Scientists at the National Cancer Institute in the US say more research is needed into the effects of low-dose radiation on cardiovascular risk, an area that remains controversial and about which there is too much uncertainty to give reliable estimates of numbers affected, compared to the well documented risks of cancer from radiation exposure.

This the conclusion of a Comment in this week's issue of The Lancet, Drs Kiyohiko Mabuchi, Parveen Bhatti, and Alice Sigurdson from the National Cancer Institute, which is part of the National Institutes of Health in Bethesda, Maryland.

There is evidence that high dose radiation of more than 30-40 Gy, such as that used to treat Hodgkin's lymphoma and breast cancer, increases the risk of heart disease. But what is less clear is the risk for people who are exposed to much lower doses, such as patients with non-cancer diseases like scoliosis and ankylosing spondylitis. While patients being treated for non-cancer conditions have been included in extensive epidemiological research, few of the studies have evaluated the effect of dose-response on cardiovascular as opposed to cancer risk, wrote the authors.

(Gy is short for gray, a unit of absorbed radiation that is equal to one joule of radiation energy absorbed by one kilogram of matter.)

What little evidence there is, suggests there could be a significant relation between low dose radiation and cardiovascular risk, but there are many difficulties because of the problems of excluding other factors that could be involved, the so-called "confounders" that obscure the statistics or make it look like there is a link when there isn't when you remove the effect of these other factors on heart disease.

For instance, there is evidence that surviving patients who were treated with radiation for peptic ulcer showed a significantly increased dose-response related risk of heart disease from cardiac doses of 1.6-3.9 Gy. And another study of workers in the UK nuclear industry also suggested a statistically significant link between exposure to low dose radiation and cardiovascular disease.

The authors suggest there are plausible biological explanations for how low dose radiation might damage cardiac tissue, for example the biological mechanisms that underlie inflammatory processes like atherosclerosis that damage the endothelial cells that line blood vessels could be similar to the effect of radiation-related tissue changes.

The authors conclude that there is too much uncertainty to be able to give reliable estimates of the number of extra heart disease cases that occur because of low-dose radiation and that:

"Further investigations are needed to sort out effects of radiation and confounders in existing and planned studies of radiation-exposed cohorts, and new laboratory studies are needed to explore biological mechanisms for low-dose radiation-related cardiovascular effects."

"The low-dose radiation effects on cardiovascular disease risk are likely to remain challenging and controversial -- even more so than the linear no- threshold arguments for cancer risk that are debated to this day -- but should not be dismissed," they added.

"Can low-dose radiation increase risk of cardiovascular disease?"
Parveen Bhatti, Alice J Sigurdson, Kiyohiko Mabuchi.
The Lancet, Volume 372, Number 9640, 30 August 2008, pages 697-699.

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Sources: The Lancet.

: Catharine Paddock, PhD