A recent discussion with a colleague on the Neurodome project centered on the acquisition of data by computed tomography (CT). Specifically, we sought volunteers for a non-medical imaging study. Volunteers were difficult, if not impossible, to obtain. Not only did we hope to find a person without cavities or implants, but we needed someone who was willing to be exposed to a certain dosage of radiation. Our conversation rapidly evolved into a treatise on radiation exposure and health risks. CT, which exposes patients to X-rays, indeed carries a certain health risk. What are these risks? How significant are they? I’d like to attempt to answer some (but not all) of these questions here.
To properly elucidate health risks related to cancer exposure, we must answer a number of important questions. What is the person’s health status? Does this person have any underlying genetic mutations? What type of study is being performed (that is, what is being scanned and for how long, along with the width/shape/angle of the beam)? For the purposes of this discussion, let us consider an otherwise healthy human undergoing standard scans.
In the links I provide below, you will note two units of radiation exposure. I’d like to clarify these so that you can more easily explore the topic independently from this post. The first unit, the gray (Gy), measures energy per kilogram. The second, the sievert (Sv), measures absorbed dose per kilogram. This is rooted in models by medical physicists that attempt to adjust for the effects of radiation dependent upon tissue type. In these models, you might be surprised by what is most likely to cause cancer after radiation exposure. While DNA damage is not insignificant, the major contributor is water. When water molecules absorb ionizing radiation (all radiation in this post is ionizing, unless otherwise specified), they emit free radicals (usually OH-), which in turn have damaging effects. Thus, the amount of water in a tissue is often related to a higher damaging dose per unit energy.
How much radiation are you exposed to in a given period of time? This handy chart should answer most of those questions. You might be surprised by the amount of exposure from certain activities. A chest CT is ~7 mSv, which is not much greater than the 4 mSv exposure from background radiation in a given year. As a former resident of central Pennsylvania, I was surprised to see that radiation exposure from the Three Mile Island incident resulted in an average of only 80 µSv. On the other hand, workers at Fukushima were exposed to a dose of 180 mSv! These doses are interesting from an academic point of view, but what real risks do they carry?
When we talk about radiation exposure, health risks come in two flavors. The first, deterministic effects, are those that result from a cumulation of radiation exposure. Below a certain threshold, adverse effects are minimal or non-existent. Above this threshold, health problems arise. The threshold differs between people and the health condition we are considering. Examples include hair loss, skin necrosis, sterility, and death. The second, stochastic effects, are those effects that have an increased probability of occurring with increased radiation exposure. The best example of this is cancer. With low exposure, one has a lower risk of cancer. With high exposure, this risk increases. We often consider this to be a linear relationship, in that a unit increase in radiation exposure results in a unit increase in cancer risk. For example, 100 mSv of radiation, increases one’s lifetime risk of cancer by 0.5%. Unlike deterministic effects, there is no threshold associated with stochastic effects. There is controversy over the linear model of cancer risk, and more research is needed.
An example against the linear model of cancer risk is exposure to radiation at high altitudes. Though this differs from a CT scan in many ways, one would still expect an increased risk of cancer to be associated with exposure to radiation at higher altitudes. However, those who live at high altitudes or those who work at high altitudes (like commercial airline pilots) do not exhibit a greater prevalence of cancer. To put this into perspective, a single round-trip flight across the continental United States results in the same radiation exposure as a chest X-ray. This begs an interesting question: How much risk do medical scans carry?
The answer, as you can see, is fairly complicated. If you want to know how much radiation exposure a particular study carries, there’s a great resource to calculate this. This website assumes the linear threshold hypothesis to be true and, as I pointed out, it very well might not be true. That being said, any stochastic risks associated with medical scans are often far outweighed by the risks of ignoring a medical condition. In the case of Neurodome, the opposite is sadly true.
That being said, I’d be a happy volunteer.