Researchers estimate that nearly 2% of future cancers could be related to computerized tomography. It is often said, “What you do not know would not hurt you.” But I have never understood the rationale behind that. In fact, as a doctor, I would argue otherwise that what you do not know can harm you a great deal.

I’m thinking of this in light of recent studies concerning radiation exposure from medical imaging tests like computerized tomography (CT) scans. Many of us don’t know that we’re exposed to ionizing radiation when we undergo a CT scan, that ionizing radiation is a carcinogen or that data links an increased risk of cancer to low-level doses that are commonly used in CT imaging.

CT scans

And while that increased risk may be small, it’s also cumulative over time — a concern for patients who receive multiple scans. The benefits of CT scans in diagnosing disease and saving lives are indisputable.

But, like any medical test or treatment, CT scans entail potential risks that should be balanced against expected benefits. Unfortunately, we’ve paid little attention to the radiation risks.

Putting the risk in perspective is difficult, considering the various yardsticks by which meaningful radiation exposure and cancer risks are measured. But, in broad terms, we can consider the constant background radiation from natural sources that we’re exposed to every day.

While a chest X-ray exposes us to a 10-day dose of background radiation, a chest CT scan delivers about 2 years’ worth. And the average 3-year dose we get from a CT of the abdomen and pelvis more than doubles when the scan is repeated with and without contrast.

It’s important to remember that the increased cancer risk from a single CT scan remains low for most individuals. Still, the risk accumulates with additional scanning, and it constitutes an unnecessary risk if the scan isn’t medically necessary.

That latter point deserves underscoring because about 30% of CT scans performed in the U.S. are unnecessary, according to estimates. And, given that we perform over 80 million CT scans annually, it’s gob-smacking to consider the extraordinary unnecessary risk we’re assuming as a population. In fact, taking this population perspective, researchers have estimated that nearly 2% of future cancers could be related to CT scans.

Given the risks, our causal attitude toward CT scans is surprising. But they’ve become the Big Mac staple of modern medical fare. As a matter of perceived need or convenience, too many doctors order them and too many patients demand them when they aren’t medically needed.

Radiation risk reduction could be pursued through various strategies, beyond the obvious one of reducing unnecessary scans. Another obvious tactic involves minimizing the amount of radiation per scan without sacrificing image quality.

You might expect that radiation doses for optimal CT images are somewhat standardized and predictably applied, given our concerns about risk and our vast experience with CT imaging over four decades. But that is not the case.

In fact, it’s been known for years that the doses used are wildly variable, both within and across institutions. For example, a study in 2009 revealed a 13-fold difference between the lowest and highest radiation doses used for identical CT procedures by four clinical sites within the Bay Area.

Last week, researchers reported on their efforts to uncover what might be responsible for ongoing variations in CT radiation dosing across 151 institutions. Published online in “The BMJ,” they concluded that dose variations were largely due to choices made by individual institutions when setting technical parameters for CT scanning — not to variables like patient size or sex, or the scanner’s make or model.

In other words, patients were being exposed to multi-fold variations in radiation dosing by the happenstance of idiosyncratic CT protocols. The lead author of the study — Dr. Rebecca Smith-Bindman of UC San Francisco optimistically concluded that, given those findings, “A single set of achievable quality standards for radiation dose should be set and applied to all hospitals and imaging facilities.”

Still, no U.S. organization is at the proverbial helm, responsible for collecting or monitoring CT radiation doses. And while California passed a law requiring documentation in the patient’s medical record of the radiation dose used for every CT scan, compliance has been inconsistent.