Carcinomatosis is a word that is sometimes used to describe a cancer that has spread widely to affect a number of different parts of the body.

There are many many different sorts of cancer. The great majority of these start from the cells which form the lining of the tubes and passageways that run through all the organs and tissues of our bodies. So, most cancers of the breast begin in the cells which line the glands and ducts of the breast, most cancers of the colon and rectum begin in the cells that line the inner surface of the bowel and most lung cancers begin in the cells which line the inside of the airways in the lung.

The cells which make up the lining tissues in the body are called epithelial cells. So cancers that start from these cells are epithelial cancers. Another name for an epithelial cancer is a carcinoma.

Carcinomas usually start from a single place in a single organ. This growth is known as the primary cancer. So, a carcinoma which starts in the lining cells of the prostate gland is a primary carcinoma of the prostate, similarly a cancer that begins in the lining cells of the inside of the bladder is a primary carcinoma of bladder.

All carcinomas may spread to other places if they are not treated (although some are much more likely to do this than others). When a cancer spreads somewhere else, that 'spread' is called a secondary cancer. So, if a primary carcinoma of the kidney sends off seedlings of tumour into the bone then these are secondary carcinomas of the bone (or 'bone secondaries') from a primary carcinoma of the kidney.

If a carcinoma has become very advanced and sent seedlings to a number of different sites in a number of different organs or tissues then it can be said to have become 'carcinomatosis'. So if, for example, a primary carcinoma of the breast had just spread to affect one or more of the nearby lymph glands under the arm, this certainly would not be 'carcinomatosis'. But if a breast cancer had become much more advanced and spread to involve lots of areas in, say, the bones, the liver and the lungs, then this could be called 'carcinomatosis'.

Although, strictly speaking, the word carcinomatosis should only be used for epithelial cancers, or carcinomas, it is sometimes used to describe other types of cancer that have spread widely throughout the body.