What are the risks of a CT scan?

In the United States and European nations, Computerized Tomography (CT) scanning is used mainly as a diagnostic tool (i.e., used only when the physician suspects any underlying disease). But in many other nations, the CT scan has also been used as a screening tool (used even in normal healthy individuals without any signs and symptoms to look for disease) in addition to diagnostic test/s.  Since CT scans were previously known to subject individuals to about 100 to 1000 fold higher doses of radiation than thought to be safe each time, it was considered that the exposure to unnecessary CT scans would increase the risk of an individual to cancer. The correlation between radiation exposure and cancer was found after reports of increased incidence of cancer in survivors of an atomic explosion.

Clinical studies show that children who were exposed to multiple CT scans were three times more likely to develop blood and brain cancer.  Also, a study looking at the incidence of breast cancer after a CT-scan of chest concluded that radiation from CT-scans and hormone therapy were the leading environmental causes of breast cancer and advised that women reduce their exposure to unnecessary CT scans.

Understandably, people are wary of any exposure to radiation. What one needs to understand is that there are many misconceptions about radiation risks from CT scans. While studies exist to show the effect of radiation on human health, experts believe that at radiation levels below 100 milliSieverts (A ‘Sievert’ is a measure of low ionizing radiation on the health of humans), the risks of cancer are too low to be directly measured.

What are the risks of medical radiation exposure?

Normally, the average yearly total radiation exposure (background radiation) for a person is about three milliSieverts. At higher elevation, the background radiation exposure can go as high as ten milliSieverts per year. Yet, there are no observed increases in cancer incidence among individuals residing at high altitudes.  Newer generation CT scans can deliver anywhere from 2 to 10 milliSieverts of radiation, depending on the region of the body scanned, the structures that need to be visualized, and the optimal image needed for diagnosis. In addition, there are certain conditions where patients require repeat scans over a period of time. The total radiation exposure from these multiple scans may add up to 20-30 milliSieverts in the majority of cases, but this is still considered a low dose of radiation. In some CT scans limited to certain regions of the body, such as a head CT, the exposure is very low, less than two milliSieverts.

Since children and young adults a have higher life expectancy, one could argue that the radiation dose would accumulate over time, eventually subjecting them to higher doses of radiation. But in a CT scan of a baby, the radiation exposure is about five times lower than that for an adult. However, there are young adults with chronic conditions who may need repeated CT scans. The cumulative exposure might be high in these individuals, and they may have an increased risk of developing cancer, as evident from several studies.

For adults, particularly elderly patients, exposure to radiation, in most cases, need not be considered a risk, since cancer takes many years to develop after the initial event.

Advice to the public about CT scans

The take-home message for the public is that radiation risks from CT scan are minimal if used judiciously and could end up saving one’s life. One should not refuse a CT exam because of the fear of radiation exposure if it can provide meaningful information to help the doctor diagnose and treat the disease.

• Young individuals who may need repeated CT scans should talk to their physicians about using Magnetic Resonance Imaging (MRI) scanning instead of a CT scan. However, while in certain conditions CT is more accurate, in others, MRI yields more accurate results. The accuracy, cost, need for sedation, and timely access should be discussed with the physician, before making a decision.
• Patients should ask their physicians the following questions:
  • Do I really need to have this test?
  • How will the test benefit me?
  • Is there an imaging test that doesn't use radiation that is just as good?
  • What is the dosage of radiation that I will be subjected to with this scan?
  • Can the physician talk to a radiologist and reduce the dosage of radiation?
  • Will I need multiple scans for this condition?
  • On an average, how long does it take to develop cancer?

Note: As stated earlier, cancer takes many years to develop after the initial event. Therefore, at least for adults, especially older patients, exposure to radiation through CT scans should not be something to worry about.

What could a physician do before ordering a CT scan for a patient?

A physician could do the following to prevent unnecessary exposure of patients to radiation (provide peace of mind to patients), without compromising on the quality of patient care:

• Weigh in the risks and benefits of CT scan.
• Try to avoid usage of CT scan as a screening tool in perfectly healthy individuals, especially children and young adults.
• Ask the radiologist whether it is possible to reduce the dose of radiation without compromising on the image quality.
• From January 2016, physicians will be required to let the patients know the total amount of radiation dosage received by the patients during the CT scan procedure.

Written by Dr. Ashish Patil