Can having a heart attack increase your risk of cancer?
Yes, it can - slightly.
But it's not the heart attack itself that is the problem. It's the barrage of tests that follow, of which an increasing number expose patients to radiation, a new study suggests.
The findings, published in the Canadian Medical Association Journal, should lead physicians to be more discerning in their use of imaging and therapeutic procedures that expose patients to low doses of ionizing radiation, said Louise Pilote, an epidemiologist at the Research Institute of the McGill University Health Centre in Montreal and lead author of the study.
"We don't want patients to get the message: 'Don't do these tests.' The message is: 'Use these tests when there is a real benefit,' " Dr. Pilote said.
Heart tests that expose patients to radiation include coronary computed tomography (CT) scans, cardiac perfusion (also known as nuclear stress tests), coronary angiography and angioplasty.
Dr. Pilote said these tests are often appropriate, and her real concern was that they are perhaps being overused for screening purposes.
She suggested that magnetic resonance imaging (MRI) and ultrasound - tests that do not involve radiation - are more appropriate.
The new study involved 82,861 patients who suffered an acute myocardial infarction, commonly known as a heart attack. Seventy-seven per cent of them underwent at least one cardiac procedure that exposed them to radiation. A total of 12,020 developed cancer one year or longer after exposure. Most of those cancers would have developed anyway.
The researchers estimate the additional risk of developing cancer from the radiation exposure amounted to about half of 1 per cent for women and one-fifth of 1 per cent for men.
Mathew Mercuri of the division of cardiology at McMaster University in Hamilton, Ont. said that while the cancer risk posed by the tests is small, it is real. Most concerning, he said, is that the risk increases with exposure to radiation, and a large and growing number of patients are now undergoing multiple procedures.
Dr. Mercuri said that, as with all medical interventions, there needs to be a balancing of benefit and risk. The problem is that the "benefits of such procedures tend to be evident. The risks, on the other hand, are not so apparent."
Radiation exposure is measured in millisieverts (mSv).
The new research showed that patients were exposed to an average of 5.3 mSv per year from heart tests, but in some exposure surpassed 30 mSv.
Tests such as CT scans and nuclear stress tests expose patients to doses ranging from five to 15 mSv. By comparison, a mammography test exposes a woman to a radiation dose of less than one mSv.
One of the main problems in calculating the cancer risks of radiation exposure is that much of the data comes from the survivors of atomic bombings in Hiroshima and Nagasaki in 1945. Residents of those Japanese cities were exposed, on average, to 200 mSv of radiation. Their cancer risk increased about 20 per cent.
It is often argued that radiation exposure from diagnostic tests is unimportant for those with heart disease because they tend to be older, and the risk of heart disease is far greater than the additional risk of developing cancer.
In the new study, the average age of heart patients was 63, but the younger the patient was, the higher was their likelihood of being exposed to radiation.
Younger males had the highest rates of exposure, and cancers of the abdomen/pelvis and thorax/breast were the most common.
Women had a sharper increase in cancer risk, likely because of their smaller body size.
A 2009 study published in the Archives of Internal Medicine estimated that about 2 per cent of all cancers in the United States - 28,800 cases - are caused by exposure to radiation during CT scans.
The total was broken down to include 14,000 cancers caused by abdominal and pelvic scans, 4,100 from chest scans, 4,000 from head scans and 2,700 from CT angiography. Two-thirds of the cancers were found in women.
Doses of radiation vary markedly by body part scanned: A patient undergoing a head scan will be exposed to as little as two mSv of radiation, while one undergoing an abdominal-pelvic scan will be exposed to 31 mSv.
CT scans are used to diagnose various conditions, including heart blockages, colon cancer, brain tumours and pneumonia. Researchers are warning increasingly that the technology brings additional risks.
For instance, a chest CT scan exposes a patient to more than 100 times the radiation dose of a chest X-ray.