To start with, cancer cells stay within the body tissue from which they have developed for example, the lining of the bladder or the breast ducts. Doctors call this superficial cancer growth or carcinoma in situ.

The cancer cells grow and divide to create more cells and will eventually form a tumour. A tumour may contain millions of cancer cells.

All body tissues have a layer (a membrane) that keeps the cells of that tissue inside. This is the basement membrane. Cancer cells can break through this membrane. If this happens, the cancer is called invasive.