Skip to content

Mass General Researcher Leads Effort to Address Cumulative Radiation Exposure

In This Article

  • Understanding patients' radiation exposure over time is important to knowing how to minimize such exposure
  • Many countries in Europe and elsewhere utilize permanent patient ID numbers, which enables them to track patients' cumulative exposure; the U.S. does not use such identifiers
  • Massachusetts General Hospital researchers looked at within-network exposures in the U.S. as proxies for total cumulative exposures and reported higher-than-anticipated results
  • The researchers make recommendations as to how to reduce exposures over time

When Madan Rehani, PhD, director of Global Outreach for Radiation Protection at Massachusetts General Hospital, moved to the United States from Vienna, Austria, where he had worked for 11 years at the International Atomic Energy Agency (IAEA), he undertook an informal experiment. He asked maybe half a dozen of radiologists to guess the highest dose of radiation a patient might receive—not from a single CT or other type of scan—but over the course of a couple of years.

Most thought it would fall somewhere in the area of 20 millisieverts (mSv)—maybe 50 mSv at most. (The average dose for a single CT procedure is in the range of one to 15 mSv.) All were surprised to hear that many patients receive upwards of 100 mSv. And not only cumulatively over two, three or even five years. In some cases, patients can receive between 50 and 100 mSv of radiation in a single day.

Challenges to Understanding Cumulative Exposure in the U.S.

Dr. Rehani has devoted decades of effort to radiation protection and patient dosimetry, focusing in more recent years on patients' cumulative exposure from tests and procedures performed over time. Much of this work has come through his association with the International Atomic Energy Agency (IAEA), based in Vienna, in which he developed a website and myriad other training materials dedicated to radiation protection, introduced the IAEA the smart card project for tracking radiation exposures of patients and more.

Many of the programs he has established or recommendations he has made otherwise have been taken up across Europe and other parts of the world, including in a number of developing nations. Notably, though, one of the largest and most developed nations has paid relatively little attention to the issue of cumulative exposure. The country in question? The United States.

Dr. Rehani offers a twofold explanation as to why the U.S. lags behind other nations in addressing cumulative exposure. First, he says, the U.S. does not have a system of permanent patient ID numbers, as many other nations do. Consequently, getting a true accounting of a patient's exposure over time will be more challenging, likely leading to less emphasis placed on cumulative exposure.

Also, in the U.S., approaches to dose reduction are often technique-focused rather than patient-focused. Since 2001, when a USA Today article titled "CT scans in children linked to cancer" raised concerns about the technology, researchers and medical device companies alike have zeroed in on the optimization of dose reduction in the scanners themselves. The cumulative impact of radiation exposure has received far less attention.

Now based at Massachusetts General Hospital, where he leads the office of Global Outreach for Radiation Protection, Dr. Rehani decided to explore the question of cumulative exposure in the U.S., and especially what can be done about it.

Within-Network Health Records as a Proxy for Overall Radiation Exposure

Even in the absence of permanent patient ID numbers, researchers can determine levels of exposure within a degree of certainty. Because patients typically get most or all of their care within a single network, the researchers can access medical health records from within that network to ascertain how much exposure individual patients have likely had. In a multi-center retrospective study reported in a series of papers published in European Radiology between April 2020 and March 2021, Dr. Rehani and colleagues did just this (see here, here, here and here).

In the most recent of these papers, they looked at CT exams performed in 279 hospitals between January 2015 and December 2019 and found a total of approximately 4.3 million CT exams conducted over roughly 3.9 million patient-days, indicating that 9.41% of the patients in the study underwent more than one exam in a single day. Nearly one in 100 patient-days (0.8%) saw exposures of 50 mSv or more; 0.03% saw exposures of 100 mSv or more. Nearly one-third of the patients who received 50 mSv or more in a single day were 50 years of age or younger.

The researchers also identified a number of CT imaging protocols associated with doses of 50 mSv or more in a single exam. All of the top 20 of these involved the chest or abdomen and pelvis; almost one-third were angiographic studies.

The Importance of Sub-mSv Doses

Where does the radiation protection community go from here? "If you ask me, what is a single action I would like to see taken?" Dr. Rehani says, "I would like to see manufacturers introduce CT scanners with sub-mSv radiation doses."

This is not a new vision, he adds. A decade ago, a working group assembled by the NIH looked into incorporating sub-mSv doses into CT technology. In the end, though, the community settled on a target of 3-mSv doses: an important step forward but not as much a reduction as Dr. Rehani feels is warranted.

Much of the debate over dosage centers on the question of background radiation: the radiation resulting from naturally occurring radioactive elements in soil, water, air and our body itself. "All of us are exposed to radiation all the time," Dr. Rehani says. "Wherever you are sitting, radiation is coming at you through the building, the floor, the table and from radioisotopes in one's own body." Background radiation is thought to hover at around 3 mSv. Many will argue that because a patient is already receiving around 3 mSv of radiation annually, the amount of radiation dose during a CT exam does not need to be any lower. This is where Dr. Rehani and others have challenged conventional thinking.

The science today, he says, suggests that any amount of additional radiation exposure, no matter how small, increases one's chances of getting cancer. He points to a 2009 study of cumulative radiation exposure in emergency department patients as an example (see also here for a discussion of elevated risks of cancer with cumulative doses from CT scans). "There is no threshold below which the exposure will not be harmful," he adds. "There is only probability, and the probability of radiation causing cancer is a function of the dose. And the effects are cumulative."

Looking ahead, Dr. Rehani plans to study exposures due to other types of exams, such as nuclear medicine exams; thus far he and colleagues have focused only on CT and fluoroscopy-guided interventions. He also hopes to encourage epidemiologists to explore the relationship between cumulative exposure and disease and how this can both impact patient outcomes and inform treatment decisions.

Learn more about the Global Outreach for Radiation Protection Program

Learn more about the Department of Radiology


The noninvasive imaging modality coronary computed tomography angiography (CCTA) offers a means to evaluate these patients with chest pain symptoms both safely and efficiently.


Radiologists may be called on to examine people suspected of concealing drugs in the GI tract, rectum or vagina. Rhea Malhotra, MD, and Ajay Singh, MD, MBBS, of the Department of Radiology, review approaches for protecting these individuals from complications and preventing the release of contraband into the community.