A population-based cohort study revealed cancer incidence was 24 percent greater in people who underwent CT imaging at least one year prior to diagnosis, according to research published online May 22 in BMJ. The researchers called for increased awareness of the risks and benefits of CT imaging among providers and increased utilization of decision tools.

John D. Mathews, MBBS, MD, PhD, DSc, from University of Melbourne in Australia, and colleagues sought to assess the cancer risk in children and adolescents after exposure to ionizing radiation from CT scans.

The researchers mined Australian Medicare records and identified 10.9 million people aged 0-19 years on Jan. 1, 1985, or born between Jan. 1, 1985, and Dec. 31, 2005. They reviewed national cancer records to determine diagnosed cancers up to Dec. 31, 2007. Average follow-up was 9.5 years.

Mathews and colleagues reported 60,674 cancers with 3,150 diagnosed in 680,211 people who underwent CT imaging. They calculated overall cancer incidence was 24 percent greater for people exposed to ionizing radiation during CT imaging than those who were not exposed.

In addition, they observed a dose-response relationship, with the incidence rate ratio (IRR) increasing by 0.16 for each additional CT scan. IRR also increased with exposure at younger ages.

The researchers acknowledged the possibility of reverse causation, meaning precancerous conditions or early symptoms triggered CT imaging in some patients.

However, the condition that this phenomenon most likely applies to is brain cancer. When the researchers excluded brain cancer from the analysis, the overall results did not change.

Mathews et al estimated approximate risks during the follow-up period per unit dose. With an average effective dose of 4.5 mSv per scan, and assuming all excess cancers except for brain cancers are attributable to CT imaging, they suggested “each Sievert of effective dose caused 0.125 cancers by 31 December 2007 in an average follow-up of 9.5 years.” They noted the likelihood of additional cancer diagnoses as the oldest participants are in their early 40s.

The results do not conclusively prove that CT imaging causes most excess cancers in exposed individuals, according to the researchers. However, they asserted that several observations support their results:

• IRR increased with increasing number of CT studies;
• The larger proportional increase in IRR after exposure at younger ages;
• The larger absolute excess incidence rate for solid cancers in females at 7.59 compared with males at 3.57;
• The correlation between the site of CT imaging and cancer site; and
• The increased risks per CT scan and per unit of radiation dose.

Mathews and colleagues recommended providers weigh the risks and benefits of CT imaging and applauded advances in recent years, including heightened awareness of the risks of CT imaging among radiologists and technical advances that help lower dose.

However, decision tools to assess the need for CT imaging remain underused and awareness of the risk-benefit balance is not universal. “It is time to alert the wider community as well as the non-radiologist physicians who order most CT scans to potential risks.”