Archives For medicine

Radiation Risks

November 8, 2013 — 1 Comment

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.

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While working on my current grant application, I was astounded by the prevalence of hearing impairment in the United States. Additionally, this begged a question: Is hearing impairment currently underdiagnosed, overdiagnosed, or neither? After perusing the literature, I found the answer to be fairly complicated. While it is believed that presbycusis (age-related hearing loss) is underdiagnosed in the U.S., the prevalence of hearing loss appears to be fairly high in this country when compared with worldwide statistics (over 30 million in the United States, with about 275 million around the world). Is this due to relatively better diagnosis in the U.S., or is there something else going on? Here, I’ll delve into that question, through the following measures:

  • Statistics on hearing impairment of all types in the United States
  • Statistics on hearing impairment of all types in various other countries
  • Comparison of screening and management in these regions

It is helpful to consider these basic data before moving on to determining any real differences between the countries. When discussing rates of change in prevalence or incidence of disease, it is helpful to first determine the effects of diagnostic bias. Nonetheless, I hope readers will leave with the impression that hearing loss is a major problem, one that will become more apparent as our population ages.

For the purposes of this post, remember that hearing loss is defined as a hearing threshold greater than 25 dB, where 0 dB is defined as the sound pressure at which young, healthy listeners hear that frequency 50% of the time. Functional impairment, however, is that which begins to impair the ability to understand conversational sound levels at 50-60 dB. A recent review of hearing loss in the United States estimated that over 10% of the population has bilateral hearing loss (>25 dB HL), and over 20% are estimated to have at least unilateral hearing loss. This staggering statistic increases to over 55% in those aged at least 70, increasing to nearly 80% by the age of 80. With an aging population in the United States, this becomes a major public health concern.

The causes of hearing impairment include genetic, drug-induced, and noise-induced hearing loss. With the increased use of overloud noise, noise-induced hearing loss has become more prevalent over time. However, nonsyndromic and syndromic genetic hearing loss accounts for about 50% of impairments in children. Remaining environmental causes include “TORCH” organisms and other neonatal infections. Nonetheless, the problem is a vast one, an issue that will grow as this population ages.

Considering the vastness of this problem, how well do we screen for it? The answer is that we do a poor job of it. Only 9% of internists offer screenings to those aged 65 and older, and only 25% of those with hearing impairment that could be treated with hearing aids actually use hearing aids. This is a failure in both screening and management. Thus, we must reiterate the prevalence of this health condition and do what we can to improve the current state of underdiagnosis and undertreatment. Thus, to answer the question above, we still do not do a stellar job in our screening of hearing loss.

How do we compare with other countries? Is hearing loss more prevalent in the United States, even though our screening programs are not ideal? It’s actually the opposite. Hearing loss is more prevalent in middle-income and lower-income countries, but the screening there is so poor that the numbers are staggeringly underreported. Compared with the rate of about 10%-20% in the United States, prevalence increases to over 25% in southeast Asia, 20-25% in sub-Saharan Africa, and over 20% in Latin America.  The WHO reports a value of about 275 million people with moderate to severe hearing impairment (note that the values listed above are for mild hearing impairment) and estimates that approximately 80% of this is in less wealthy nations. If we include all those with any type of hearing impairment (including mild, >25 dB HL), the number rises to 500-700 million people around the world (with 30-40 million in the United States). There is also very little information regarding hearing aid use in low and middle income countries (excluding Brazil), since many of these countries tend toward worse management than what we have in the United States.

When discussing the “global burden of disease,” hearing impairment hits the nail on the head. It is a health condition that affects all countries and in much the same way. Though there is a lower prevalence in high-income countries, consider that 1 in 5 people will succumb to some form of hearing loss. We must therefore implement increased standards for screening and management of this condition.

As promised before, I plan to write on topics related to my experience in medical school, graduate school, and the combination of the two. For those who do not know, I am a student in an MD-PhD program (thus the “MudPhud” in the title of the blog). The classic paradigm is one that follows a 2-4-2 model of training. Our particular program follows the following pattern:

  • 2 years of medical school – These are the preclinical years, where we study biochemistry, histology, pathology, physiology, pharmacology, and related topics. It is mostly lecture-based, though our school utilizes a problem based learning (PBL) model. A few graduate school courses are taken in parallel with medical school. 
  • 3.5-4.5 years of graduate school – We then transition to graduate school, where a few courses are taken in the first year of graduate school (third year in the program). After rotating in multiple labs during the previous years, we settle into a lab and perform research for the following years. This ends with the defense of a doctoral dissertation.
  • 1.5 years of medical school – These are the clinical years, where students practice on the wards in each of the required fields. This portion of training culminates in graduation from the medical school and thus the MD-PhD program.
  • After the program – Students take multiple paths, ranging from medical residency to a postdoctoral fellowship to work in industry. Most will go on to residency.

The challenge in the transition from medical school to graduate school is not an easy one. In medical school, one must acquire large quantities of data and share this knowledge at regular intervals (usually on written exams). One could consider it like a very fast treadmill where you do not have access to the controls. The treadmill will continue to push you, but you may feel challenged to keep up. Or you might not feel this challenge. To be honest, I did not find this to be too fast, but the challenge for me was the lack of control over my schedule, from an emotional standpoint. During this time, you build a rapport with a large group of classmates who will later become colleagues. The shared experience of medical school creates solidarity among this group.

In graduate school, things change. You are now on your own, in a place where you are now at the bottom rung once again. It is exciting on one hand, because you can now choose what to study and how to direct your education. On the other hand, you may feel lost. As opposed to a treadmill, this is more like jogging through a forest, where vision is limited. You can take breaks to reorient yourself, and you can move at your own pace. However, it is difficult to know whether you are making progress, how fast you should be moving, or whether you are completely lost. Your friends in medical school are now moving on, and you no longer share the rapport you previously had with them. This creates a distance, and it is often emotionally trying.

For as challenging as the graduate school transition might be, the benefits outweigh the drawbacks. You are now able to study what truly fascinates you. You have control over your schedule, and you determine your own pace. You have access to a vast array of resources, and you can take on additional projects outside of your program. For example, I found myself volunteering with mentorship programs, science fairs, and even with a community clinic. The challenges you face in graduate school make each success far more rewarding than if they were easy. A simple rotation or a year-long research project cannot create the same level of suspense, mostly due to their limited timelines and more structured projects. Failure begets learning. Success begets inspiration.