Is radiation anxiety more harmful than radiation?

NEU-ISENBURG. Ramsar is a small Persian town on the Caspian Sea and has a special feature: the largest natural radioactive background radiation was measured there in an inhabited area.

It reaches up to 260 millisieverts (mSv) per year, which is ten times more than the maximum permissible annual exposure of workers in nuclear power plants and 26 times more than the exposure of an average CT scan (10 mSv). The reason is hot, radon-containing springs.

Nevertheless, the people in Ramsar do not seem to be doing badly: Nothing is known about increased cancer rates, and the lymphocytes of residents from the particularly polluted districts are apparently also particularly robust.

For example, after in vitro irradiation, they show significantly fewer chromosomal aberrations than those of residents in low-radiation neighborhoods. It seems that they have adapted to the increased radiation exposure (Health Physics 2002; 82:87).

Outdated model

The two radiation oncologists Dr. Jeffry Siegel and Dr. James Welsh from the University of Chicago cite examples like these in order to dispel an old idea: that there is no safe dose of radiation and that every single alpha particle and every gamma quantum ultimately increases the risk of cancer (Technol Cancer Res Treat 2015; online March 30).

This idea, scientifically referred to as the "linear no-threshold model" (LNT), has long since made a career and largely dominates our idea of ionizing radiation, although it contradicts Paracelsus' old insight that the dose makes the poison.

Also, apparently no one really trusts the human body that its cells, which were once formed in the Archean in an environment with ten times higher natural radioactivity, have learned to deal with a certain amount of radiation over the eons.

Such considerations do not justify radiologists neglecting radiation protection: they should not expose patients to more ionizing radiation than is absolutely necessary. But it is precisely on this point that opinions differ: How much radiation is necessary from a medical point of view and how much can be expected of patients?

This question becomes relevant when patients forgo an examination due to a "radiophobia" fueled not least by media reports and die of a tumor that could still have been removed if detected in time.

Doctors may also reduce the dose for fear of too much radiation exposure and thus achieve a suboptimal diagnostic result, which can lead to further radiological examinations and thus "ironically even increase the total radiation exposure of the patients," Siegel and Welsh write.

Ultimately, the big problem is that the carcinogenic risk of low-dose radiation is difficult to determine. For the two radiation oncologists, there is a simple reason for this: such a risk does not exist at all.

In the LNT model, the dangers of high radiation exposure are only downcalculated. "So far, however, there are no clear data according to which low doses (below 100 to 200 mSv) induce tumors," they write.

Limit values for harmful radiation?

In her opinion, the linear model, as used by regulatory authorities and safety commissions worldwide, is simply wrong. They also justify this with data from the Life Span Study (LSS) of Japanese atomic bomb survivors.

The more radiation the survivors had received, the higher the cancer rate - but this correlation only applied to relatively high doses.

However, at doses below 0.2 Gy or 200 mSv, linearity is no longer shown, and this is confirmed in a new analysis of the LSS data.

Below 0.5 Gy, the curve takes on a more concave shape - which, depending on the calculation and reference model used, intersects the abscissa and thus slips into the negative range. However, a negative cancer risk would mean in plain language: Low radiation doses are more likely to protect against tumors.

The LSS data thus supported a hormetic model (J or U curve) known for many other noxious agents, Siegel and Welsh write: With increasing dosages, a health-promoting effect is first observed, and from a certain dose onwards, the risk of damage begins to increase linearly or exponentially.

The two radiation oncologists also lack plausible evidence from biology for the linear model. Although it is true that any ionizing radiation can damage DNA and trigger mutations, this effect pales in comparison to the spontaneous mutation rate in the body at low doses.

On average, up to 30 mutations per cell per year are to be feared due to background radiation, but the spontaneous mutation rate due to factors such as thermal stress and oxidative stress is around two and a half million times higher.

"The point is: Normally, the body can easily deal with this mutation rate through an adaptive response, so a little more radiation should not overwhelm this system," they write.

The experts include DNA repair mechanisms, the production of antioxidants or apoptosis as part of this adaptive response of the body.

Such mechanisms are probably very old and arose at a time when the radioactive background radiation on our planet was much higher. It is unlikely that today's cells have forgotten these mechanisms.

How does the body react to changes?

The two radiation oncologists also reject the idea that individual mutations can trigger a tumor.

Although mutations are necessary to create a tumor, the body's response to the changes, such as the immune system's ability to recognize and eliminate tumor cells, is just as important.

This is made clear, for example, by the increased cancer rate among HIV patients and immunosuppressed organ recipients.

Siegel and Welsh also do not like some epidemiological studies that suggest an increased cancer rate in children with frequent CT examinations: It is more likely that a tumor leads to increased CT examinations, i.e. that there is a reverse causality.

So if one now assumes a limit value for the harmfulness of ionizing radiation, where should it be? In the opinion of the two experts, both the LSS data and the investigations at relatively high background radiation suggest that doses below 100 to 200 mSv per year are harmless.

However, one should not rely on such speculation. In Ramsar, only a very small part of the population is highly contaminated with radon, so there is no need for statistically reliable statements about the risk of cancer.

They would also have liked to see the aforementioned lymphocyte experiments confirmed once again. Since they were initiated by researchers from Iranian nuclear energy agencies, they may not convince everyone.

Researchers must provide facts

However, this is precisely what points to the real problem: the discussion about the risks of low radiation doses moves in ideological trenches. Instead of relying on mathematical models, researchers should provide facts. We finally need to know which radiation doses are dangerous and which are not.

And this does not only apply to radiology: According to the nuclear lobby organization World Nuclear Association, more than 1000 people died as a result of the forced evacuations around Fukushima: These figures refer to an excess mortality among the 160,000 evacuees that cannot be further justified.

They may rightly be doubted, but it would be tragic if more people would actually die from the consequences of radiation fear than from the radiation itself. In Japan, areas with significantly lower pollution than in Ramsar were evacuated.

As the world continues to rely on nuclear energy, we will continue to see exploding reactors with some regularity in the future. We should then at least know for sure how far away from disaster it is still safe to live.