CT Scans and Cancer: What Are the Risks?

The doctor orders a CT scan for your recurring abdominal pain. When you ask about radiation, she assures you that the dose is low and that CTs are safe. But are they? Is there a connection between CT scans and cancer?

Read on to learn about the increasing use of CT scans, the cancer risks associated with even just one scan, and why children are especially susceptible to radiation hazards.

The Real Cost of a CT Scan

Computed tomography, or CT, scans can provide valuable diagnostic information, but at a cost—they deliver ionizing radiation, a known human carcinogen. The radiation breaks chemical bonds in tissue molecules, which frees charged ions that can damage DNA and ultimately increase the risk of cancer.

We are exposed to a small level of background radiation all the time, naturally occurring radiation in our environment from cosmic rays, rocks and soil, and even other living things. Background radiation accounts for around 3 mSv (millisievert, the unit for radiation measurement) per year, per person. To get an idea of how CT radiation compares, consider the following estimated doses delivered for different scans: (1, 2, 3)

• 10 mSv for an abdominal CT
• 10 mSv for a CT colonography
• 12 mSv for a whole-body CT
• Up to 106 mSv for a trauma patient’s CT scans
• 1–10 mSv for a coronary arterial calcification scan

But there are important nuances associated with these estimates. First, radiation doses for typical scans vary widely by medical center. A retrospective cross-sectional study found up to 13-fold variations in radiation dose for commonly performed scans, depending on the type of scan and where it was performed. (And the centers weren’t spread across the country—they were concentrated within the San Francisco Bay Area.) (4)

Second, the dose delivered is not necessarily the dose each organ receives. The amount of radiation absorbed by each organ, taking into consideration its radiosensitivity, is known as “effective dose” and is measured in milligrays (mGy). For example, during a CT angiogram, the breast can receive an organ-specific dose of 51 mGy, while the lungs receive 64 mGy. (5) A mGy is approximately equal to a mSv, which means that the breast tissue can receive an effective radiation dose equal to 17 years’ worth of background radiation during one angiogram.

What’s the link between CT scans and cancer? Are the radiation doses high enough to cause concern? If you have a scan scheduled or are considering one, check out this article for more information on the risks involved with this procedure.

What Are the Risks When It Comes to CT Scans and Cancer?

In an attempt to quantify cancer risks from CT scans, the National Research Council in 2006 calculated that every 10 mSv of radiation exposure increases the lifetime risk of cancer by 0.1 percent. (6) Other studies have calculated that the risk of a single CT scan causing cancer later in life can range from 1 in 10,000 to 1 in 2,000 to as high as 1 in 80. (7, 8)

Some of the factors that influence cancer risk include the following:

• Type of CT scan: As I mentioned above, some scans emit more radiation than others. Scans for a trauma patient, for example, can emit up to 106 mSv.
• Number of CT scans: More scans equal more radiation and a greater cancer risk.

• Which organs are exposed: Some organs, like the esophagus, breast, thyroid, and bladder, are more susceptible to radiation damage, while others like the pancreas and prostate are less so. (9, 10)
• Age during exposure: Damaged cells have more time to develop into cancerous cells in younger patients. The lifetime risk of cancer from 11 common CT scan types for a 20-year-old is about twice as high compared to the risk for a 40-year-old. (11) And the risks are even higher for children (discussed below).
• Sex: In general, females are more susceptible to radiation damage than males.

Currently, the lifetime risk of cancer for any individual is around 1 in 5, or 400 in 2,000. Though it may sound miniscule, the risk of a later cancer as a result of single CT scan goes up to 401 in 2,000. (12) That’s a small, but non-negligible, risk, especially when considering the harms and costs to overall public health. If growing numbers of people are getting CT scans, that seemingly tiny fraction of an increase—from 400 to 401—essentially means an increase in cancer diagnoses across the population.

Many people and organizations believe that this small cancer risk is well worth the benefits of a CT scan, which might be medically necessary if it’s truly the only way for a doctor to assess and treat a life-threatening condition, especially in an emergency. Small doses of radiation might even be beneficial through hormesis, an advantageous response to an acute stressor, similar to the benefit of intense exercise. The body can often repair DNA damage from small doses of radiation, but we don’t know how this ability varies among people of different ages and health statuses. (13)

Risks Are Much Higher for Children

In a large British study of over 175,000 children, those who were exposed to a cumulative radiation dose of at least 30 mGy had over the three times the risk of developing leukemia and over 2.5 times the risk of developing brain cancer compared to children exposed to 5 mGy or less. (14) Multiple CT scans during childhood increase the risks of other cancers, too, in the 10 years following the first scan. (15, 16)

In 2018, a massive meta-analysis reviewed 18 studies and reported that CT scans in children correlated with increased brain tumor risk in a dose-dependent manner, meaning that the risk increased along with increased radiation exposure. (17)

Other points of view argue that several studies failed to properly adjust for confounding factors, and that once adjustments were made for family history, reasons for scan, and other factors, the cancer risk from CT scans in children was no longer significant. (18, 19) Regardless, it’s hard to argue against lowering carcinogen exposure in young children whenever possible.

Overuse and Too Much Radiation: The Danger of CT Scans

Although the FDA oversees the approval of CT scanners, how the scanners are used is largely controlled by each individual medical center. CT scans have become a routine diagnostics procedure, often chosen over lower-quality x-ray images and longer scan times in MRIs. In the ER, doctors regularly order multiple CTs before even meeting a patient.

Recent efforts have been made to reduce unnecessary CTs and lower the average radiation dose per scan. The FDA manages a program called the Initiative to Reduce Unnecessary Radiation Exposure from Medical Imaging. (21) It educates doctors and patients on the risks and benefits of scans, increases patient awareness of radiation exposure, and encourages lower doses and fewer CT scans.

The American College of Radiology Dose Index Registry allows medical centers to compare their CT radiation dose indices to others in an anonymous database. (22) In studies of facilities who consulted the registry data, median dose reductions of up to 30 percent were observed.

Other research has demonstrated that higher resolution scans are not always necessary to correctly diagnose. Two radiologists at Massachusetts General Hospital determined that only one-quarter of the usual radiation was required to correctly identify abnormal growths in lung tissue. (23) Likewise, in pediatric CT scans, only one-half to one-quarter of the common radiation dose may be necessary to achieve the same diagnostic utility. (24) For blunt trauma to the chest in pediatric patients, a CT scan might not even be necessary unless an x-ray is very inconclusive. (25)

Use Your BRAIN

BRAIN, an acronym tool prevalent in the birthing and doula communities, is a great exercise for making any medical decision.

• B – Benefits: What are the benefits of doing this procedure? Will the results change the treatment decision?
• R – Risks: What are the risks?
• A – Alternatives: Are there any alternatives?
• I – Intuition: What is your intuition saying?
• N – Now/Nothing: What happens if we do nothing for the time being? Can the decision be delayed for now?

If you are looking for an alternative, rapid-sequence MRI or ultrasound may be effective choices for you. (26) If a CT is necessary, ask if it is possible to scan a smaller area, reduce the radiation emitted, and/or decrease the resolution. In children, make sure the exposure parameters will be adjusted for their smaller weight and size.

Compared to the lifetime risk of cancer for an individual, a CT scan adds a very small but still significant risk. Obviously, many other factors are at play in determining one’s overall cancer risk, including lifestyle, diet, environmental toxin exposure, and stress. Still, it makes sense to lower your risk where you can.