Blood clots and cancer

Blood clots are a normal strategy that your body employs to prevent bleeding and help heal from injuries. However, when they form within blood vessels, it can lead to serious medical problems. When this happens, it is called thrombosis.

Cancer-associated thrombosis (CAT) is the general term for problems with blood clots connected to cancer. It is the second leading cause of death for cancer patients—the first being the cancer itself.[4] Cancer patients are about 4 times more likely than the general population to develop venous thromboembolism (VTE – a kind of thrombosis).[5]

If you have cancer, it’s important to review the typical signs of thrombosis, and to contact your health care professional right away if any of them appear. Cancer-associated thrombosis is often overlooked as a cause of illness and death by both patients and health care professionals, but it needs to be taken seriously.

Why is cancer linked to thrombosis? Partly because of the effects of the cancer itself, and partly because of effects of cancer therapies such as chemotherapy and surgery. Other factors such as obesity and immobility are also important. See the list of risk factors below.

The terms DVT (deep vein thrombosis) and VTE (venous thromboembolism) appear frequently in information on cancer-associated thrombosis, because it is these specific kinds of thrombosis that cancer patients are at risk for. Venous thromboembolism is when a blood clot occurs in a vein (as opposed to an artery). Deep vein thrombosis, a kind of VTE, is when a blood clot occurs in a vein deep within the body, typically in the legs and less often in the arms. Some more definitions are given in the glossary below.


  1. Signs of cancer-associated thrombosis
  2. Risk factors for cancer-associated thrombosis
  3. How can the risk of CAT be reduced?
  4. Glossary of terms
  5. Links to resources
  6. References

Signs of cancer-associated thrombosis

Cancer-associated thrombosis generally comes in the form of either deep vein thrombosis (DVT), if it occurs within a limb, or pulmonary embolism (PE), if it occurs within a lung.

Signs of deep vein thrombosis (DVT) include:[6]

  • Swelling or a sense of heaviness in the leg or arm
  • Pain, tenderness, or cramping in the limb that isn’t caused by injury
  • Redness or brownness around the area where the clot forms
  • A feeling of warmth in the leg or arm, or skin that is warm to the touch

Symptoms of pulmonary embolism (PE) include:[1,6]

  • Shortness of breath
  • Pain in the chest that worsens when breathing or coughing
  • Feeling faint, light-headed or dizzy
  • Coughing up blood
  • Faster than normal or irregular heartbeat

Risk factors for cancer-associated thrombosis

Cancer type

The type of cancer you have can influence your risk of developing thrombosis.

Cancers of the pancreas, stomach, brain, lungs, uterus, ovaries, kidneys, and blood (e.g. lymphoma and myeloma) are most often associated with thrombosis. [1]

Breast cancer and skin cancers, on the other hand, are associated with a relatively low risk of VTE.[5]


Chemotherapy is associated with a significantly greater risk of VTE. In cancer patients undergoing chemotherapy, the incidence of VTE is 6 ½  times greater than in the general population—compared to 4 times greater for cancer patients overall.[5]

Certain drugs used for chemotherapy are more likely to cause thrombosis than others. Some combinations of drugs are associated with a greater risk even though the individual drugs are not. Anti-angiogenic drugs, which are thought to work by preventing the formation of blood vessels in tumours, have been found in one study to be connected to a greater risk of VTE.[3,5]


Surgery for cancer is associated with increased risk of VTE.

  • The risk of developing VTE in cancer surgery patients can be as high as 50%.[4] (Some studies have found lower numbers.)
  • Twice as many patients undergoing surgery for cancer develop VTEs as do patients undergoing surgery for other reasons.[3]
  • Note that surgery in general is a risk factor for VTE, whether for cancer or not.

Anticoagulants are recommended in most cases to prevent VTE in cancer patients undergoing surgery. Their use can reduce the risk of VTE to anywhere from half to one fifth of what it otherwise would be.[4]

Compression socks can also help prevent VTE.[4]


Use of central venous catheters (CVCs) is related to greater risk for venous thromboembolism (VTE). These catheters are often used for chemotherapy. The incidence of VTE among cancer patients who have catheters, though, is quite variable, suggesting that more specific factors play strong roles.[3] One study found that 5% of patients with long-term venous catheters developed cancer-associated thrombosis.[5]


In general, sitting down or staying still for an extended period of time is associated with greater risk for VTE. This applies especially to cancer patients because surgery and hospital stays often involve prolonged periods of immobility.[5]

Moving around and exercising the leg muscles can help reduce the risk for thrombosis that comes from immobility.[2]

Additional factors[1]

  • Having had a blood clot in the past
  • A family history of blood clots or inherited clotting disorder
  • Broken bone or severe muscle injury
  • Severe physical trauma, like a car accident
  • Serious medical conditions, such as heart and lung diseases or diabetes
  • Being overweight or having obesity
  • Birth control methods that contain estrogen, or using hormone therapy with estrogen
  • Pregnancy
  • Smoking

How can the risk of CAT be reduced?

Anticoagulants. Anticoagulant drugs, or blood thinners, are often used by doctors to prevent thrombosis where they believe a risk to exist—for cancer patients undergoing surgery, for example.[4]

Anticoagulants are also used to treat VTE. They do not actually dissolve the blood clots, but prevent blood from clotting further, so that the clots can go away on their own. This is a gradual process that may take a few weeks or a few months.[6]

Physical activity. Moving around can help reduce the risk for VTE from being sedentary. It doesn’t have to be strenuous activity—doing it frequently is more important. The CDC recommends:[2]

  • Getting up and moving around as soon as possible after having been confined to bed—for example, after surgery.
  • Walking around a bit at least every 2-3 hours.
  • Exercising your leg muscles while sitting.

Staying hydrated. Dehydration can increase the risk of clotting because it causes your blood to be thicker.[6]

Quitting smoking. Smoking can damage blood vessels and is a known risk factor for thrombosis.[6]

Glossary of terms

Here are some quick definitions of common terms for blood clotting.

Thrombosis: This is the general term for the formation of blood clots within a blood vessel (in contrast with blood clots that form to prevent bleeding). The problems it causes depend on where it occurs—primarily, whether it occurs in a vein or in an artery. Thrombosis that occurs in an artery (arterial thrombosis) can lead to a stroke or a heart attack. Thrombosis that occurs in a vein (venous thrombosis) is the kind that cancer patients are particularly at risk for.

Embolism: When a piece of something gets stuck in a blood vessel and blocks it. In general, the piece that gets stuck can be anything; here, though, we are interested in cases where it is a blood clot. This is called thromboembolism.

VTE – venous thromboembolism: This is when a blood clot forms in a vein and blocks it. It is a category of problems that includes deep vein thrombosis (DVT) and pulmonary embolism (PE), the two main blood clot problems that cancer patients are at risk for.

DVT – deep vein thrombosis: This is when blood clots form within a vein located deep within the body—typically in the legs, sometimes in the pelvis, and less often in the arms.

PE – pulmonary embolism: When a blood clot blocks a blood vessel in the lungs. Either the clot has formed in the lungs, or it has formed in another place—like from a case of deep vein thrombosis in the legs—and travelled to the lungs from there.

The U.S. CDC (Centers for Disease Control and Prevention) has excellent webpages providing information for cancer patients about blood clots.

CancerClot is a website with extensive and accurate information for patients on the relationship between deep vein thrombosis and cancer. It is funded by the pharmaceutical company LEO Pharma.

Thrombosis Canada is a not-for-profit organization that promotes excellence in care for thrombosis patients, mainly by providing resources for healthcare professionals and supporting research. It offers a large catalogue of educational materials for clinicians and patients.

A white paper with recommendations on how to reduce the negative impact of CAT on cancer patients: Cancer Associated Thrombosis, A Neglected Cause of Cancer Death (2016).

Video resources

Thrombosis Canada’s YouTube channel has an excellent series of video interviews with Dr. Simon Noble, a palliative care specialist whose research focusses on cancer-associated thrombosis.

  1. Watch Dr. Simon Noble speak about cancer-associated thrombosis (CAT)
  2. Background on Dr. Simon Noble and his research on cancer-associated thrombosis (CAT)
  3. Overview of blood clots and cancer-associated thrombosis (CAT)
  4. Signs and symptoms of cancer-associated thrombosis (CAT)
  5. Where to find information on cancer-associated thrombosis
  6. Management of cancer-associated thrombosis (CAT)
  7. Lifestyle changes that can help prevent blood clots
  8. Key findings from interviewing patients living with cancer-associated thrombosis (CAT)



  1. Centers for Disease Control and Prevention [CDC] (2020). “Venous Thromboembolism (Blood Clots) and Cancer.” Retrieved August 2020 from
  2. CDC (2020) “What is Venous Thromboembolism?” Retrieved August 2020 from
  3. CancerClot (2019). “DVT risk factors.” Retrieved August 2020 from
  4. Brose, K. M. J. and A. Y. Y. Lee (2008). “Cancer-associated thrombosis: prevention and treatment.” Current Oncology 15 (Supplement 1): S58–S67.
  5. Kasthuri, Raj (2018). “More Information About Blood Clots and Cancer.” Retrieved August 2020 from
  6. Thrombosis Canada (2019). “Cancer-Associated Thrombosis: What Every Patient With Cancer Needs to Know.” Retrieved August 2020 from