Adaptive Responses to Radiation Exposure in Cath Labs May Protect Physicians

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Exposure to radiation in the catheterization lab induces changes at the cellular level that may contribute to homeostasis, thereby offering some level of protection against radiation-induced genetic damage. However, the researchers and others say it would be wise to make any change in radiation protection protocols based on the small study, published online August 24, 2011, ahead of print in the European Heart Journal.

Gian Luigi Russo, PhD, of the National Research Council (Avellino, Italy), and colleagues compared blood samples from 10 healthy interventional cardiologists (age 38 ± 5 years) with those of 10 age- and gender-matched laboratory and hospital workers who were not exposed to radiation on their jobs.

Compared with their unexposed colleagues, the radiation-exposed physicians had a 1.7-fold increase in levels of glutathione and a 3-fold increase in levels of hydrogen peroxide. Glutathione is an antioxidant that protects against cell damage from oxygen-containing reactive oxygen species, while hydrogen peroxide indicates the amount of oxidative stress caused by such species (table 1).

Table 1. Cellular Response to Radiation

 

Unexposed Group
(n =  10)

Exposed Group
(n = 10)

Glutathione, mM

12.37 ± 1.22

20.61 ± 2.16

Hydrogen Peroxide, µM H2O2 equivalents

2.21 ± 1.03

6.51 ± 1.55


The radiation-exposed interventionalists also had higher levels of caspase-3, both at baseline and following 2 Gy in vitro radiation challenge. Caspase-3 is a downstream effector of apoptotic processes. Increases in caspase-3 suggest a possible defense mechanism for killing off damaged or potentially cancerous cells.

The effective annual dose of radiation ranged from 1.5 to 8.4 mSv, which is equivalent to approximately 200 chest X-rays per year. The lifetime radiation exposure, meanwhile, ranged from 20 to 100 mSv, the equivalent of approximately 1,000 to 5,000 chest X-rays. Although the doses are below the maximal allowable limits set by the International Commission of Radiation Protection, the study authors say their results show that even these low levels “can have clear biological and biochemical effects, as suggested here by the increased level of circulating [reactive oxygen species].”

Mechanism Behind Cellular Changes Unclear

In a press release, Dr. Russo said it is unclear whether the cellular changes are “adaptive, beneficial modifications, or the harbinger of clinically relevant adverse changes, since increased DNA damage, oxidative stress, and apoptotic activity have been involved in the development of a variety of diseases."

But he stressed that the findings do have clinical and scientific implications.

“Interventional cardiologists should make every effort in their daily practice to minimize their exposure, as we know that if there is a radioprotection culture in the catheterization laboratory, then the same activity can be done with a dose reduction of 90% for doctors, staff, and patients,” Dr. Russo explained. “From the scientific perspective, invasive cardiologists today have a unique opportunity to clarify the effects of chronic low dose exposure.”

To address this question, interventional cardiologists are conducting the Health Cath Lab study in Italy, he reported.

In an editorial accompanying the study, Tommaso Gori, MD, PhD, and Thomas Münzel, MD, of the University Medical Centre of Mainz (Mainz, Germany), say the study is interesting but suffers from some clear limitations. These include:

  • Small size
  • Incomplete insight on the mechanisms involved
  • Differences in body mass index
  • Lack of information on other cardiovascular risk factors such as smoking that may have a larger effect on cancer occurrence than radiation exposure

“Although the mechanism of these phenomena is complex and incompletely understood, the production of reactive oxygen species triggered by ionizing radiation appears to play a central role,” they write. “Reactive oxygen species may damage cell structures and, most importantly, DNA.”

The editorial also raises the issue of radiation hormesis—the theory that ionizing radiation, in doses just above those encountered in natural background levels, may somehow spur activation of repair mechanisms that protect against diseases such as cancer. Interestingly, the situation is akin to ischemic preconditioning, whereby exposure to short-term sublethal ischemia actually reduces damage associated with more prolonged ischemia.

Radiation Protection Still Top Priority

Nevertheless, Drs. Gori and Münzel concur that despite the apparent favorable effects of radiation on the cardiologists in the study, keeping radiation levels ‘as low as reasonably achievable,’ or ALARA, should still be the top priority.

In a telephone interview with TCTMD, Charles Chambers, MD, of Penn State Hershey Medical Center (Hershey, PA), agreed.

“This study is an opportunity to raise awareness about radiation exposure in the cath lab and the importance of protecting yourself,” he said. “The ICRP [International Commission on Radiological Protection] is revisiting the issue of eyeglasses to protect against cataracts. So protection is still a very important issue that is front and center for interventional cardiologists as well as others who have daily radiation exposure.

“There have been concerns—legitimate concerns—that we have not been as conscientious in the past about this issue as we should have,” Dr. Chambers added. “But with studies like this and dose-management papers from [the Society for Cardiovascular Angiography and Interventions], I think the awareness is there and we will continue to improve.”

Small But Worthy of Further Investigation

Stephen Balter, PhD, of Columbia University Medical Center (New York, NY), told TCTMD in a telephone interview that while the study is one piece of evidence about the cellular effects of radiation, it is too small and does not include a wide enough range of ages and experience to draw many conclusions.

“The changes, if they are radiation induced, are suggestive of biological protective processes,” he said. “But the controls in this study were not in the cath lab . . . so you don’t know if there was something else in the environment that may have been triggering these effects.” Another possibility, he suggested, is reaction to contrast media.

Dr. Balter said a larger study might help tease out exactly what is happening in the cath lab. But he added that there are certain areas of the world where the natural background radiation is reaching levels similar to those seen in the study, so results may vary depending on who is included and where they live and work.

“The bottom line is we just don’t know what is going on, but this is a reasonable, careful pilot study and it’s certainly worth pursuing,” he said. “It also puts very much into doubt that you can predict with mathematical certainty how many extra cancers will be caused by CT scans, because it’s a very complex interaction between cancer induction by radiation and various bodily repair processes mounted to attack the invasion.”

 


Sources:
1. Russo GL, Tedesco I, Russo M, et al. Cellular adaptive response to chronic radiation exposure in interventional cardiologists. Eur Heart J. 2011;Epub ahead of print.

2. Gori T, Münzel, T. Biological effects of low-dose radiation: Of harm and hormesis. Eur Heart J. 2011;Epub ahead of print.

 

 

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Disclosures
  • Drs. Russo, Gori, Münzel, and Balter report no relevant conflicts of interest.

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