Carcinoid tumour: forming over 80% of appendiceal tumours, carcinoid tumour of classical type is usually small (< 1 cm diameter) and found as an incidental finding at the appendiceal tip with or without associated appendicitis. It can be a histological finding only amidst the inflammation, or macroscopically discernible as a firm, pale-yellow mass replacing the lumen and wall. It has a variable nested and tubular pattern of uniform neuroendocrine cells that are positive with chromogranin antibody. Despite showing transmural, serosal and lymphovascular spread, appendicectomy is usually totally therapeutic and recurrence is only seen in a very small number of cases where the lesion is greater than 2 cm diameter or there is involvement of the appendiceal base, caecal wall, mesoappendix or local lymph nodes. Conversely, the much less common mucin-rich goblet cell carcinoid (adenocarcinoid/crypt cell carcinoma) more frequently involves the appendiceal base with potential for nodal metastases, local invasion of the caecal pouch and transcoelomic peritoneal spread. Because of this, goblet cell carcinoid may require consideration for right hemicolectomy. Due to the difficulties in distinguishing between carcinoid tumour and inflammatory fibrotic reaction the appendiceal tip and base are sampled and separately identified as part of the routine blocking procedure to assess adequacy of tumour excision if present.
Adenoma: tubular, tubulo-villous or villous and more often sessile than polypoid comprising low- or high-grade dysplastic epithelium. It may be associated with synchronous/metachronous polyps or tumours elsewhere in the colorectum, FAPC, mucocoele (see below) or adenocarcinoma of the appendix or adjacent caecal pouch.
Adenocarcinoma: a relatively unusual lesion that may be mucinous and cystic, secondary involvement of the proximal appendix from the caecal pouch is more common than a primary appendiceal lesion. More recently, attention in the literature has focused on the association with ovarian mucinous borderline tumours (see pseudomyxoma peritonei). Very occasionally, mixed adenocarcinoma-carcinoid tumours occur. Other cancers metastatic to the appendix are from ovary, stomach, breast and lung.
Mucocoele: macroscopic distension of the appendiceal lumen by abundant mucus often with marked thinning of the wall. Obstructed or non-obstructed in character, the former represents a retention cyst lined by attenuated and atrophic but normal mucosa. Non-obstructed mucocoele is due to oversecretion of mucus by an abnormal mucosal lining that can be either hyperplastic, adenomatous or adenocarcinomatous in nature. Extrusion of mucus through the wall to the serosa results in pseudomyxoma peritonei which is localised to the periappendiceal tissues or generalised in the peritoneal cavity. The latter is refractory to surgical debridement with reaccumulation over a prolonged time course of months to years resulting in bowel obstruction and death. It is due to spillage of either atypical or frankly malignant appendiceal epithelium into the peritoneal cavity whereas mucocoele due to benign hyperplastic or cystadenomatous epithelium more often results in a self-limited localised reaction.
It is now recognised that there is a strong association between generalised pseudomyxoma peritonei, appendiceal mucinous tumours and bilateral ovarian mucinous borderline tumours, with the latter regarded as metastases from the appendiceal lesion.
Prognosis: carcinoid tumours less than 2 cm diameter are generally adequately treated by local appendicectomy. Those that are larger, involve the base or are of goblet cell type may require right hemicolectomy. Prognosis of mucocoele depends on the nature of the underlying mucosal epithelium and degree of spillage into the peritoneum. Adenocarcinoma treated by appendicec-tomy alone does worse (20% five-year survival) than when right hemicolectomy is performed (60-65% five-year survival) - outlook is tumour grade and stage dependent.
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