CT Scans and Radiation Exposure

Abstract & Commentary

By Mary Elina Ferris, MD

Synopsis: Increasing use of CT scans has led to an increase of radiation exposure to its recipients, potentially causing up to 2% of new cancers, especially in children.

Source: Brenner DJ, Hall EJ. N Engl J Med. 2007;357:2277-2284.

CT scans deliver a larger radiation dose than conventional X-Rays, for example 50 times more from an abdominal CT scan compared to a single anterior-posterior flat abdominal X-Ray (KUB). For neonates this dose may be even doubled. Radiation doses to a particular organ are mathematically calculated in theory and depend on a number of factors, including size of the patient and number of scans. Radiation-induced damage to DNA or other molecules is usually repaired rapidly by the body's cells, but chromosomal mutations can occur which can potentially induce cancers.

Most scientific information about radiation-induced cancers comes from studies of 25,000 survivors of the atomic bombs dropped on Japan in 1945, who have now been followed more than 50 years. Although their exposure was in a single dose, rather than cumulative, and involved total body radiation rather than specific organs, their outcomes have been used to determine safety levels for exposure. Their average radiation exposure was equivalent to 2-3 CT scans, and significant increases in subsequent cancer risk have been found. Studies of workers in the nuclear power industry who have been exposed to the equivalent of one CT scan and above also show a gradually increasing cancer risk, similar to the atomic bomb survivors. Growing children are thought to be more radiosensitive since they have a larger number of dividing cells. Data from pediatric atomic bomb survivors show more cancer risk also because they have more years of life to express potential cancers.

Lifetime cancer risk from estimated organ radiation doses have been calculated for a single typical CT scan of the head or abdomen, ranging from 0.08% for a neonate to zero for a 70-year-old. Although individual risk is small, with the widespread use of CT scans in the population it may account for 1.5 to 2% of new cancer diagnoses. Increasing use of multiple CT scans would thus theoretically lead to even more cancers attributable to this exposure.

Commentary

The potential hazards of radiation exposure from CT scans is an increasing concern as the utilization of these scans rises exponentially, from 3 million in 1980 to over 62 million in the U.S. today. In Japan they are utilized even more frequently, where there are 3 times more CT scanners per population than in the U.S. For children, CT scans provide the advantage of the brief time of the procedure (often less than 1 second), which can avoid the need for anesthesia to keep the child from moving during the study. CT scans are also being increasingly used in asymptomatic adults for screening purposes.

As alarming as this radiation exposure may appear, it's also important to note that no large epidemiologic studies linking actual CT exposure to subsequent cancers have yet been done, although some are underway. CT scans can provide crucial diagnostic information and have saved many lives; the authors of this study note that the "use of CT represents probably the single most important advance in diagnostic radiology." Nonetheless, many CT scans may be ordered without clear scientific evidence and as part of defensive medicine. Clinicians need to be aware of the potential carcinogenic exposure from multiple CT scans, and should carefully consider the risk-benefit ratio when choosing CT scans for diagnostic assistance.