Cataracts are formed when cells are damaged or die in the lens of the eye. The dose level which leads to cataract induction depends on the dose fractionization, ie. how many weeks/months was the dose delivered. The radiation dose which has been shown to cause cataract is between 2mGy (single fraction) -5.5 mGy (dose distributed over 3 months or more). The ICRP has an even more conservative estimate of the threshold that is currently 0.5 mGy.
One of the possible areas of harm from radiation is an increased risk of forming a cataract within the eye.
Cataract are very common for older individuals. Relatively high levels of radiation lead to cataracts induction, in a deterministic process.
Below we describe what a cataract is and what radiation doses are required to induce a cataract within the eye.
Anatomy of the Eye
Within the eye the light passes through a lens where it is projected onto the retina. The image from the retina is then transferred via the optic nerve to the brain to be interpreted.
It is the lens itself that we will focus our discussion on here. If there is anything opaque inside the lens, this will reduce our ability to have clear vision. The definition of a cataract is anything that is opaque within the lens.
Cataracts are formed when cells are damaged or die within the lens, as there is no means to clear waste within the lens. This is because the lens does not communicate with the liquid surrounding it.
Cataracts can become more severe over the time. Thus, having a simple scoring system is useful to classify the progression of a given cataract. This rating system is on a scale of 1 to 4. In the case of 1, there are small cataracts present on the back of the lens (posterior side). With level 2, some anterior is being formed of opaque regions. In case of level 3, these regions become larger and the most severe cataracts of level 4 have very significant coverage both of anterior and posterior.
Cataract as Deterministic Radiation Effect
Cataract is an example of a deterministic effect of radiation. In the section above we have a comparison discussion of deterministic vs stochastic effects. Remember that there is a lower dose below which a deterministic effect will not have any impact.
So if the doses are low, the deterministic effects will not take place. Therefore, with very low radiation doses, there is not an increased risk of cataracts induction.
The available data for cataracts induction risk comes from external beam radiation therapy patients where a significantly higher dose of radiation is delivered compared with diagnostic imaging procedures (see Figure).
One of the important aspects of radiation therapy treatment is the dose fractionation (i.e. how the radiation dose is distributed over multiple treatments).
If the treatment was given in just one dose, then cataract was observed if the dose was increased above 2 Gy.
When giving the treatment over many different fractions in a period that’s three weeks to three months, then the lowest dose that could cause cataracts there would be 4 Gy, but in some people you could get up to 10 Gy without causing a cataract.
Finally, when doing a longer fractionated treatment over three months, the minimum dose that can cause a cataract was 5.5 Gy. And for some individuals they can receive a dose of up to 10.5 Gy before developing a cataract.
This is a prime example of the desire to perform fractionization in radiation therapy treatment plans as the healthy tissue can be spared to a significant extent when separating out the dose in multiple fractions.
Cataract Latency Period
In the figure above a demonstration of a deterministic curve is shown. In this case, the increased severity is really measured in a decreased latency time. So as the radiation dose is raised cataracts will be induced sooner.
If the radiation dose is within 2.5 to 6 Gy, then on average it will take eight years before that cataracts will form. And with a radiation dose of 6.5 to 11.5 Gy, it will take only four years (i.e. half as much time) for a cataract to develop. Again, in this deterministic behavior, the increase in severity here is actually the reduced latency.
Astronauts are exposed to significantly higher radiation doses than us common earth dwellers. This is because the atmosphere protects us from radiation.
Astronauts get high doses of heavy ions (such as neutrons) that led to cataracts in many astronauts.
One thing to note is that the RBE for neutrons and for heavy ions is between 10 and 50.
Since the eye is made mostly out of water which is effective at stopping neutrons and these heavy particles are more damaging there is a higher likelihood of cataracts induction. This is just one of the potential dangers of being an astronaut.
Cataracts in the context of diagnostic radiology
Finally, the most important aspect from our standpoint is the impact of diagnostic radiation doses on cataract induction.
The radiation dose from a diagnostic exam which follows the clinical standard of care is below the dose which can induce a cataract.
As we discussed above for cataract induction, a single dose of radiation must be more than 2 Gy. For instance for a non-contrast head CT the ACR upper limit is 0.08 Gy or 80 milli Gray.
Then a relatively higher dose CT exam is called a head perfusion exam where the CT scanner goes around the head several times and takes multiple time frames. And that exam has an upper limit of 500 milli Gray or 0.5 Gy.
You can see that both the standard head exams and even the perfusion head exams which are at the higher end of the radiation dose from a CT scanner, are below the 2 Gy level. The radiation doses for diagnostic radiology exams are below the threshold for cataracts induction from the studies on Radiation Therapy patients presented above.
Rad Take-home Point:
- Radiation that induces cataracts has deterministic effect
- Cataracts have latency period that is related to the radiation dose
- Radiation dose of diagnostic exams is lower than the dose of cataracts induction from radiation therapy studies
More Recent Recommendations/Studies
The data presented above was from the early 1970s and a deterministic relationship for cataracts has been the accepted model with a threshold of multiple mGy as described above.
Recently the ICRP lowered the estimated acute threshold dose to 0.5 mSv. However, this lowering is this is not universally believed to be accurate based on a review of the literature in Health Physics .
There has also been an article in American Journal of Epidemiology suggesting that there is an elevated risk of cataract induction in Radiologic Technologists from the period of 1983-2004. This study incorporated other risk factors such as smoking, BMI, diabetes, etc into the analysis. This epidemiological study contradicts the data above and suggests that there is no lower threshold for cataracts induction (more similar to a cancer induction model).
While there is not a clear consensus on the best way to model cataracts induction it is suggested as always to keep the radiation doses in diagnostic exams As Low As Reasonably Achievable (ALARA) and to use best practices as radiographers/ radiologic technologists to reduce your x-ray exposure.