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Low-dose, long-term radiation exposure: Does the body adapt for protection?
Avellino, Italy - Biochemical changes in response to radiation exposure among a small group of interventional cardiologists suggests that individuals might adapt to low doses of radiation by upregulating several processes, leading to the release of antioxidants and cellular apoptosis [1]. The data show that radiation exposure at a level considered safe by regulatory authorities can induce a biological and cellular response that might offset the hazards of radiation, report researchers.
Despite the results, which are published online August 23, 2011 in the European Heart Journal, investigators urge caution in interpreting the findings.
"Adaptive response to [low-dose] radiation exposure is an old story that occasionally emerges when papers like ours are published," lead investigator Dr Gian Luigi Russo (National Research Council, Avellino, Italy) told heartwire. "I can tell you that adaptation has been proven in many in vitro studies on cell lines and animal models experimentally irradiated. Many controversies exist, mainly due to the radiation doses or dose rates, to the cell types, or to the species under investigation. It is more difficult to observe this phenomenon in a human population of professionally exposed subjects. For this reason, we must be very careful in interpreting the data obtained."
The study included 10 interventional cardiologists, average age 38 years old, with a median radiation exposure of 4 mSv per year. Russo said this effective annual dose corresponded to an average dose equivalent of 200 chest X-rays per head per year, with a lifetime exposure that ranged between 20 to 100 mSv, or approximately 1000 to 5000 chest X-rays. These doses are below the maximal allowable limits set by the International Commission of Radiation Protection. Investigators measured several markers of oxidative metabolism in plasma, erythrocytes, and lymphocytes and observed an increase in hydrogen peroxide, a biochemical marker indicative of oxidative stress. This increase in oxidative stress, however, was countered by two specific types of cellular defenses, explained Russo. In the 10 interventionalists, glutathione (GSH), a measure of antioxidant response, was increased 1.7-fold compared with age- and gender-matched controls.
In addition, the cardiologists exposed to radiation also had significantly higher levels of caspase-3 activity in the lymphocytes. Caspase-3 is responsible for the activation of cell apoptosis, providing a mechanism to eliminate damaged cells from the body, said Russo.
Don't fear or be sloppy with radiation
With evidence of cellular adaption to chronic low-dose radiation exposure, the investigators caution that clinicians should continue to strictly protect themselves against radiation, with shields and aprons, as well as by limiting patients' exposure to radiation. While the present study shows that biochemical changes occur at the cellular levels, it remains to be determined whether the changes are adaptive behavioral modifications or hint at other clinically relevant pathologies.
"I think that professionally exposed people shouldn't fear or be sloppy about radiation," Russo told heartwire. "They must follow and, where possible, increase the culture of radioprotection. This is the only way to protect themselves and patients until scientists clarify the medical and biological consequences of a chronic exposure (decades) to low-dose radiation." One such study, led by Dr Eugenio Picano (National Research Council, Pisa, Italy), is currently under way. The study is testing the long-term effects of radiation exposure to cardiologists on different clinical end points, including cancer.
In an editorial accompanying the published study [2], Drs Tommaso Gori and Thomas Munzel (University Medical Center, Mainz, Germany) point out that the theory of "radiation hormesis" is not unlike ischemic preconditioning, where a exposure to short-term sublethal ischemia reduces the damage that might be caused by more prolonged ischemia. Like preconditioning, the radiation-hormesis theory suggests that the repair mechanisms observed in this trial, as well as others, not only compensate for the toxic effects of low-dose radiation but also might prevent disease caused by exposure to other risk factors.
Despite this, the editorialists state that such research does not "limit our responsibility toward patients, our colleagues, and ourselves, and we still need to aim at occupational radiation doses at levels 'as low as reasonably possible.'
"The beauty of modern medical images, the personal sense of self-achievement that follows a complex, prolonged interventional procedure, must be balanced by their costs, clinical utility, and risks-not least, that of prolonged operator exposure to radiation," conclude Gori and Munzel.
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