What is cancer?
Cancer is a disease of the cells in the body. There are many different types of cell in the body, and many different types of cancer which arise from different types of cell. What all types of cancer have in common is that the cancer cells are abnormal and multiply 'out of control'. Some cancers are more serious than others, some are more easily treated than others (particularly if diagnosed at an early stage), some have a better outlook (prognosis) than others.
So, cancer is not just one condition. In each case it is important to know exactly what type of cancer has developed, how large it has become, whether it has spread, and how well it usually responds to treatment.
What are cells?
Normal body cells
The body is made up from millions of tiny cells. Different parts of the body such as organs, bones, muscles, skin, and blood are made up from different specialised cells. All cells have a centre called a nucleus. The nucleus in each cell contains thousands of genes which are made up from a chemical called DNA.
The genes are like 'codes' which control the functions of the cell. For example, different genes control how the cell makes proteins, or hormones, or other chemicals. Certain genes control when the cell should multiply, and certain genes even control when the cell should die.
Most types of cell in the body divide and multiply from time to time. As old cells wear out or become damaged, new cells are formed to replace them. Some cells normally multiply quickly. For example, you make millions of red blood cells each day as old ones become worn out and are broken down. Some cells do not multiply at all once they are mature, for example, brain cells.
Normally, your body only makes the right number of cells that are needed.
Abnormal cells
Sometimes a cell becomes abnormal. This occurs because one or more of the genes in the cell has become damaged or altered. The abnormal cell may then divide into two, then four, then eight, and so on. Lots of abnormal cells may then develop from the original abnormal cell. These cells do not know when to stop multiplying. A group of abnormal cells may then form. If this group of cells gets bigger, it becomes a large clump of abnormal cells called a tumour.
What are tumours?
A tumour is a 'lump' or 'growth' of tissue made up from abnormal cells. Tumours are divided into two types - benign and malignant.
Benign tumours
These may form in various parts of the body. Benign tumours grow slowly, and do not spread or invade other tissues. They are not 'cancerous' and are not usually life-threatening. They often do no harm if they are left alone. However, some benign tumours can cause problems. For example, some grow quite large and may cause local pressure symptoms, or look unsightly. Also, some benign tumours that arise from cells in hormone glands can make too much hormone which can cause unwanted effects.
Malignant tumours ('cancers')
Malignant tumours tend to grow quite quickly, and invade into nearby tissues and organs which can cause damage. The original site where a tumour first develops is called a primary tumour. Malignant tumours may also spread to other parts of the body to form 'secondary' tumours (metastases). This happens if some cells break off from the primary tumour and are carried in the bloodstream or lymph channels to other parts of the body. These secondary tumours may then grow, invade and damage nearby tissues, and spread again.
Note: not all cancers form solid tumours. For example, in cancer of the blood cells (leukaemia) many abnormal blood cells are made in the bone marrow and circulate in the bloodstream.
What causes cancer?
Each cancer is thought to first start from one abnormal cell. What seems to happen is that certain vital genes which control how cells divide and multiply are damaged or altered. This makes the cell abnormal. If the abnormal cell survives it may multiply 'out of control' into a malignant tumour. 医学全在.线提供
We all have a risk of developing cancer. Many cancers seem to develop for no apparent reason. However, certain risk-factors are known to increase the chance that one or more of your cells will become abnormal and lead to cancer. Risk factors include the following:
Chemical carcinogens
A carcinogen is something (chemical, radiation, etc) that can damage a cell and make it more likely to turn into a cancerous cell. As a general rule, the more the exposure to a carcinogen, the greater the risk. A list of known and suspected chemical carcinogens can be found at http://physchem.ox.ac.uk/MSDS/carcinogens.html Well known examples include:
Age
The older you become, the more likely that you will develop a cancer. This is probably due to an accumulation of damage to cells in the body over time. Also, the body's defences and resistance against abnormal cells may become less good as you become older. For example, the ability to repair damaged cells, and the immune system which may destroy abnormal cells, may become less efficient with age. So, eventually one damaged cell may manage to survive and multiply 'out of control' into a cancer. Most cancers develop in older people.
Lifestyle factors
Diet and other lifestyle factors may increase or decrease the risk of developing cancer. For example:
Radiation
Radiation is a carcinogen. For example, exposure to radioactive materials and nuclear 'fallout' can increase the risk of developing leukaemia and other cancers. Too much sun exposure and sunburn (radiation from UVA and UVB) increase the risk of developing skin cancer.
The larger the dose of radiation, the greater the risk of developing cancer. But note: the risk from small doses such as from a single X-ray test is very small.
Infection
Some viruses are linked to certain cancers. For example, people with persistent infection with the hepatitis B virus or the hepatitis C virus have an increased risk of developing cancer of the liver. However, most viruses and viral infections are not linked to cancer.
Immune system
People with a poor immune system have an increased risk of developing certain cancers. For example, people with AIDS, or people on immunosuppressive therapy. 医学 全在.线提供www.med126.com
Your genetic make-up
Some cancers have a strong genetic link. For example, in certain childhood cancers the abnormal gene or genes that may trigger a cell to become abnormal and cancerous are inherited. Other types of cancer may have some genetic factor which is less clear-cut. It may be that in some people their genetic make-up means that they are less resistant to the effect of carcinogens or other factors such as diet.
