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How can cancer kill you?

Firstly, it is important to say that not all cancers kill you. Overall, more than 50% of people diagnosed with cancer live for more than 5 years. Some cancers have survival rates of more than 90%. Secondly, cancer at an early stage does not kill you. So, a lot of effort is put into early diagnosis so that treatment is much more likely to be effective. The way in which cancer can cause death varies and there is no single answer to this question. The answer really depends on the type of cancer and the parts of the body affected by the cancer. 

Some types of cancer can spread to take over part of the body that carries out an essential function. For example, if a cancer is growing in part of the digestive system, it can prevent the digestion and absorption of food. It can block the digestive system so that food cannot go through the intestines. If food cannot pass through, then nutrients from the food can't be absorbed. 

If cancer affects the lungs, then eventually there is not enough healthy lung tissue to allow enough oxygen to be absorbed into the body to keep you alive. Or the cancer can block off part of the lung. This part then collapses and often becomes infected. A person with an advanced cancer does not always have the strength to fight off such an infection, even with strong antibiotics, and so the infection can eventually lead to death. 

The human body has very finely balanced limits of certain body salts and chemicals. If the cancer has spread to the liver or the bones, this can upset the body's delicate chemical balance. The liver is the chemical factory of the body. It carries out many different functions and is very important in maintaining the balance of body chemicals. 

For example, there has to be a certain amount of calcium in the blood. If the calcium levels go up or down it upsets the whole chemical balance. If cancer cells are in some of the bones in the body, a lot of calcium may be released into the bloodstream. Normally the body has systems to correct this sort of imbalance. But when the imbalance becomes too great the systems don't work any more. Some treatments can bring calcium levels back to normal, but these only work for a limited time. Then unfortunately the calcium levels will rise in the blood until the person becomes unconscious and eventually dies. 

If cancer cells take over the bone marrow, there will eventually not be enough healthy bone marrow to make blood cells. This can cause a lack of red blood cells (anaemia) which means that they can't carry enough oxygen around the body. It can also cause a drop in white blood cells. As white blood cells fight infection, it becomes more and more difficult for the body to keep bacteria and viruses under control. A drop in platelets prevents the body from controlling any abnormal bleeding. If a blood vessel in a vital part of the body is damaged - for example, in the brain - then the bleeding can cause a stroke. The stroke may be fatal, as the body cannot control the bleeding. 

Some cancers make particular substances which upset the body balance. This can cause problems such as severe weight loss or dehydration, which will eventually overwhelm the natural balancing systems of the body. 

Many treatments can control cancer for a long time even if they can't cure it. But if a cancer continues to grow, then unfortunately it can become too much for the body to cope with and eventually the treatment can no longer control the cancer. 

Although this is a difficult subject for people to talk about (including for some doctors and nurses), it may help you to ask your specialist doctor or nurse about how you or your relative may die. It is something most people worry about at some point. Talking about the way the cancer may affect your body can help to lessen at least some of those worries. Many people are relieved to find out that they (or their relative) are likely to become unconscious shortly before they die. What you imagine may happen before death is often far worse than the reality. It is important to remember also that very good pain control is available and no one with cancer should die in pain.

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Updated: 14 March 2014