Exposure to ionizing radiation at high doses causes cancer. But there is uncertainty at low doses typically received by radiation workers and patients undergoing diagnostic radiation procedures. To reduce this uncertainty, any study must include a very large number of exposed individuals over a long period of time. Such a landmark study of 3,08,297 radiation workers employed in France, the UK and U.S., published online in The Lancet Haematology, (June 21, 2015) provided strong evidence of positive association between protracted low-dose radiation exposure and leukaemia.
In the WHO International Agency for Research on Cancer (IARC) coordinated study, they used the radiation dose due to external exposure to individual workers who wore dose measuring devices during their service. The study followed them up to 60 years after exposure. The average annual dose was 1.1 mGy.
The Nature on June 30, 2015 stated that the study has now provided the strongest support yet for the idea that long-term exposure to low-dose radiation increases the risk of leukaemia, although the rise is only minuscule.
“The finding will not change existing guidelines on exposure limits for workers in the nuclear and medical industries, because those policies already assume that each additional exposure to low-dose radiation brings with it a slight increase in risk of cancer,” Nature added
“The health risk of low-dose radiation is really very tiny, but the public is very concerned,” Bill Morgan told the journal. He heads a systems-biology programmed in low-dose radiation at the Pacific Northwest National Laboratory in Richland, Washington, and chairs the committee on radiation effects at the International Commission on Radiological Protection (ICRP) in Ottawa, Canada.
“It is a solid, unusually large study of individuals exposed to very low doses of ionizing radiation,” Nature quoted epidemiologist Jørgen Olsen, director of the Danish Cancer Society Research Center in Copenhagen.
The study drew flak for some of its limitations.
In his comments published on line in Nature , Dr Mohan Doss Associate Professor, Fox Chase Cancer Center , U.S., pointed out that the authors did not take into consideration the increase in medical radiation dose (e.g. from CT scans) that occurred during 1944-2005, the period of the study. The annual per capita medical dose increased from 0.25 mSv in the 1960s, 0.5 mSv in 1980, and 3 mSv in 2006.
“They have ignored medical radiation dose, which would be larger than the occupational dose (or be of similar magnitude). Therefore, their conclusion should be dismissed,” he argued.
In an accompanying comment , this writer conceded that Dr Mohan Doss raised a valid point.
“When the absolute magnitudes of doses to individuals are very small, variations in dose components can distort the picture. Medical radiation dose is a variable component over the past decades. To a lesser degree, contribution from natural radiation doses also matters. The variations of this component over time may not be significant. But differences from country to country and those in the same country are considerable,” this writer added
According to United Nations Scientific Committee on the Effects of Atomic Radiation, overall, natural background variation is shown to vary by a factor of 13.
Ideally, the researchers must compute radiation dose for each individual worker. Obviously this is virtually impossible. Conclusions drawn from epidemiological studies involving tiny doses suffer from this inescapable limitation.
The present study questions the existence of a threshold dose. As the absolute risk is very small, it is unnecessary to search for a threshold.
Scare mongers may use the study. A specialist may appreciate the nuances; the lay public will not realize that the rise in risk is only minuscule. For instance, Dr Jørgen Olsen, estimated that each accumulation of 10 mSv of exposure raises a worker’s risk of leukaemia by 0.002 per cent.
Thus far, scientists are not able to clearly draw the dose response relation in the low dose region of a few mSv to a few tens of mSv. In any case, this limitation is not serious in view of the low risk. With the available knowledge, the stakeholders which include scientists on either side of the aisle will have to take decisions based on acceptable risk as is the case in any activity.
ksparth@yahoo.co.uk
