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REEXAMINING THE LINEAR NO-THRESHOLD MODEL

You’ve probably gotten an X-ray or CT scan at some point in your life. Most likely, your doctor referred you, but then you might start doubting his/her competence. How can a doctor consciously send you to get zapped by gamma rays and X-rays, effectively nuking your cells and DNA? You can’t feel or see radiation but just hearing the word you’ll probably start picturing hazard symbols, nuclear disasters and maybe you’ll even hear the faint ticking of a Geiger counter. Is it all really that scary?

A short introduction on the linear no-threshold model

 The linear no-threshold (LNT) model was proposed in the early 20th century and the first government agency to adopt it was the National Academy of Sciences (NAS) in 1956. Ever since that time, the LNT model has been the subject of controversy. Every couple of years a story breaks about how inaccurate, misleading and simply untruthful the LNT model is. Most recently, a team from Loyola University took a stab at debunking this decades old theory.

 To start off, the LNT theory is a risk model used by practically all government health agencies and nuclear regulators to form a strict policy of dose limits for workers of nuclear facilities and the general public. At its core, the LNT theory proposes that cancer risk is directly proportional to radiation exposure, hence the inclusion of linear in the name. The second half of the name comes from the simple conclusion that can be made from the linear progression. If cancer risk is proportional to radiation exposure, then there is no safe threshold for radiation. This is where many biologists, epidemiologists and others chime in to say that there is no way to prove low-dose radiation exposure of <100 millisieverts (mSv) per year causes cancer. We’ll explore this argument a little later.

How does radiation cause cancer?

 Ionizing radiation is a known and well-quantified risk factor for cancer. To fully grasp the LNT model, one must first have a basic understanding of how radiation can cause cancer. The lowest dose of ionizing radiation is one nuclear particle that goes through one cell. Either the nuclear particle goes through the nucleus and damages the DNA molecule, or it doesn’t affect it at all. The most damaging is believed to be double-strand DNA breaks. The fact that radiation is a carcinogen is backed up by studies of humans (epidemiology), studies of plants and animals (experimental radiobiology) and studies of cells (cellular and molecular biology). To understand the health effects of radiation, the information from these sources must be combined and studied.

Competing theories

 Since many believe the LNT model to be insufficient and incapable of properly assessing cancer risk below 100 mSv, several theories have been proposed and tested.

Photo courtesy of the Canadian Nuclear Safety Commission (CNSC)

The hypersensitivity model claims that there are higher risks associated with low-dose radiation compared to LNT. The threshold theory implies that risk is completely absent below certain levels, a bold statement in my opinion. An even more extreme and controversial theory is radiation hormesis, which claims that low-dose radiation is beneficial and may even prevent cancer.

The reader might be wondering if there is any evidence to support the other theories, and the answer is yes. There is evidence to support them, but it’s limited and in many cases it’s either biased or conclusions aren’t supported by the results.

Do we need the LNT?

  All countries with regulatory nuclear agencies and commissions, with a few exceptions, stand by the LNT because it is supported by a mountain of evidence. These agencies base their recommendations and policies on hard science. Even though most of them have at one point or another concluded that the evidence of cancer risk at sub-100 mSv doses is inconclusive, they don’t see a reason to change their stance.

  Is the LNT model really so bad? It has been the tried and true basis for decades of radiation protection measures. If it didn’t exist, there’s no telling what we’d be up to our ears in; solid tumors, ridiculous working conditions for workers at nuclear facilities and possibly more genetic defects than we could ever imagine.

  Anti-LNTers cry out that there is needless spending and unnecessary expenses associated with nuclear energy. But what if take into consideration the rare, yet evident threat of a nuclear disaster, like Chernobyl or the more recent Fukushima meltdown. They could’ve been many times worse if radiation protection was set up according to any of the other theories. Want to jump start your immune system to prevent cancer? In the delusional hormesis world, a doctor would send you to Chernobyl for a rejuvenating radiation cleanse.  If radioactive waste wasn’t disposed of properly, we could have radioactive sludge seeping into our water supply. Doesn’t it make more sense to be as careful as possible?

The experiment that can finally prove or disprove LNT

Because we are constantly exposed to background radiation and other carcinogens, it’s next to impossible to prove that low-dose radiation from say a yearly X-ray or CT increases cancer risk. Although there are studies showing such a correlation, there are as many studies showing the opposite. That’s why in 2006, scientists attending the Ultra-Low-Level Radiation Effects Summit came up with the perfect experiment to test the LNT model. They plan on building a special laboratory that can test the effects of no radiation on lab animals and cell cultures and compare them to control groups exposed to natural radiation levels. 

LNT- Food for thought

The linear no-threshold theory is a theory not a law. Can it be wrong about the effects of cancer risk at low doses of radiation? Of course it can, it’s a theory after all. But it’s simple, accurate beyond 100 mSv and there’s a general scientific consensus that accepts it. Getting an X-ray or CT scan shouldn’t be scary if it’ll have some diagnostic value for you. For those that argue that LNT policies require exorbitant spending, perhaps cutting military expenditure would be more practical. We can argue back and forth all day, but in the end, I believe that keeping radiation levels as low as reasonably achievable should be our priority.

By Dr. Yuriy Sarkisov, BiMedis staff writer

09.03.2016

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