LET (Linear Energy Transfer), RBE (Relative Biological Effectiveness) and OER (Oxygen Enhancement Ratio) are important terms in radiation biology and relate to the relative damage that will occur with radiation under different circumstances. As LET increases there are more energetic electrons deposited closely together and thus damage to DNA is more likely. Therefore, even with the same radiation dose a high LET radiation will cause more damage (x-rays are low LET; alpha particle and neutrons are high LET). RBE is a measure of the damage that will occur in comparison with x-rays for the same radiation dose, and high LET radiation will have high RBE. Oxygen enhancement ratio (OER) is the ratio of the radiation dose needed to cause the same biological damage when there is oxygen absent to when there is oxygen present. For more information on these radiation biology terms see the video below and the rest of this blog.
Linear Energy Transfer (LET) and Relative Biological Effect (RBE)
Energy released in a given distance is called Linear Energy Transfer (LET) that measures change of energy per change in distance.
Different types of radiation have different LET. Radiation types with high LET deposit more energy locally, i.e. over a very short distance. When large amounts of energy are deposited very locally this leads to increased biological damage to human tissue.
X-rays are low LET so fewer electrons generated that will cause radiation damage in a given distance compared with other high LET sources like neutrons or alpha particles.
In radiology departments, typically we’re primarily concerned about x-rays, but in other disciplines there are also concerns about the relative radiation damage effects for other radiation sources such as neutrons, alpha particles, and protons.
This info-graphic summarizes LTR, RBE and OER, and more detail on each is given below.
In case of high LET radiation, these particles deposit more energy over shorter distances. This has a very dramatic effect for causing damage to the DNA that effects the organs within the human body.
On the other hand, photons usually deposit their energy more gradually over longer distances. Thus, photons are less damaging at the same radiation dose level.
Since x-rays have lower LET they require higher radiation dose to have the same damage as higher LET radiation like α particles.
The way that this effect is quantified is a concept called the Relative Biologic Effect (RBE). RBE is the relative biologic effect. In this case, we’re talking about the biological damage from different types of radiation.
The reference for RBE is x-rays from a 250 kVp source. Based on what we discussed above with LET we know that heavy particles will deposit energy more locally and cause more damage to the DNA.
We note that the RBE is dependent upon many factors such as the fractionization of the radiation, dose level, end-point quantified for the damage (e.g. LD-50), and the biological specimen (i.e. animal cells may differ significantly from plants).
That being said, the most important point for RBE is the LET of the radiation.
In this figure we can see an example (i.e. made up) plot of the radiation damage due to α particles compared with x-rays.
It is clear that given the same dose the α particles are more damaging than x-rays.
The x-rays are used as a reference when calculating the RBE.
If we use this example data and we use LD-50, we can see that the RBE for α particles is about RBE=6/3=2. This is just a sample RBE calculation, and in reality, the RBE for α particles can vary and can be much higher than this.
When radiation dose is calculated there is a conversion factor based on the type of radiation. These radiation weighting factors convert from equivalent dose to effective dose.
The difference in radiation damage for the same dose described here by the LET and RBE is the reason that weighting factors are needed for different types of radiation.
Since x-rays are low LET and used as the reference for RBE calculations it is clear why the weighting factor to convert from equivalent dose to effective dose is just 1.0.
Rad Take-home Point: Linear Energy Transfer (LET) measures the change of energy per change in distance. X-Rays are low LET and α-particles are high LET. Relative Biologic Effect (RBE) provides a method to compare how damaging different types of radiation are, given the same dose, and it is calculated using x-rays as a reference type of radiation.
Oxygen Enhancement Ratio (Oxygen Effect)
When x-rays go through the human body, the primarily interactions are via the Compton and Photoelectric effects. These interactions both produce energetic electrons that can cause damage to the body.
Damage to the DNA is caused by direct action of the electrons, or indirect action of Free Radicals as discussed above. About two-thirds of the damage is caused by indirect action.
In cases when free radicals are causing the breaks in the DNA, those breaks can be repaired.
The breaks can be repaired more easily in the case that there’s no oxygen nearby. When there is local oxygen, the damage cannot be repaired as easily and thus is said to be fixed.
We can look at survival fraction curves as we discussed above and compare with and without oxygen. Example curves for the survival fraction as a function of the dose are shown in this Figure.
Typically, what we’ll do is take a look at the LD50 (Lethal Dose to 50% of the population). Here we have some sample numbers on the curves just to get an idea of the meaning of Oxygen Enhancement Ratio (OER).
In this example, in a well-oxygenated environment it takes about 3 Gray to cause death in 50% of the cells. Then in a low oxygen environment, it takes about 6 Gray.
Next we can calculate something called the oxygen enhancement ratio (OER). This oxygen enhancement ratio is basically just saying, “What’s the difference due just to the fact that there’s oxygen locally near the DNA?”
We look at the LD50 in air, and the dose needed without oxygen (that condition is called hypoxia). Then we take the ratio of that to the dose needed with oxygen.
For the example given here the dose needed in hypoxia to the dose needed in air is nearly 6/3. So, the oxygen enhancement ratio for this case is 2.
Therefore, in this example case roughly twice as much damage takes place in the case of a well-oxygenated environment.
Rad Take-home Point: There is more permanent damage from free radicals in well-oxygenated environments as the repair mechanisms are not as effective.
This is quantified as an oxygen enhancement ratio (OER) that is typically greater than 1.
| Radiation Biology Abbreviation | Full Term | Meaning | Impact |
| LET | Linear Energy Transfer | Higher LET more electrons generated close together, that can damage DNA. | Higher LET more DNA damage. X-rays have low LET. |
| RBE | Relative Biological Effect | RBE is used to account for radiation sources with difference LET, where x-rays are the reference. | X-rays are the reference case and alphas and neutrons will have higher RBE. |
| OER | Oxygen Enhancement Ratio | The ratio of dose needed without oxygen, to the dose needed with oxygen to have the same effect | Radiation dose will have more impact if there is local oxygen near damage site. Research has been conducted for radiation therapy manipulated OER, but not significant impact for x-ray radiography or CT exams. |