Answer to Question #672 Submitted to "Ask the Experts"

Q

I will be receiving a full-body CT scan next month and would like to do everything possible to lessen the effect of the radiation exposure. I've read that certain antioxidants, specifically melatonin and green tea extract, can somewhat protect against some forms of radiation. In addition to taking these before the CT scan, are there other things I can take or measures I can take to help protect from cellular damage ?

A

I think that the effect of green tea extract that is being presumed here is based on the idea that it contains compounds that act as free radical scavengers. It is true that when radiation damage is produced in biological materials (that is, in radiation therapy, where the object is to kill tumor cells) roughly two-thirds of the damage is done by free radicals. Free radicals are highly reactive chemical species that are produced by ionizing radiation as it interacts with water, fats, and some other molecules found in living systems. A free radical scavenger is one type of pharmaceutical known as a "radioprotector." Radioprotectors don't provide full protection from radiation, but the best ones may increase the dose required to elicit an effect (such as reddening of the skin) by as much as a factor of three. That is, three times as much radiation would be required to achieve the same effect when the radioprotector is present in the system compared to when it isn't present. However, the discussion above assumes that there has been measurable damage. Computed Tomography doses are on the order of a few rem (tens of millisieverts) or less. Some of the best (most reliable in a statistical sense) data on radiation bioeffects from single exposures come from studies of the Atomic Bomb survivors. Even at much larger doses than your scan will produce, the probability of induction of cancer by radiation is still small. Of 86,000 survivors followed from 1950 to 1990, about 400 cases were found beyond what would have been expected from natural incidence in a population. That may not sound small, but the natural incidence of cancer in a population of 86,000 is about 30,000. The average dose to this population was 200 rem (2000 millisieverts). The Atomic Bomb survivor studies revealed no excess cancers in subjects who received less than about 35 rem (350 millisievert). The CT dose is well below that level. No study of patients who have received CT scans has ever shown an increase in cancer or, indeed, any effect of radiation of which I am aware. So, I think that there is no point in administering radioprotective agents. If there were, we would do it in diagnostic radiology. The problem is that most radioprotective agents become toxic when used at concentrations that are high enough to have a significant effect. Astronauts carried a radioprotective compound to the moon in the 1970s. The agent they carried had a risk of producing dangerously low blood pressure. However, it would have been taken if a solar flare occurred while they were there. If that had happened, they would have received doses of ionizing radiation (mostly from high-energy protons) that would have been close to lethal doses. It certainly wouldn't hurt to have some green tea. I'm a big fan of it myself. I suspect that it won't have much effect—except for that pleasant taste, soothing effect, and also that it seems to alleviate my sinus problems. Our bodies already contain sulfhydryl compounds, which are free radical scavengers. So, in a sense, we are already somewhat "protected." An intentional ingestion of enough sulfhydryl compounds to produce a noticeable increase in radiosensitivity is usually accompanied by toxic side effects. Major research programs to investigate these and other agents were carried out by Walter Reed Army Hospital beginning shortly after World War II to try to come up with agents that could protect soldiers from the effects of nuclear weapons. I'm not familiar with the action of melatonin in regard to radiation effects. It may be that there is some confusion here with regard to the effects of visible light on biorhythms, which I believe are influenced by the levels of melatonin present in the body. I don't know about toxicity issues related to melatonin, but I doubt that it would significantly change the risk of low-dose radiation. The real issue here is benefit versus risk. If there is a potential medical benefit to be had as a result of information obtained from your CT scan (something that you could discuss with your referring physician or the radiologist to whom you have been referred), then it probably exceeds the small, hypothetical risk of the radiation. If there is no benefit, then even though the risk is small, the risk would be unacceptable. That's a discussion you should have with a physician. E. Russell Ritenour, Ph.D.