Basic guideline: The basic tenet of radiation protection that radiation doses to workers should remain As Low As Reasonably Achievable remains.
Researchers at the Imperial College, London predict a link between low dose radiation exposure and heart disease (PLoS Computational Biology, October 2009). Their mathematical model suggests that the risk would increase as the dose increases.
Cardiovascular effect of high levels of radiation has been known in patients who underwent radiotherapy. Specialists found higher risk for cardiovascular disease among women treated for left-sided breast cancer when compared with women treated for right-sided breast cancer, during the period more than 10 years after diagnosis.
Emerging evidenceThere is emerging evidence of excess risk of heart disease at much lower doses occurring after a long time after exposure among the A-bomb survivors included in the Life Span Study project and in some groups of radiation workers. The likely mechanism to explain the effect was not clear.
The US National Academy of Science’s Committee for Biological Effects of Ionizing Radiation (BEIR committee) in its latest report has acknowledged the existence of such effects; it asserted that the available data are inadequate to quantify the risk, if it exists.
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in its latest report published in 2008, clarified that it is necessary to assess the epidemiological evidence of radiation effects other than cancer at low doses because the phenomena is potentially important for radiation risk assessment at these dose levels, and there is considerable lack of consistency among the available epidemiological data.
“The Committee considered it important to focus on cardiovascular disease as the major endpoint of interest, because cardiovascular disease is among the most common diseases in many populations worldwide and thus may be important for radiation risk assessment,” the report cautioned.
The Imperial College team suggested that radiation kills monocytes (a type of white blood cell) in the arterial wall, which would otherwise bind to monocyte chemo-attractant protein 1 (MCP-1). The resultant higher levels of MCP-1 cause inflammation which leads to cardiovascular disease.
According to the researchers, the predicted consequence of several small radiation exposures is to cause mean concentration of MCP1 to increase linearly with cumulative dose.
“The radiation-induced risks predicted by the model is consistent with those observed in a number of occupationally observed groups,” they claimed.
The implicationsAccording to them, if their model is valid, it will have substantive implications in radiological protection, which currently does not take cardiovascular diseases into consideration.
In an interview to BBC, Professor Steve Jones, of Westlakes Research Institute who formerly worked for British Nuclear Fuels (BNF) and who has published his own research on the links between radiation and circulatory disease in nuclear workers conceded that the results of the mathematical model were interesting. He cautioned that its finding cannot be taken as definitive as it is based very largely on mathematical modelling.
“However it does propose a plausible biological mechanism and, most importantly, a mechanism that is testable by experiment.” he clarified to the BBC.
BBC quoted Professor Richard Wakeford, of the University of Manchester who also formerly worked for BNF, thus: “More research like this is needed if the results of epidemiological studies are to be properly understood, but there is still some way to go before it may be reliably concluded that low-level radiation can increase the risk of circulatory disease.”
According to BBC, Professor Dudley Goodhead, former director of the MRC Radiation and Genome Stability Unit, noted that this paper puts forward a highly complicated mathematical model, which makes many assumptions, to explore one possible causal mechanism.
“Such conclusions should not be drawn without laboratory validation of the key assumptions,” he asserted.
For many years, the model proposed by the Imperial college team may remain just that; a model with no practical implications.
The basic tenet of radiation protection that radiation doses to workers should remain As Low As Reasonably Achievable (ALARA principle) remains.
Scientists from the Regional Cancer Centre, Thiruvananthapuram and Bhabha Atomic Research Centre did not find any excess cancer in the high background radiation areas (HBRA) in Kerala. It will be worthwhile to look for excess cardiac diseases in HBRA.
RAJA RAMANNA FELLOW, DEPARTMENT OF ATOMIC ENERGY