Yes, it's true that you have a higher risk of developing blood clots if you have cancer, although most people with cancer don't develop blood clots.

If a blood clot develops it is most often in the veins deep in the legs. This is called a deep vein thrombosis or DVT. The first sign of a DVT is usually swelling in the leg, sometimes with pain or tenderness in the calf, or numbness or tingling in the foot. The leg may also be warmer and a different colour from usual (red, purple or bluish). But, sometimes there are no symptoms.

Less commonly a blood clot can develop in the lungs. This is called a pulmonary embolus (PE). It can occur when a fragment breaks off from a clot in the leg and travels in the bloodstream to the lungs. The symptoms can include pain in the chest or back that gets worse when you breathe deeply, feeling breathless or coughing up blood. A blood clot in the lungs is a serious condition and can be life threatening. So, if you suspect you might have a clot in your leg or lungs it is important you see a doctor right away.

There are a number of factors that can affect your risk of developing a blood clot:

• the type of cancer you have
• whether you are currently having cancer treatment
• immobility - not being able to move around
• older age
• dehydration - not having enough fluid in your body and blood
• certain medical conditions.

The more of these risk factors a person has the more their risk is increased.

Cancer

We don't know all the reasons why having cancer increases your risk of DVT. But it seems that in many cancers, the balance of chemicals that control blood clotting is changed.

Cancers that can increase the risk of blood clots include cancers of the bowel, brain, kidney, lung, pancreas, stomach, ovary and womb, as well as blood cancers like lymphoma and leukaemias.

Some cancers can cause an increase in the number of clotting cells (platelets) in your blood making you more prone to clots. The stage of a cancer may also be a factor. People with more advanced cancer seem to have a higher risk than people with early stage cancer. Some studies have also suggested that people with cancer are more at risk of developing a DVT within the first six months after their diagnosis.

Cancer treatments

Many cancer treatments can increase the risk of blood clots, including:

• surgery
• chemotherapy
• central venous catheter
• hormonal therapy
• steroids
• other treatments such as thalidomide

Surgery

Surgery can increase your risk because, if you aren't able to move around for long periods, the blood flow in your veins slows down. This increases the chance of a clot developing. During and after an operation you are immobile for longer periods than usual, which increases the risk of DVT. This is one of the reasons you are encouraged to start moving around as soon as possible after an operation.

The National Institute for Clinical Excellence (NICE) has produced guidelines on how to cut the risk of blood clots after surgery (see http://guidance.nice.org.uk/CG46).

They say that, people having an operation and who have a higher risk of blood clots, because they have active cancer or are having cancer treatment, should be given preventative treatment.

There are two types of preventative treatment. They work by:

• stopping blood collecting in the legs
• slowing the rate at which blood clots

You will usually be given thigh length compression stockings to wear. They improve the circulation of blood in the deep veins of the legs. And so, help to stop blood collecting in the legs.

You will also be given treatment with a special type of heparin called low molecular weight heparin (LMWH) or with a drug called fondaparinux. This is injected into the layer of fat under the skin (subcutaneously) once or twice daily. It works by slowing the rate at which blood can clot.

Chemotherapy

Studies have found that people with cancer are more at risk of DVT while they are having chemotherapy. Several different types of chemotherapy drugs seem to have this effect.

Usually people having chemotherapy won't be given treatment to prevent blood clots. This is because there isn't clear evidence to show that this is safe or effective for most people. But, some people, who are at high risk because of the treatment they are having or because of other medical conditions, may be given medication to reduce blood clotting.

Having a central venous catheter (CVC), is also a risk factor for DVT. Flushing the catheter weekly can help to prevent blood clots forming. Some people may also be given anti-clotting treatment such as warfarin or heparin while they have a CVC in place.

Hormonal therapy and other cancer treatments.

Some hormonal treatments such as tamoxifen are also known to increase the risk of blood clots. Other drugs used in cancer treatment such as steroids, thalidomide and some newer targeted therapies can also increase the risk.

Immobility

If your ability to move around is affected by, for example, disability, or fatigue this can increase your risk of a clot forming. Being confined in one position during a long journey (three hours or more) whether on a plane, train, car or bus is also a common cause of blood clots. If you are planning to go on a long journey speak to your doctor about whether there are any particular precautions you should take.

Age

Older age can be a risk factor for blood clots. So a person, under the age of 45 years would be less at risk of developing a blood clot than an older person with the same risk factors.

Reducing your risk

Although you can't remove all risk of a blood clot developing, there are some things you you can do to lower your risk.

• Drink plenty of water.
• Take short walks regularly
• If you can't get up to walk around for a few hours keep your blood flowing by taking deep breaths, and exercising the muscles in your legs every so often.

Although many cancer treatments can increase the risk of blood clots, for most people the benefits of treatment greatly outweigh the risks. If you are worried about your risk of blood clots talk to your cancer specialist. They will be able to talk through your own personal risk factors and things that may be done to reduce your risk.