Debate on the linear-no-threshold hypothesis, a pragmatic relationship defining the relationship between levels of radiation and their effects, continues despite us having become better at measuring radiation. Why?
The Merriam-Webster dictionary defines ‘phobia’ as “an exaggerated, usually inexplicable and illogical fear of a particular object, class of objects, or situation.” The Mayo Clinic explains, “A phobia is an overwhelming and unreasonable fear of an object or situation that poses little real danger but provokes anxiety and avoidance.” Since 1980, Fred Culbertson has maintained an alphabetic list of phobias. Presently, it starts with ablutophobia, a “fear of bathing”, and ends with zoophobia, a “fear of animals”. Radiophobia finds a place in the list as well. Its cause is very complex. Regulators, and promoters of nuclear energy and applications of radiation particularly in medicine, face radiophobia as an insurmountable sentiment. In spite of this, they must develop suitable radiation protection standards and enforce them effectively without fear or favour.
Researchers have studied the possible role of radiation in inducing cancer, genetic defects, birth defects, etc., within a few years of the discovery of X-rays by Wilhelm Roentgen in 1895. A 2016 UNEP booklet, which reviewed radiation effects and sources, stated that “by the end of the 1950s, at least 359 early radiation workers (mainly doctors and other scientists) had died from their exposure to radiation, unaware of the need for protection.” They used X-rays carelessly.
Most extensive use of radiation in medicine did not cause any phobia until late the 1980s, when specialists started using computerised axial tomography (CT) units widely. Admittedly, many physicians used this new tool indiscriminately and carelessly. For instance, several papers published during the early 2000s in the American Journal of Roentgenology highlighted the impact of choosing for children and small adults the same technique factors used for adults, leading to unnecessarily high doses. The US Food and Drug Administration had published a special advisory on the topic in 2001.
This notification emphasised “the importance of keeping radiation doses during CT procedures as low as reasonably achievable, especially for paediatric and small adult patients, who may sometimes receive more radiation than needed to obtain diagnostic images. To prevent this, we want to stress the importance of adjusting CT scanner parameters appropriately for each individual’s weight and size, and for the anatomic region being scanned.” The notification recommended measures to reduce the dose without sacrificing CT’s clinical benefits.
Legacy of reckless use
The fact that CT units expose patients to doses that are many hundred times more than conventional X-rays added to the anxiety. During 2012 and 2013, there were claims that some children who underwent CT examinations suffered excess cancers. Though reviewers found glaring shortcomings and challenged their conclusions, extensive media coverage of the bad news persisted.
The wrong notions may tempt patients to avoid clinically indicated and beneficial CT studies. The fear of radiation and consequent careless evacuation of people in Fukushima reportedly caused more deaths than those due to earthquake and tsunami together in the region.
Looking back, I realise that specialists in radiation protection and health physics must share the blame for not arresting radiophobia among the public. We remained in the comfort zone of our workplace, totally isolated from the public. (Disclosure: The author is a former Secretary of the Atomic Energy Regulatory Board, India.) We discussed radiation protection and health physics among ourselves, convincing the already convinced! The discussions remained within the four walls of scholarly seminar halls and learned academies.
We did virtually nothing when the public identified atomic reactors with cancer and a spectre of deformed children. We did not explain the history of radiation protection or the highly conservative and very scientific way we arrived at the radiation protection standards. As government servants, we considered giving public lectures on these topics or writing about them in newspapers to be risky.
Specialists unanimously agree that radiation at high dose-levels can cause cancer. At low dose levels, the results are not conclusive. Most specialists accept the linear-no-threshold (LNT) dose-effect relationship as a practical approach in radiation protection. This means that the dose-effect relation is linear without a threshold; any dose however small will have some deleterious effect. We knew the limitations of the concept. Application of LNT without appreciating that it is only a pragmatic concept to enforce radiation protection leads to unreasonable fear about radiation.
This adversely affects many beneficial uses of nuclear energy, including nuclear power generation. The correctness or otherwise of the LNT hypothesis is now a controversial topic. During 2005-2006, the nuclear regulatory community faced a very serious dilemma.
The word of two important reports
On March 30, 2005, the French Academy of Sciences and the French National Academy of Medicine published a report titled Dose-effect relationships and estimation of the carcinogenic effects of low doses of ionizing radiation. In 2006, the US National Research Council National Academy of Sciences (NRC/NAS), the National Academy of Engineering and the Institute of Medicine published a similar report titled Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2.
The reports showed some important differences.
Briefly: The NRC/NAS asserted that the effect of ionising radiation is proportional to dose, and there is no threshold dose below which the effect is zero. It means that even low levels of ionising radiation may cause harm. The report thus endorsed the LNT hypothesis, which is the cornerstone of radiological protection. The committee did not accept the extreme view that low doses are substantially more harmful than estimated by the LNT model.
On the other hand, the French Academy of Sciences and the French National Academy of Medicine, two equally scholarly bodies, jointly raised doubts about the validity of using the LNT concept to evaluate the cancer-causing risk of low doses. They conceded that the LNT concept is a convenient, pragmatic relationship useful for radiation protection, but not a model, based on data.
“Since it is not based on biological concepts of our current knowledge, it should not be used without precaution, for assessing by extrapolation of the risks associated with low and even more so, very low doses,” the scientists warned.
Unlike the BIER VII report, the report from the French Academies did not attract much media attention.
These reports arrived at different conclusions on LNT concept, seemingly using the same set of peer-reviewed papers on the topic. I brought this issue to the notice of Richard Monson, the chairman of BEIR-VII, and Maurice Tubiana, chairman of the committee that wrote the report of the French Academies.
Evan Douple of NAS/NRC responded thus:
“As director of the Board on Radiation Effects Research that provided oversight to the BEIR VII Report, I received a copy of your email message from Dr. Monson and he asked me to respond to you. You raise an important point in that when two learned academies come to opposite conclusions on an important scientific issue, it is often difficult to decide which is the most prudent decision or conclusion to accept. This is especially difficult for the non-scientific public and the news media. Analyses of the risks associated with the exposures to low-dose ionizing radiation are an example of this and a clear indication that this is an especially difficult and highly technical issue to resolve”.
“There could be several reasons why the two reports might have led to different conclusions”, he added. They are the approach used by the committee, the timing of the report release and the information available or used by the committee, as well as the composition of the committee and the charge given to them. Douple went on to clarify:
“BEIR VII did look at all of the latest biological data that might have some impact on the mechanisms of radiation-induced carcinogenesis. An earlier French report had concluded that those biological phenomena (bystander effects, genomic instability, adaptive responses, hormesis, etc.) must be operative in humans and they concluded that they must result in lowering risks of health effects at low doses. The BEIR VII committee has been following closely the work in this area and did not assume that those phenomenological observations in cell systems under defined experimental conditions necessarily were operative in humans or translated into animals or people at relevant environmental doses and under realistic scenarios. The BEIR VII report contains a detailed description of the phenomena and points out why at this time one should not extrapolate directly from those data to human health effects.”
He then deftly threw the ball back into the regulator’s court: “Ultimately, a policy decision must be made by regulatory and governing bodies and persons such as yourself must make those decisions based on levels of risk that your government is willing to accept.”
The French report identifies with those who believe in radiation hormesis – the concept that radiation exposure is beneficial. The report argued that the present data suggest the existence of a practical threshold. However, the researchers could not define a threshold dose or provide the evidence for it.
The report also targeted a paper titled Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know, which appeared in the Proceedings of the National Academy of Sciences (November, 25 2003). Its authors included virtually all the famous specialists in the field across the world! And it appears that the NAS/NRC committee drew inspiration from this paper.
I sent the extracts of the French criticism of this paper to the lead author, David J. Brenner, Center for Radiological Research, Columbia University. He addressed one major point. Evidently, it appears that there is no agreement when they deal with biological effects at low dose levels of interest in radiation protection. After receiving Brenner’s criticism from me, Tubiana, who chaired the French committee, promised to address the issues in some papers later. He did it and sent me those papers promptly.
In 2011, Edward Calabrese, an environmental toxicologist at the University of Amherst and a hormesis enthusiast, claimed that Hermann J. Muller, a famous radiation geneticist, knowingly lied in his Nobel Prize lecture in 1927. Calabrese believes that Muller thus promoted the LNT concept.
Finally, the balance of evidence gathered by me suggested that the edifice of radiation protection is not built on a lie.
Nonetheless, quibbling on radiation protection principles continues. Specialists can now measure a single double strand break (DSB) in DNA, an event that is likely to have profound influence on carcinogenesis. These unparalleled technological advances may encourage public concerns disproportionate with any harm. Some experiments showed that presence of radiation is necessary for cells’ normal growth as well.
Many specialists feel that they may be able to develop a model based on exposing organisms to near-zero levels of radiation, a model based on sound science.
The results of such research may have profound impact on the practice of radiation protection. It may lead to increasing the levels of radiation considered safe; it may affect the economics of decommissioning nuclear facilities, long-term storage of radioactive waste, construction of nuclear power facilities among others. Then again, this also requires drastic changes in public perception. So, what is the way forward?
A better understanding of radiation risk
“We need to educate the public regarding the importance of ‘acceptable levels of risk’ – levels that are believed to include risks, but risks for adverse effects that are so small that one would not be able to observe and measure an excess of the effects with a realistic study. Only then will the fear and paranoia associated with radiation effects gradually become less and less and sources for energy production can be fairly and objectively be evaluated,” Douple, once the associate chief of research at the Radiation Effects Research Foundation, Hiroshima, proposed in an email response.
This is certainly a pragmatic and acceptable approach. An overemphasis on hormesis as well as oversimplification of some reassuring observations by nuclear enthusiasts may not be appropriate. At least not yet.
The US Department of Energy’s low dose radiation research has elucidated many new concepts. Low-dose responses trigger processes that seem to be protective even as higher doses turn damaging. Moreover, the data from the studies in the low-dose region do not support the LNT hypothesis.
Have we reached a stage in which our radiation protection standards are based on best science? Many extensive studies of tens of thousands of radiation workers show that the impact, if any, of low dose exposures is acceptably low. In spite of being the most studied agent, why public considers radiation differently?
When we use antibiotics, we trust the tests done to study them. We feel assured that, when clinically indicated and the doses are right, benefit far outweighs the possible harm. Scientists must convince the public that radiation risks are better studied than risks from drugs.
On February 24, 2015, an organisation called Scientists for Accurate Radiation Information submitted a six-page petition to the NRC to amend its present Standards for Protection Against Radiation, based on science and evidence that contradicts the LNT hypothesis. NRC has received three more petitions that are similar. and per procedure, NRC collected the views of the public on the petitions. The petitioners wanted the NRC to reject the LNT model in favour of a model based on radiation hormesis. If the recommendation of NRC’s Advisory Committee on the Medical Uses of Radiation on the issue is any indication, the NRC is unlikely to amend the rule now.
Recently, Bill Sacks, a former official of the US FDA, and others published a thought-provoking article severely criticising the agencies that accepted the LNT concept.
The regulatory agencies will stick to LNT as it remains a practical model. The LNT-baiters, including Calabrese and his followers, are not able to give precise threshold doses for various effects. The dose levels to radiation workers achievable are so low compared to the dose limits recommended by the International Commission on Radiological Protection that the risk from them is negligible. And negligible risk is no risk at all. That we cannot rule out beneficial effects of radiation is also a comforting thought. In medically needed radiation procedures, benefits far outweigh the risks. The dose reduction is important but it is of less concern.
Low dose research must continue. We must disseminate to the public correct information on recent advances in radiation biology, including their limitations. Specialists in radiation protection must accept a very proactive attitude and engage with the public and media in radiation safety related matters. We must discuss relative impacts of various sources of energy. A discerning public will make the right choice.
K.S. Parthasarathy is a former Secretary of the Atomic Energy Regulatory Board.