The latest Life Span Study (LSS-Report 14) of A-bomb survivors by Dr Kotaro Ozasa, Radiation Effects Research Foundation (RERF), and others, published in Radiation Research Journal this year, noted that the risk of all causes of death among the survivors increased with radiation dose over the entire range of doses with no threshold observed. The RERF study supported the Linear No Threshold (LNT) concept which is basic to radiation protection.
This means that harmful effects of radiation increase with radiation dose and even small radiation doses can cause some finite harm. Some scientists challenge the validity of LNT concept.
Writing in Radiation Research (on-line July 20, 2012) Mohan Doss, Brian L. Egleston and Samuel Litwin, Fox Chase Cancer Centre, Philadelphia argued that the functional forms the RERF authors chose for dose dependence, were not flexible enough and might have led them to the conclusion of a zero-dose threshold.
They showed that there is too much variability in the data used by the RERF authors to suggest that the threshold for the harmful effect of radiation is zero.
RERF researchers observed that the radiation risk estimates for intermediate doses were lower than those for the linear model. Professor Doss argued that this observation is consistent with radiation hormesis or ‘beneficial’ effect of radiation (Dose-Response, 2012).
He noted that RERF’s formalism ignored the potential for a large systematic bias in the measured baseline cancer mortality rate. He showed that if we correct the bias, the excess relative risk for intermediate doses can lower to negative values.
Whether low dose of radiation will cause harm or not remains controversial. The U.S. National Academy of Sciences and The French Academy of Sciences do not agree on the matter.
The Answers
The biological mechanisms of repair at dose levels of a few mSv proposed by scientists appear to be at best a guess work. So how can we rely on the theory that cellular repair will be absolutely error free?
“A presumption in such a concern is that but for the low dose radiation we would be fine, and would not have any cellular damage. This presumption is far from the truth, as the natural cellular processes do lead to a certain amount of cellular damage all the time,” wrote Prof Doss in his email.
“The increased defences triggered by the low dose radiation not only repairs the damage from the low dose radiation, but also prevents damage that would have occurred naturally in the subsequent period, while the defences are elevated.
Thus, the total damage that occurs (from low dose radiation and from endogenous causes) is much less than what would have occurred from endogenous causes alone.”
But if a few mis-repaired or un-repaired cells survive, can they not develop into a clone of malignant cells? Is there any conclusive evidence that it will not happen?
“Low dose radiation boosts the defences and would get rid of many more of the naturally transformed cells compared to no radiation. Thus, the net result is reduced mutations from low dose radiation,” he noted.
The most importantly, the radiation dose below which one need not have any concern about (harmful) radiation effects is not clear.
For instance, the report of the French Academy of Sciences stated that “on the basis of our present knowledge, it is not possible to define the threshold level (between 5 and 50 mSv) or to provide the evidence for it.”
“Based on the atomic bomb survivor data, I would not be concerned below ~300mGy instantaneous dose.
The threshold for increased cancer is probably at about 600 mGy) or higher for instantaneous dose, so that leaves plenty of margin.
(mGy and mSv are the same for x rays, gamma rays and electrons),” he stressed. “If the dose is given gradually over a period of time, the threshold would be much higher.”
But followers of the International Commission on Radiological Protection (ICRP) may shudder at this response as ICRP dose limit is 20mSv per year averaged over five years with no dose in any year exceeding 50 mSv.
Professor Doss conceded that the dose limits for radiation protection to be enforced by regulators will need to be set based on further study.
K.S. PARTHASARATHY
Former Secretary, Atomic Energy Regulatory Board
(ksparth@yahoo.co.uk)
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