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Multi cellular organisms, like humans, are made up of billions of cells. These cells need to divide and copy themselves for a variety of reasons. For example:
Mitosis is closely controlled by the genes inside every cell. Sometimes this control can go wrong. If that happens in just a single cell, it can replicate itself to make new cells that are also out of control. These are cancer cells. They continue to replicate rapidly without the control systems that normal cells have. Cancer cells will form lumps, or tumours, that damage the surrounding tissues. Sometimes, cancer cells break off from the original tumour and spread in the blood to other parts of the body. When a tumour spreads to another part of the body it is said to have metastasized. They continue to replicate and make more tumours. These are called secondary tumours.
Medicines that are used to treat cancer are sometimes aimed at killing cells that are rapidly dividing by mitosis. They inhibit the synthesis or function of DNA - this type of treatment is called chemotherapy. More modern medicines target specific cancers in different ways. Many inhibit the growth signals for that type of cell.
There are many different types of cancer. They depend on which type of cell was the original one that started to replicate out of control. This means that there is not just one treatment for cancer. Treatments may include a combination of surgery, medicines and radiation therapy (radiotherapy).
As researchers have come to understand more about cancers, new and targeted therapies are constantly being developed. For example, a type of breast cancer that is influenced by the hormone oestrogen can be treated with hormone therapy that blocks the action or synthesis of oestrogen. Other medicines can block growth signals to the cancer cell and so slow the development of a tumour or block the growth of new blood vessels into tumours. This effectively 'starves' the cancer cells of the nutrients they need to grow.