20 positive/7 negative. The intra-cytoplasmic vacuoles of lobular carcinoma are PAS-AB positive.

— treatment is right hemicolectomy and regional lymphadenectomy. Prognosis reflects the histological grade of tumour and Dukes' classification, with an overall 5-year survival rate of 60-65% with hemicolectomy but only 20% for appendicectomy alone.

Metastatic carcinoma

— peritoneal spread: colorectum, ovary, stomach.

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