Appendix Pathology
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Direct answer
A small but exam-favoured share of appendices removed for suspected appendicitis turn out to harbour tumours — most often a well-differentiated neuroendocrine tumour (carcinoid) at the tip, less often a mucinous neoplasm that distends the organ into a mucocele or an adenocarcinoma including the goblet cell type. The neuroendocrine tumour is the commonest appendiceal neoplasm, and its size dictates surgery: under 1 centimetre, appendicectomy alone is curative; beyond 2 centimetres, right hemicolectomy is performed, with the 1-to-2-centimetre band individualised. Rupture of a low-grade appendiceal mucinous neoplasm (LAMN) seeds gelatinous implants throughout the peritoneum, producing pseudomyxoma peritonei — the "jelly belly" — which is why a mucocele must never be allowed to spill.
What you must remember
- Carcinoid (well-differentiated neuroendocrine tumour): the commonest appendiceal tumour, arising at the tip in about two-thirds of cases, usually found incidentally; carcinoid syndrome is rare unless hepatic metastases are present.
- The size rule: under 1 centimetre — appendicectomy suffices; over 2 centimetres — right hemicolectomy; between 1 and 2 centimetres, decide by mesoappendiceal invasion, lymphovascular invasion and involvement of the caecal base.
- Grading: well-differentiated neuroendocrine tumours are graded by proliferation — Ki-67 up to 2 per cent for G1, 3 to 20 per cent for G2.
- LAMN: low-grade mucinous epithelium growing in a pushing (non-destructive) fashion with dense wall fibrosis, flattening the lining and producing a mucocele; high-grade disease is termed HAMN.
- Mucocele is a gross description, not a diagnosis: the distended, mucus-filled appendix may reflect a simple retention cyst, LAMN, or a mucinous cystadenoma or carcinoma — histology settles it.
- Pseudomyxoma peritonei: gelatinous mucin and mucinous epithelium coating the peritoneum, classically from a ruptured appendiceal LAMN; ovarian involvement is common in women and historically misread as a primary ovarian tumour.
- Goblet cell adenocarcinoma (formerly goblet cell carcinoid): submucosal nests of goblet and signet-ring-like cells in older adults, behaving more aggressively than classic carcinoid, spreading to peritoneum and ovary, and managed like an adenocarcinoma with right hemicolectomy.
- Non-neoplastic pathology: diverticulitis of the appendix (elderly, high perforation risk), Enterobius infestation and endometriosis also involve the appendix; acute appendicitis itself is a separate topic.
The incidental finding on the histology form
Consider three specimens that arrive the same week. The first is a 26-year-old's laparoscopic appendicectomy for classic right iliac fossa pain; histology reports a 0.6-centimetre well-differentiated neuroendocrine tumour at the tip, completely excised, without lymphovascular invasion or mesoappendiceal spread. The reasoned next step is nothing further — surveillance only — because subcentimetre tip tumours are cured by appendicectomy, and a completion hemicolectomy would add surgical risk without survival benefit.
The second reports a 1.6-centimetre tumour with invasion into the mesoappendix. This falls in the individualised band: invasion beyond the appendix wall, lymphovascular invasion or a positive caecal margin tips the decision towards right hemicolectomy, and most surgeons offer it here. The third specimen is different in kind — a 60-year-old with a cystic right iliac fossa mass on CT reported as a mucocele. The operative principle is en-bloc removal without rupture, because spillage of LAMN mucus implants the peritoneum; if the tumour has already perforated and pseudomyxoma peritonei is found, management shifts to specialist cytoreductive surgery with hyperthermic intraperitoneal chemotherapy, and long-term surveillance follows mucin recurrence.
High-yield viva angles
Examiners return to the ovarian decoy: a woman operated on for bilateral ovarian mucinous tumours whose peritoneum is studded with jelly must have the appendix examined, because most pseudomyxoma peritonei is appendiceal in origin — calling the ovarian disease primary is the historical error. The second angle is the tumour that causes appendicitis: a carcinoid or a mucinous neoplasm obstructing the lumen produces ordinary acute appendicitis, which is exactly how incidental tumours are discovered, so the surgeon's motto is to histologically examine every appendix. Third, the renaming of goblet cell carcinoid to goblet cell adenocarcinoma in current WHO classification reflects its behaviour — mixed endocrine and exocrine differentiation, but clinical management as an adenocarcinoma.
Frequently asked questions
Which is the commonest tumour of the appendix, and where does it arise?
The well-differentiated neuroendocrine tumour (carcinoid), typically at the tip, most often an incidental finding in an appendicectomy specimen.
What size thresholds govern surgery for appendiceal carcinoid?
Under 1 centimetre — appendicectomy alone; over 2 centimetres — right hemicolectomy; the 1-to-2-centimetre band is individualised by mesoappendiceal and vascular invasion.
What is a low-grade appendiceal mucinous neoplasm?
A LAMN — low-grade mucinous epithelium proliferating in a pushing pattern with wall fibrosis, causing mucocele formation and, on rupture, peritoneal seeding.
Why must an appendiceal mucocele be removed intact?
Rupture spills mucinous neoplastic epithelium and mucin into the peritoneum, seeding pseudomyxoma peritonei, which is difficult to eradicate.
What is the "jelly belly" of pseudomyxoma peritonei?
Gelatinous mucin with implanting mucinous epithelium throughout the peritoneal cavity, classically of appendiceal LAMN origin, treated with cytoreduction and hyperthermic intraperitoneal chemotherapy.
How does goblet cell adenocarcinoma differ from a classic appendiceal carcinoid?
Its submucosal goblet and signet-ring-like cells spread to peritoneum and ovaries, behave aggressively, and are managed with right hemicolectomy as for adenocarcinoma.