There are always questions about radiation in medical imaging. People have mammograms, bone density tests and computed tomography (CT) scans. I want to know if radiation increases the risk of developing cancer. For most women, there is little risk from routine x-rays such as mammograms or dental x-rays. However, many experts are concerned about the explosion in the use of high-dose tests such as CT and nuclear imaging. In 1980, there were only 3 million CT scans, but more than 80 million CT scans are performed each year in the United States.

There are good reasons for this trend. Computed tomography and nuclear imaging have revolutionized diagnosis and treatment, eliminating the once common need for exploratory surgery and many other invasive and potentially dangerous procedures. When needed, the benefits of these tests far outweigh the risk of radiation-related cancers, and the risks of a single CT or nuclear imaging test are very small. But are we tracking the health problems of the future?

The effect of ionizing radiation is increasing

Radiation from X-rays, CT scans, and nuclear imaging is ionizing radiation. That is high-energy wavelengths or particles that penetrate tissues and expose the body's internal organs and structures. Ionizing radiation can damage DNA, and while cells repair most of the damage, they do their job perfectly, sometimes leaving small areas of 'misrepair'. As a result, a few years later, a DNA mutation appeared that caused cancer.

We are constantly exposed to small amounts of ionizing radiation from nature: cosmic rays, mainly from the sun, and radon, a radioactive gas produced by the natural decay of uranium in soil, rocks, water, and building materials. How much you are exposed to this background radiation depends on several factors, including altitude and room ventilation. However, the average figure is 3 millisieverts (mSv) per year. Although exposure to ionizing radiation from natural or background sources has not changed since about 1980, total radiation exposure per capita in the United States has nearly doubled, and experts say the main cause is the increased use of medical imaging. The proportion of total radiation generated by medical facilities has increased from 15% in the early 1980s to 50% today. According to a report published by the National Radiation Protection and Measurement Council in March 2009, CT alone accounts for 24% of all radiation exposure in the United States.

Radiation measurement

When talking about radiation measurements, most people remember the classic Geiger counter with a crescendo. However, the Geiger counter only measures the intensity of the radiated emission. Effects on human tissues and health are more difficult to measure. Sieverts (Zv) and millisieverts (mSv) are useful here. Often used when comparing imaging procedures, this device takes into account the biological effects of radiation and depends on the type of radiation and the delicacy of the affected body tissue. With this in mind, millisieverts describe what is called an 'equivalent dose'.

Ionizing radiation and the risk of cancer

It has long been known that children and adolescents who receive high doses of radiation to treat lymphoma or other cancers are more likely to develop new cancers later. However, there are no clinical studies to guide our thinking about the cancer risk of medical radiation in healthy adults. Much of what we know about the dangers of ionizing radiation comes from long-term studies of survivors of the atomic bombs dropped on Hiroshima and Nagasaki in 1945. The study shows a small but significant increase in cancer in people exposed to the blast, including 25,000 Hiroshima survivors who received less than 50 mSv of radiation. This is a figure that can be obtained with three or more CT scans. 

High dose imaging

In the United States, increased exposure is due to CT and nuclear imaging and requires higher radiation doses than conventional X-rays. For example, a chest x-ray gives 0.1 mSv, and a chest CT gives 7 mSv - 70 times. This excludes routine follow-up CT scans.

In a 2009 study at Brigham and Women's Hospital in Boston, researchers assessed cancer risk from CT scans in 31,462 patients over the age of 22. The increased risk for the group as a whole was small. It was 0.7% higher than the overall US lifetime cancer risk of 42%. However, for patients who had multiple CT scans, the increased risk was higher, from 2.7% to 12%. (In this group, 33% had at least 5 CT scans, 5% had at least 22 scans, and 1% had at least 38 scans.)