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# Tumours of the appendix

- Lesion

The commonest appendix tumour is a small harmless neuroendocrine nodule at its tip; mucus-making tumours can fill the abdomen with jelly.

Updated Nov 12, 2026

## Key points

- Neuroendocrine tumour: bulbous swelling at the tip, usually under two to three centimetres; nodal spread uncommon, distant spread very rare.
- Low-grade appendiceal mucinous neoplasm (LAMN): mucinous epithelium, extracellular mucin, pushing (not infiltrative) margins; excellent prognosis when confined.
- High-grade mucinous neoplasm (HAMN) has more atypia and a more variable outcome; an infiltrative pattern means mucinous adenocarcinoma.
- GNAS mutation (raising Gαs and mucin output) and KRAS mutation are common; APC, TP53 and SMAD4 usually intact in LAMN.
- Pseudomyxoma peritonei: semisolid mucin filling the abdomen from peritoneal spread; treated by cytoreductive surgery and heated intraperitoneal chemotherapy; recurrence is common.
- Mucinous neoplasms occur mostly in the sixth decade.

## What it is

Appendiceal tumours are usually found by chance in an appendix removed for appendicitis. The well-differentiated neuroendocrine (carcinoid) tumour is the commonest and is almost always benign. Mucinous neoplasms, from low grade to adenocarcinoma, produce extracellular mucin that can rupture the appendix and spread over the peritoneum as pseudomyxoma peritonei. Conventional adenomas and adenocarcinomas also occur and can cause obstruction that mimics appendicitis.

## Appendiceal tumours

| Tumour | Typical finding | Behaviour |
| --- | --- | --- |
| [Well-differentiated neuroendocrine tumour](https://durgical.com/en/conditions/carcinoid-syndrome/) | Yellow nodule at the tip, found incidentally | Almost always benign |
| Low-grade mucinous neoplasm (LAMN) | Mucin-distended appendix, pushing margins | Excellent if confined; can cause pseudomyxoma |
| High-grade mucinous neoplasm (HAMN) | As LAMN with high-grade atypia | More variable |
| Mucinous or non-mucinous adenocarcinoma | Infiltrative invasion | Malignant; peritoneal and distant spread |

## Clinical notes

- Peritoneal spread of mucinous tumours of the appendix (or colon) causes abdominal distension from mucin.
- An appendiceal mass can be misread as an ovarian tumour in women; mucinous ovarian tumours with pseudomyxoma usually come from the appendix.

## Memory aid

Tip nodule: relax. Mucus balloon: watch the peritoneum.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the most common tumour of the appendix?

The well-differentiated neuroendocrine (carcinoid) tumour, usually at the tip and almost always benign.

## What is pseudomyxoma peritonei?

Semisolid mucin filling the peritoneal cavity from peritoneal spread of a mucinous neoplasm, usually of the appendix.

## Which gene mutation is characteristic of appendiceal mucinous neoplasms?

GNAS, encoding Gαs, which promotes mucin production; KRAS is also common.

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## Connections

## Related 6

- [Gastrointestinal neuroendocrine (carcinoid) tumours](https://durgical.com/en/general-pathology/gastrointestinal-neuroendocrine-tumours/)
- [Acute appendicitis](https://durgical.com/en/general-pathology/acute-appendicitis/)
- [Diseases of the peritoneum](https://durgical.com/en/general-pathology/diseases-of-the-peritoneum/)
- [Colorectal adenocarcinoma](https://durgical.com/en/general-pathology/colorectal-adenocarcinoma/)
- [Benign vs malignant tumours: the comparison table](https://durgical.com/en/general-pathology/benign-vs-malignant-tumours-the-comparison-table/)
- [Pathways of spread and the usual metastatic sites](https://durgical.com/en/general-pathology/pathways-of-spread-and-the-usual-metastatic-sites/)

## Related conditions 4

- [Acute appendicitis](https://durgical.com/en/conditions/acute-appendicitis/)
- [Carcinoid syndrome](https://durgical.com/en/conditions/carcinoid-syndrome/)
- [Ovarian cancer](https://durgical.com/en/conditions/ovarian-cancer/)
- [Peritonitis](https://durgical.com/en/conditions/peritonitis/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Appendiceal Cancer](https://www.cancer.gov/pediatric-adult-rare-tumor/rare-tumors/rare-digestive-system-tumors/appendiceal-cancer) (National Cancer Institute, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/tumours-of-the-appendix/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/tumours-of-the-appendix/