The Million Person Study: Whence It Came and Why

A screenshot of the article “How Sound is the Model Used to Establish Safe Radiation Levels?” on the website physicsworld.com.

Last fall, physicsworld.com published an editorial by Robert Crease asking “How Sound is the Model Used to Establish Safe Radiation Levels?” This question is addressed in Chapter 16 of Intermediate Physics for Medicine and Biology, and I have discussed it before in this blog. Crease begins

Ionizing radiation can damage living organisms, that’s clear. But there are big questions over the validity of the linear no-threshold model (LNT), which essentially states that the risk of cancer from radiation and carcinogens always increases linearly with dose. The LNT model implies, in other words, that any amount of radiation is always dangerous and that zero risk is present only at zero dose.

Crease notes that alternative models are the threshold model in which there is a minimum dose below which there is no risk, and the hormesis model which says that small doses are beneficial by triggering repair mechanisms. He explains that by adopting such a conservative position as the linear no-threshold model we may cause unforeseen negative consequences.

What sort of negative consequences? One of the most urgent and dire health hazards faced by humanity is climate change. Addressing the danger of a warming climate, with all its implications, must be our top priority. Climate change is caused primarily by the emission of greenhouse gasses such as carbon dioxide that result from the burning of fossil fuels to generate electricity, warm our homes, power our vehicles, or make steel and concrete. One alternative to burning fossil fuels is to use nuclear energy. But nuclear energy is feared by many, in part because of the linear no-threshold model, which implies that any exposure to ionizing radiation is dangerous. If, in fact, the linear no-threshold model is not valid at the low doses associated with nuclear power plants and nuclear waste disposal then the public might be more accepting of nuclear power, which may help us in the battle against climate change. Crease concludes

One of the many reasons for the need to study the validity of LNT is that convictions of its accuracy continue to be used as an argument against nuclear power plants, in connection with their operation as well as their spent fuel rods. Nuclear power may be undesirable for reasons other than this. But the critical need to find a workable alternative to fossil fuels for energy production requires an honest ability to assess the validity of this model.

In my opinion, determining if the linear no-threshold model is valid at low doses is one of the greatest challenges of medical physics today. It’s a critical example of how physics interacts with medicine and biology. We need to figure this out. But how?

Screenshot of The Million Person Study website.

One way is to conduct an epidemiological study of low-dose radiation exposure. But such a study would have to be huge, because it’s looking for a tiny effect influencing an enormous population. What you need is something like The Million Person Study. Yes, medical physics has its own “big science” large-scale collaboration. The Million Person Study’s website states

There is a major gap in epidemiological understanding, however, of the health effects experienced by populations exposed to radiation at lower doses, gradually over time.

The foundation of the Million Person Study is to fill that gap, using epidemiological methods of assessing rate and quality of mortality on a study group of one million persons exposed to this type of radiation.

The website notes that there are many reasons to assess the risk of low doses of radiation, including determining 1) the side effects of medical imaging procedures such as computed tomography, 2) the danger of nuclear accidents or terrorism (dirty bombs), 3) the safety of occupations that expose workers to a slight radiation dose, 4) the hazards of environmental exposure such as from radon in homes, and 5) the uncertainty of space and high altitude travel such as when sending astronauts to Mars. The Million Person Study not only focuses on the level of exposure, but also on the duration: was it a brief exposure as if from an nuclear accident, or a low dose delivered over a long time?

The cover of a special issue of the International Journal of Radiation Biology about The Million Person Study.

Want to learn more about The Million Person Study? See the paper by John Boice, Sarah Cohen, Michael Mumma, and Elisabeth Ellis titled “The Million Person Study: Whence it Came and Why,” published in the International Journal of Radiation Biology in 2022 (Volume 98, Pages 537–550). Its abstract is printed below.

Purpose: The study of low dose and low-dose rate exposure is of immeasurable value in understanding the possible range of health effects from prolonged exposures to radiation. The Million Person Study (MPS) of low-dose health effects was designed to evaluate radiation risks among healthy American workers and veterans who are more representative of today’s populations than are the Japanese atomic bomb survivors exposed briefly to high-dose radiation in 1945. A million persons were needed for statistical reasons to evaluate low-dose and dose-rate effects, rare cancers, intakes of radioactive elements, and differences in risks between women and men.

Methods and Materials: The MPS consists of five categories of workers and veterans exposed to radiation from 1939 to the present. The U.S. Department of Energy (DOE) Health and Mortality study began over 40 years ago and is the source of ∼360,000 workers. Over 25 years ago, the National Cancer Institute (NCI) collaborated with the U.S. Nuclear Regulatory Commission (NRC) to effectively create a cohort of nuclear power plant workers (∼150,000) and industrial radiographers (∼130,000). For over 30 years, the Department of Defense (DoD) collected data on aboveground nuclear weapons test participants (∼115,000). At the request of NCI in 1978, Landauer, Inc., (Glenwood, IL) saved their dosimetry databases which became the source of a cohort of ∼250,000 medical and other workers.

Results: Overall, 29 individual cohorts comprise the MPS of which 21 have been or are under active study (∼810,000 persons). The remaining eight cohorts (∼190,000 persons) will be studied as resources become available. The MPS is a national effort with critical support from the NRC, DOE, National Aeronautics and Space Administration (NASA), DoD, NCI, the Centers for Disease Control and Prevention (CDC), the Environmental Protection Agency (EPA), Landauer, Inc., and national laboratories.

Conclusions: The MPS is designed to address the major unanswered question in radiation risk understanding: What is the level of health effects when exposure is gradual over time and not delivered briefly. The MPS will provide scientific understandings of prolonged exposure which will improve guidelines to protect workers and the public; improve compensation schemes for workers, veterans and the public; provide guidance for policy and decision makers; and provide evidence for or against the continued use of the linear nonthreshold dose-response model in radiation protection.

 Lead on Million Person Study, and thank you for your effort. We need those results!