In simple terms
Normally, blood clotting is a healthy defence mechanism that prevents excessive bleeding when you get injured. Thrombosis occurs when a clot forms inside a blood vessel without an external injury, blocking the vital flow of blood. Cancer cells can alter the natural balance of your blood, making it much stickier and more prone to clotting. Clots often form in the legs, causing sudden swelling, warmth, and discomfort. If a clot breaks loose and travels to the lungs, it can cause sudden breathlessness, making thrombosis a serious condition that requires swift evaluation.
Key takeaways
- Involves inappropriate clot formation that occludes venous or arterial blood flow.
- Presents most frequently as deep vein thrombosis (DVT) or pulmonary embolism (PE).
- Cancer inherently increases clot risk by making the blood more hypercoagulable.
- Warning signs include unexplained leg swelling, calf pain, chest pain, and breathlessness.
- Treated safely and effectively using anticoagulant medications (blood thinners).
Definition
Thrombosis is the pathological development of a blood clot (thrombus) inside an artery or vein, leading to vascular occlusion. In the oncology setting, venous thromboembolism (VTE) is the most frequent manifestation, comprising deep vein thrombosis (DVT)—usually developing within the deep veins of the leg or pelvis—and pulmonary embolism (PE), a life-threatening event occurring when a thrombus dislodges and occludes pulmonary arterial circulation.
Malignancy promotes a prothrombotic state through the secretion of procoagulant proteins (such as tissue factor), mechanical compression of blood vessels by tumours, inflammatory cytokine release, endothelial damage caused by chemotherapy, and extended periods of immobility.
Why it matters
Venous thromboembolism is one of the leading causes of preventable morbidity and mortality in cancer patients. Recognising symptoms early—such as unilateral limb swelling, pain, sudden breathlessness, or chest discomfort—enables rapid intervention with anticoagulant medications, preventing critical complications. Knowing about thrombosis risk also helps patients engage in proactive prevention, such as remaining active when possible, wearing compression stockings during hospital admissions, or using prescribed preventive blood thinners.
Related biomarkers and tests
Diagnosis involves clinical risk assessment (e.g., the Khorana score) and diagnostic imaging: compression ultrasonography for suspected DVT, and CT pulmonary angiography (CTPA) for suspected pulmonary embolism. Blood D-dimer tests are occasionally measured, though cancer frequently elevates baseline levels.
Related cancers
Thrombosis can complicate any malignancy, but risk is particularly elevated in pancreatic, gastric, lung, ovarian, brain, and metastatic colorectal cancers, as well as multiple myeloma (especially when treated with immunomodulatory drugs).
Related treatments
Once diagnosed, thrombosis requires immediate anticoagulation therapy, typically utilising low-molecular-weight heparins (LMWH) or direct oral anticoagulants (DOACs). Anticoagulation must be carefully balanced against any concurrent bleeding risks or low platelet counts during ongoing cancer therapy.
Frequently asked questions
What are the common warning signs of a blood clot?
A clot in the leg (DVT) usually causes swelling, tenderness, warmth, or redness, often in just one limb. If a clot travels to the lungs (pulmonary embolism), it can trigger sudden shortness of breath, sharp chest pain when breathing, and a rapid heart rate. These require urgent emergency medical assessment.
Why does having cancer increase my risk of blood clots?
Cancer cells release substances that activate the clotting system, making blood thicker and more prone to forming clots. Treatments like surgery and chemotherapy, the use of central venous catheters, and spending prolonged periods resting in bed further compound this risk.
How is thrombosis treated in someone with cancer?
Treatment relies on anticoagulant medications, often referred to as blood thinners. Doctors usually prescribe direct oral anticoagulants (DOACs) or low-molecular-weight heparin injections for several months, while continuously balancing clot prevention against individual bleeding risks.
References
- 1.Blood Clots and Cancer— National Cancer Institute
- 2.Venous Thromboembolism in Patients with Cancer— American Society of Clinical Oncology
- 3.ESMO Clinical Practice Guideline on Venous Thromboembolism in Cancer Patients— European Society for Medical Oncology
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Last reviewed August 1, 2026
Medical disclaimer
Educational information only. GetOnco is software, not a medical provider, and does not diagnose disease or recommend treatments. Always discuss your situation with qualified healthcare professionals.