What are appendix cancer and Pseudomyxoma Peritonei (PMP)?

Appendix Cancer

Appendix cancer occurs when cells in the appendix become abnormal and keep growing and form a mass or lump called a tumour.

The type of cancer is defined by the particular cells that are affected and can be benign (non-cancerous)
or malignant (cancerous). Malignant tumours have the potential to spread to other parts of the body through the blood stream or lymph vessels and form another tumour at a new site. This new tumour is known as secondary cancer or metastasis.

Types of appendix cancer

The most common types include:

Mucinous adenocarcinoma – These start in epithelial cells that line the inside of the appendix. They can
produce mucin (a jelly-like substance) and spread to other parts of the body, including the peritoneum,
which is a sheet of tissue that lines and protects organs in the abdomen (belly).

Neuroendocrine tumours (NETs) – These form in neuroendocrine cells inside the appendix. The  neuroendocrine system is a network of glands and nerve cells that make hormones and release them into the bloodstream to help control normal body functions. Appendiceal NETs are often found at the tip of the appendix.

Goblet cell carcinoma (GCC) – These have features of both a NET and adenocarcinoma but behave more like an adenocarcinoma, which can be more aggressive.

Colonic-type adenocarcinoma – These may behave like colon (large bowel) cancer and are often found at the base of the appendix.

Pseudomyxoma Peritonei (PMP)

Pseudomyxoma peritonei (PMP) is a rare tumour that grows slowly and causes a build-up of mucin (a jelly-like substance) in the abdomen and pelvis, giving rise to the name “jelly belly”. Several other diseases may also be associated with “jelly belly” including mucinous adenocarcinoma, or may resemble features of PMP including mucinous tumours in the bowel.

PMP often starts in the appendix but can also start in other organs such as the large bowel and ovary. While it doesn’t spread to other parts of the body, PMP can put pressure on important organs as it continues to grow and this may cause problems.

How common are appendix cancer and PMP?

Appendix cancer is rare with 0.12 cases per 1,000,000 people each year being reported for primary malignancies (cancer that first develops in the appendix). The most common types are seen in middle-aged people, with the typical age at diagnosis about 40 to 60 years. There is an almost equal risk for males and females for mucinous adenocarcinoma and GCC. Colonictype adenocarcinoma is diagnosed slightly more often in men and appendiceal NETs are diagnosed more often in women.

PMP is also rare with about 1 or 2 cases per 1,000,000 people each year. It is more likely to be diagnosed in people aged 40 years or over. Women may be diagnosed slightly more often and at an earlier stage than men, after a mass or lump is found in their ovary.