Cancer treatment and the disease itself create a state of heightened coagulation, increasing the risk of developing blood clots. This complication, known as cancer-associated thrombosis, is a major cause of illness and is considered the second leading cause of death for individuals with cancer. Chemotherapy is a known contributing factor that significantly elevates the likelihood of a clotting event. Patients undergoing chemotherapy face a risk of blood clots up to 23 times higher than that of the general population.
The Confirmed Link and Core Definitions
Chemotherapy significantly increases the risk of blood clots. This elevated risk centers on Venous Thromboembolism (VTE), the umbrella term for clots that form in the veins. The most common form is Deep Vein Thrombosis (DVT), which occurs when a clot forms in a deep vein, most often in the lower leg, thigh, or pelvis.
If a DVT breaks free and travels through the bloodstream, it is called an embolus. If the embolus lodges in the arteries of the lungs, it causes a potentially fatal blockage known as a Pulmonary Embolism (PE). A PE is considered the most dangerous consequence of a DVT because it severely restricts blood flow to the lungs.
Biological Mechanisms Driving Clot Risk
The increased propensity for clotting is driven by hypercoagulability, which aligns with the Virchow triad: changes in blood flow, damage to the vessel wall, and alterations in blood composition. Chemotherapy and cancer interact to fulfill all three conditions.
One mechanism involves direct damage to the blood vessel linings, or endothelium. Many chemotherapy agents are cytotoxic, inadvertently injuring the delicate cells that line the veins. This injury triggers the body’s repair mechanisms and exposes pro-clotting proteins beneath the vessel wall surface.
Cancer cells actively contribute by releasing substances that promote clotting. These cells express high levels of Tissue Factor, a potent initiator of the coagulation cascade that generates fibrin, the protein mesh that forms a clot.
Additionally, both the malignancy and chemotherapy induce systemic inflammation. Inflammation elevates circulating markers and cytokines that accelerate the production of clotting factors by the liver. This chemically alters the blood, making it stickier and more prone to forming a thrombus.
Identifying Specific Patient Risk Factors
While chemotherapy raises the baseline risk, certain variables significantly elevate an individual’s likelihood of developing VTE. The type and stage of the malignancy play a large role. Cancers of the pancreas, stomach, and lung carry the highest risk levels, and patients with advanced or metastatic disease face a greater risk than those with localized tumors.
Specific chemotherapy agents are known to be more thrombogenic than others:
- Platinum-based drugs, such as cisplatin.
- Hormonal therapies, such as tamoxifen.
- L-asparaginase.
- Thalidomide.
- The anti-angiogenic drug bevacizumab.
Patient-specific factors also compound the treatment risk. These include a history of prior blood clots, being older than 65, or having a high body mass index. Immobility, whether due to hospitalization or fatigue, slows blood flow and allows clotting factors to accumulate. Furthermore, the presence of a central venous catheter (port or PICC line) introduces a foreign body into a large vein, which can cause local irritation and injury, promoting clot formation at the insertion site.
Recognizing Signs and Symptoms of Thrombosis
Recognizing the signs of a developing blood clot is important because immediate medical attention is necessary. Deep Vein Thrombosis (DVT) symptoms usually involve the affected limb, most commonly an arm or a leg.
A patient may notice sudden, unexplained swelling or a painful, cramping sensation that does not resolve. The skin over the affected area often feels warm to the touch and may appear reddish or discolored. Tenderness along the course of the deep vein may resemble a pulled muscle. DVT symptoms typically affect only one limb at a time.
Signs of a Pulmonary Embolism (PE) require urgent attention, as this is a medical emergency. Common symptoms include the sudden onset of shortness of breath, sharp chest pain that worsens when taking a deep breath or coughing, an unexplained rapid heart rate, and lightheadedness. Any combination of these symptoms warrants an immediate call for emergency medical services.
Prevention and Treatment Strategies
To manage this elevated risk, medical professionals employ strategies focused on both prevention and treatment. Prevention, or prophylaxis, involves a personalized risk assessment, often using a tool like the Khorana score to identify high-risk ambulatory patients before chemotherapy begins. This scoring system considers factors such as the cancer type and certain blood test results to predict VTE likelihood.
For patients identified as high-risk, preventative anticoagulation therapy is often recommended, usually involving a low-molecular-weight heparin or a direct oral anticoagulant. General preventative measures also include maintaining hydration and ensuring regular physical movement to prevent blood from pooling.
For those who have confirmed clots, the treatment strategy shifts to therapeutic anticoagulation. Blood-thinning medications are administered to stop the existing clot from growing larger and to prevent new ones from forming. The standard duration for this treatment is typically at least three to six months, and often continues as long as the cancer is active or the patient is receiving treatment. This approach manages the clot while balancing the risk of increased bleeding associated with the medication.