Medically reviewed by Susan Kerrigan, MD and Marianne Madsen
If you or your child are facing a CT scan, it’s natural to worry. After all, the scan can deliver 300 times the radiation of an X-Ray. You may have heard that there is a slight risk of developing cancer years later. Slight is not zero. Do the benefits outweigh the risks? If you’re wondering what the risks are and if you’ll get cancer, here’s what you need to know.
Surrounded by Radiation
Whether you’re taking a stroll or puttering around the house, radiation is unavoidable. Traveling at light speed, electromagnetic waves from the sun can warm our skin or give us cancer decades later. X-Rays, gamma rays, and ultraviolet radiation carry the most energy –– sufficient to remove electrons when they interact with atoms. The smallest particle of an element, atoms boast protons and neutrons within their central core or nucleus. Negatively charged electrons revolve around a positively charged nucleus. Radioactivity is a release of energy as a result of instability within an atom due to unstable blends of protons and neutrons. When the energy from radiation removes an electron, it is “ionized.”
This ionizing radiation is what causes tissue damage. Radiation transforms water molecules into free radicals capable of damaging DNA. If a cell dies as a result of interaction with a free radical it’s called “deterministic effect.” If the DNA is permanently altered, that’s called a “stochastic effect.” This damage can cause genetic changes or cancer. While deterministic effects are predictable (for example, we know how much radiation it takes to turn your skin red), stochastic effects are not. Because they are random, it is impossible to know how much radiation someone can absorb before developing cancer. A very small amount can still lead to the disease with one person while someone else might absorb relatively large amounts without issues. That’s why cancer is so unpredictable.
Radiology’s safety has improved markedly since its beginnings with the development of the X-ray 125 years ago. In fact, one study determined that the cancer risk for radiologists who graduated from medical school after 1940 was no greater than for psychiatrists. Unfortunately, in the 21st century, cancer risk for patients getting CT scans may have increased.
To CT or Not to CT
From its introduction in a medical journal in 1972, the CT (computed tomography) scan revolutionized diagnostics. By creating a composite of multiple X-Rays, the CT scan can deliver a 3-D representation of bones, organs, and tissues. Scans can be made from the throat to the legs. It has been instrumental in the declining death rate from cancer over the last 50 years. Since its introduction, the CT scan has led to better outcomes from surgeries, traumas, injuries, strokes, and heart attacks. Yet its widespread use is still controversial.
Because CT scans can deliver hundreds of times more radiation than an X-Ray, they may increase your risk of developing cancer by around one in 2000. CT scans account for over three-quarters of the effective radiation dose delivered from all imaging procedures. Yet only around 11% of X-Ray based examinations are CT scans. Because there is some linkage between CT scan use and solid organ cancers and leukemia (especially in those who have the scans as children) the recommendation is to avoid getting more than a cumulative dose of 100 mSv. A CT scan of the abdomen and pelvis, for example, delivers 7.7 mSv––about what you’d get normally in 2.5 years. So the suggested threshold is fairly high.
The issue lies not only with multiple scans but with scans of children. In one Australian study of over 600,000 people, those who had CT scans in childhood had a greater likelihood of developing cancer. After adjusting for other variables like age or gender, the overall cancer incidence was 24% greater for exposed than for unexposed people. In the U.S. over a three-year period around 40% of US children were exposed to at least one medical imaging examination with ionizing radiation. In one example, an 11-year-old girl suspected of having appendicitis was subjected to a CT scan when ultrasound could have been used. It revealed a nodule on her lung (a common and generally benign occurrence in the Middle West). Not only did she have a follow-up chest CT scan but when her family decided against a lung biopsy, the recommendation was for follow-up CTs every two years. She has already received around 20 mSv.
It’s not just children who are subjected to multiple tests. In a 2015 interview with Consumer Reports, Dr. James Duncan, M.D., a radiologist at Washington University in St. Louis, described having a CT scan for possible kidney stones. When the scan was supposedly complete, it instead began again––because the technician running the machine incorrectly thought the first scan didn’t capture images of the top of Duncan’s kidneys. Ironically, he was preparing to give a lecture on reducing radiation exposure from medical imaging.
It’s estimated that in the U.S. alone, around two percent of all future cancers will be linked to CT scans. That’s as many as 15,000 deaths a year. Of course, CT scans may be necessary for you or your child. You and your doctor need to weigh the risks and benefits of a CT scan, just like any other type of procedure. Many doctors don’t discuss radiation risks with patients contemplating CT scans. Some don’t completely understand it themselves. One study revealed that emergency room doctors in particular had “poor knowledge about radiation doses and risks.” Don’t assume your doctor can explain the radiation risk and don’t demand a CT test when other examinations will suffice. MRIs, for instance, don’t rely on radiation for their imaging. The CT scan is a modern marvel, but it should be used with caution.
Written by John Bankston
References
- What is Radiation?
- Atom
- How to Understand and Communicate Radiation Risk
- Long-term Mortality in 43 763 U.S. Radiologists Compared with 64 990 U.S. Psychiatrists
- Computed tomography and patient risk: Facts, perceptions and uncertainties
- Radiation Dose in X-Ray and CT Exams
- Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians
- An Appeal for Safe and Appropriate Imaging of Children
- Doctors’ knowledge of the doses and risks of radiological investigations performed in the emergency department