Is It Time to Move Beyond the Linear No-Threshold Theory?

LNT is a model used in radiation protection to measure or estimate the ionization radiation leading to cancer.

Linear no-threshold or LNT is a model used by researchers, scientists, and health professionals in radiation protection to measure or estimate the ionization radiation leading to cancer.

According to the LNT model, radiation doses above zero can increase the risk of chronic conditions, including cancer and heritable diseases. For instance, the LNT theory or model suggests that smaller radiation doses can cause genetic mutations and cellular damage with increased cancer risk.

Several international scientific institutions, including the International Commission on Radiological Protection, the National Council on Radiation Protection, and the National Academy of Sciences, have acknowledged and endorsed the LNT theory.

graph with linear threshold model

However, scientific bodies also have concerns about the efficiency of this theory of model. On the other hand, scientists can apply the linear-quadratic theory or model to cell killing, mutation, chromosome aberration formation, transformation, and other endpoints.

Today’s article will discuss whether it is appropriate to move from the LNT model and focus on the linear quadratic model when estimating ionization radiation and other biological or radiological functions. Read on!

Are there any Problems with the LNT Model?

Although the LNT model has several applications and advantages, scientists argue that this theory does not focus on human defense mechanisms. For instance, the human body manufactures specific enzymes that play a crucial role in genetic repair with 99.99% efficiency.

Enzymes manufactured by the human body for DNA repair have 99.9% efficiency rates for single-stranded DNA damages. Likewise, it has 90% efficiency rates for damages caused to the double-stranded DNA.

Because the LNT model does not consider the natural human defense system, scientists question its accuracy in determining the effects of ionization radiation. Besides, a growing body of research evidence shows that low-level radiation can stimulate natural cell death, also known as apoptosis.

So, radiation can change the cell cycle timing and disrupt mitosis, a process that divides a cell into two. However, low-level radiation increases the cell cycle timing, meaning the cell can identify the damages caused to its DNA.

It means the damaged cell can initiate the process of apoptosis (programmed cell death) before mitosis. Thus, the cell prevents itself from becoming cancerous. Bear in mind that this is something the LNT theory does not consider.

Many biological response models to radiation suggest that the LNT theory does unreasonable at low doses. For example, according to the radiation hormesis model, cells can benefit from low-level ionization radiation.

Similarly, human cells have an adaptive phase response mechanism, meaning the low-level ionization radiation can condition cellular processes and allow them to develop better responses to higher radiation doses.

Therefore, the LNT model does not align with the natural response of the cell. However, there is a need for more research to confirm whether moving beyond this model is appropriate.

What is the Linear Quadratic Model? Is it Effective?

The linear-quadratic model is one of the most powerful tools in biology and radiology that lets scientists measure the relationship between the delivered dose and cell survival.

Radiation biology scientists and researchers extensively use this model to identify, predict, and analyze cellular responses to ionization radiation in vitro and in vivo, making it a universally accepted model/theory for calculating the effects of low doses on human cells.

The linear-quadratic model is a biological and mechanistic model that provides researchers with experimental parameters analyze ionization radiation. The model can predict cell survival and death rates based on low doses.

Final Words

Several studies show that the linear-quadratic model plays a crucial role in fractioned radiotherapy, allowing scientists to determine the relationship between iso-effective and fractionated doses.

So, you can use this model to estimate efficiency and toxicity after changes in ionization radiation doses per fraction and total dose. Unlike the LNT model, the quadratic model considers the natural defense mechanism of human cells, producing more accurate data than the LNT model.

Yet, there is a need for more research on LNT and the quadratic model before deciding to move beyond the linear no-threshold theory.

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