Cancer-associated thrombosis (CAT) is one of the most common yet preventable causes of death among cancer patients. With over 200,000 Canadians being diagnosed with cancer each year, knowing about causes, prevention, and treatment of CAT will be critical to the quality of life and long-term survival of these individuals.

CAT refers to blood clots that can occur in people living with cancer. They may be related to the cancer, the treatment, or other factors such as the patient’s age and genetic risk factors
Two examples of CATs are deep vein thrombosis (DVT), which occurs in legs or arm, and pulmonary embolism (PE), which occurs when a blood clot breaks off and travels to the lungs. “It’s quite common as about 20- 25 percent of cancer patients develop blood clots at some point during their treatment,” says Dr. James Douketis, Staff Physician in Internal Medicine and Thromboembolism at St. Joseph’s Health Care Hamilton, Professor of Medicine at McMaster University, and President of Thrombosis Canada.

Despite its seriousness, CAT is often given secondary consideration in cancer care. “It tends to fall under the radar because most of the emphasis is on the treatment of cancer per se, but it’s something that deserves more attention because it’s a common, preventable cause of mortality in cancer patients,” says Dr. Douketis.

Missed or delayed diagnosis

Missed or delayed diagnosis of CAT occurs for many reasons. One is that patients are not adequately informed. “They’ve not necessarily been given this information when undergoing treatment so they don’t know what to look for,” says Jackie Manthorne, President and CEO of the Canadian Cancer Survivor Network, a patient and survivor group that is working to close the CAT information gap.

Another reason is that the signs and symptoms of blood clots in cancer patients may not present as usual or classical symptoms. “The symptoms can be masked or hard to tease out, making it challenging for doctors and other health care professionals to recognize,” says Dr. Douketis. For example, patients thinking they have a pulled muscle or a touch of pneumonia may actually have a blood clot.

Other times, diagnosis can be delayed because “those symptoms are ignored or just attributed to the treatment, like a side effect of the chemotherapy,” says Dr. Jay Easaw, Medical Oncologist with the Cross Cancer Institute in Edmonton.

It is therefore critical to recognize the signs and symptoms because some blood clots can be lethal, particularly when they occur in the lungs. “Patients with cancer are at a higher risk of developing these clots, so the sooner the patient recognizes the symptoms and gets proper treatment, the better,” says Dr. Easaw.

Symptoms of CAT might be leg or arm swelling and discolouration, persistent chest pain, shortness of breath, and pain or tightness when taking a deep breath.

Diagnosis, treatment, and prevention

Testing for CAT may be done through ultrasound of the legs for a suspected DVT and a computed tomography (CT) scan for suspected PE. 

Treatment and prevention can typically include using blood thinners, which work by stopping blood from clotting quickly, with the dose determined by the situation. “For prevention of a blood clot, the dose used is about one-third the dose used to treat blood clots,” says Dr. Douketis.

Dosing is critical because blood thinners can also be risky for cancer patients. “These patients may be at increased risk for bleeding because of their cancer, and the effects of their cancer treatment, so that can make the blood thinning treatment more challenging,” says Dr. Douketis.

Survival rates for PE are over 95 percent if the blood clot is identified and treated quickly.

With cancer rates expected to rise as the population ages, so too will the need for education on this potentially life-threatening complication. “Patients need to be empowered with knowledge of DVT so they can raise it with their health care team before their cancer treatment starts,” says Manthorne.

A multi-stakeholder approach is being advocated by many health care professionals and patient support groups to make CAT an integral part of cancer care and ensure patients receive optimal treatment. Increasing preventive measures, early diagnosis, and appropriate treatment of any kind of venous thromboembolism for people living with cancer will help reduce the burden of CAT, including death.

“Greater awareness, research, and advocacy that facilitate open conversations between Canadians and their health care professionals are essential and will help save lives,” says Manthorne.