Friday, February 5, 2016

Questioning The Accuracy Of The LNT Model

Diagnostic Imaging

CT Scanner: The LNT model used for the last seven decades to estimate cancer risk is not accurate at low dosages common to medical imaging, say researchers at Loyola University. Catharine Paddock writes for Medical News Today: “The researchers say the model that is used to estimate the potential cancer risk of low-level radiation from medical imaging machines—such as this CT scanner—is wrong and should be abandoned.”
Photo Credit & Source: Medical News Today


An article, by Catharine Paddock, in Medical News Today says that some medical researchers are questioning the validity of a decades-old theoretical model—Linear no-threshold model (LNT)—which essentially says that there is no safe threshold when it comes to exposure to radiation. The model also says that the sum of many small exposures equals one large exposure. In other words, there is a cumulative effect on the human body.

It was John William Gofman [1918-2007], an American scientist (nuclear and physical chemistry), who first advocated for such a model to estimate actual cancer risks from exposure to low levels of radiation. The LNT model, Wikipedia notes “is considered the  foundation of the international guidelines for radiation protection.”

Yet, it ought to be thrown out, at least when it comes to establishing guidelines for medical imaging. says James Welsh of Loyola University’s Stritch School of Medicine. In “‘No evidence that CT scans, X-rays cause cancer’” (February 4, 2016), Paddock writes:
Writing in the American Journal of Clinical Oncology, the researchers describe how the linear no-threshold model (LNT)—first proposed over 70 years ago—is used to estimate cancer risks from low-dose radiation, such as medical imaging.
But—say James Welsh, a radiation oncology professor in the Stritch School of Medicine at Loyola University, Chicago, IL, and colleagues— risk estimates based on the LNT model are only theoretical, and, as yet, "have never been conclusively demonstrated by empirical evidence."
They say persistent use of the LNT model by regulators and advisory bodies leads to unfounded fears and money being wasted on unnecessary safety measures. As a result, many doctors are averse to recommending and using the most appropriate imaging procedures for their patients, and many patients are unnecessarily scared to undergo them.
This was my fear, as well; I, too, had read and been told that CT scans and x-rays have a cumulative effect on the body, increasing the risk of cancer. A recent article in Scientific American says just as much. [see here]. The article points out the benefits of using low-dose scans, where such scans deliver 75 percent less radiation than typical scans while giving sufficiently accurate results. The article also notes that physicians order too many CT scans and as a result patients receive unnecessary doses of radiation. This is a concern. Obviously, ionizing radiation (such as gamma rays) cause damage to the living cells’ integrity and ability to repair itself. It destroys DNA bonds.

A too large dose of radiation can be harmful, and even lethal at high enough levels of exposure, such as when one is in close proximity to a nuclear blast or the site of a nuclear accident. High-dose radiation is anything greater than 500 millisieverts (mSv). As an example, at Chernobyl (April 26, 1986), 134 plant workers and firefighters received high radiation doses (800 to 16,000 mSv), suffering acute radiation sickness: 28 persons died within the first three months.

What can turn lethal when control is lost can be beneficial when sufficient control is maintained. Such is the thinking behind safely harnessing the power of ionizing radiation as a diagnostic tool. The unanswered and debated question is determining a safe dose for medical imaging, and the most important question is determining the cumulative effects on the human body. This requires answering to everyone’s satisfaction, because a fear of over-exposure to radiation will prevent persons from having diagnostic tests that use radiation.

These tests, after all, do save lives, and CT scans are a wonderful diagnostic tool. The benefits over-all outweigh the risks, and one wonders what would happen if the use of CT scans diminished greatly. This is true in my case, where an emergency CT scan (about 10 mSv of radiation, the FDA reports) revealed I had a large tumor in my colon. I have had a half-dozen scans since this initial one a few years ago. Combined, this is well under the threshold of high-dose radiation, thus I should not be overly concerned. To be honest, I am not overly concerned, but I do want to know more. I would like more assurance, if this is at all possible.

The study’s paper reasons that low-dose radiation, which includes and is comparable to normal natural background radiation. does not contribute to cancer. (In Toronto, it is 1.6 mSv/y). The reason is that the human body, Paddock writes, “is able to repair damage caused by low-dose radiation— something that has evolved over millennia in humans and other organisms that are continually exposed to naturally occurring radiation in the environment.”

It would seem rational to think that multiple low doses over long intervals would allow the body to repair itself, something that is not possible with a single high dose. Yet, as much as there is truth in this statement, I continue to find this argument lacking in reassurance for the following reason. The dose that a human gets from a  CT scan (even of the head at 2 mSv, let alone the chest at 7 mSv) is much higher than the annual background radiation in the city in which I reside (1.6 mSv), or for that matter the average annual background radiation of any major city in the world. The Canadian annual average is 1.77 mSv; the U.S. annual average is 3.00 mSv; and in Japan it is 1.50 mSv. The worldwide annual average is 2.4 mSv. 

Again, this is noteworthy. Equally compelling, we still do not have sufficient knowledge on the cumulative effects of low-dose radiation. Although a study published in the journal Nature (June 30, 2015) suggests that even tiny doses above natural background radiation slightly elevate the risk of leukemia. The large-scale study focused on nuclear-industry workers, but the results could be extrapolated to both health-care workers and patients. For this reason and others,  I am not sure that the LNT model ought to be abandoned just yet.

I am sure, however, that this is not the end of the discussion.

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For more, go to [MNT]