## Tumour types differ in their ability to invade surrounding tissue

The types of tumours are distinguished according to their ability to spread into the surrounding tissue (Figure 9.1). A non-malignant (also called benign) tumour is usually small and likely to grow for years. A benign tumour has a regular shape and does not interfere with surrounding tissue because it is separated from it by a fibrous capsule that often allows its complete surgical removal. At the same time, the molecules that provide the connections between the individual cells hold the tissue together and prevent it from migrating from its place of origin. It generally poses relatively little risk to the host as long as its size does not threaten the organ from which it originated or if it does not release a large amount of biologically active substances, such as hormones. What makes a tumour malignant is its ability to invade surrounding tissue. This is due to rapid growth due to accelerated cell division. As a result, a large amount of DNA mutations accumulate, further altering the properties of the cells. Intercellular communication and cellular cohesion are also disrupted. The shape of a malignant tumour is irregular and narrow as it invades and damages surrounding tissue. At the same time, due to the high demand for nutrients and oxygen, a solid malignant tumour begins to form new blood vessels that supply the tumour with oxygen and nutrients.

A metastasis occurs when some cells are torn away from the primary tumour and become lodged elsewhere in the body (Figure 9.2). Tumour cells can migrate through the blood and lymphatic systems to very distant parts of the body, where they become established and form another tumour. Approximately one in every 1,000 to 10,000 tumour cells that enter the bloodstream colonizes other tissues. Metastases are the most dangerous in terms of the severity of consequences for the body, as evidenced by the fact that they are responsible for up to 90% of deaths from various types of cancer.