Radiation Cataracts (Radiologic Technologists)

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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.

Below we describe what a cataract is and what radiation doses are required to induce a cataract within 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.

Astronauts
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.

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.

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.