Medical imaging has experienced tremendous growth since the discovery of X-rays: the x-ray-based CT scanner, advances in positron emission tomographic (PET) imaging, the development of single photon tomographic imaging (SPECT), and the use of magnetic resonance for imaging (MRI). These procedures, some of which require the use of radioactive isotopes, have led to improved diagnoses and treatment for a wide range of disorders.
One thing that hasn’t kept pace is a reporting requirement for errors that occur when using radioisotopes — radioactive substances used to diagnose and treat disease.
Scientific and technical developments must be properly regulated to ensure a high level of safety for patients. In the U.S., the Nuclear Regulatory Commission (NRC) regulates the use of radioactive isotopes for the diagnosis and treatment of disease, but the pace of scientific development makes this a challenging task.
The use of radioactive isotopes can significantly improve a doctor’s ability to diagnose and treat disease and the benefits of using radiation or radioactive isotopes far outweigh the risks — though high doses of radiation, of course, represent a potential health hazard.
The validity of the images that emerge from the use of radioactive isotopes depends on their being injected cleanly into a vein. If the vein is punctured or ruptured, some of the radioactive material may be deposited — the technical term is infiltrated — into the tissue around the injection site instead of circulating in the blood. Often the injection site is not in the imaging field of view, so it’s possible that the physician interpreting the images may not know that an infiltration occurred.
If the infiltration is large enough, the results of the procedure may be compromised. An unknown infiltration can lead to an incorrect assessment of the extent of disease and inaccurate assessment of treatment, both of which can negatively affect patient care.
Infiltrations also expose the tissue around the injection site to unintentional radiation. A large infiltration will expose the tissue to significantly more radiation that a clean injection with no infiltration.
To learn from mistakes and protect patients during medical procedures that use isotopes, the NRC began requiring hospitals to report isotope misadministration in 1980. In 2002, the term misadministration was changed to “medical event” and the NRC established risk-based reporting limits of 500 millisieverts (mSv) to patient tissue. A sievert is a measure of radiation that includes the type and the amount of radiation absorbed in a certain mass. During a normal PET procedure, the radiation dose to the tissue of the average person is approximately two-thousandths of a sievert (2 millisieverts). The NRC concluded that when a patient unintentionally receives 500 millisieverts in a single irradiation, it should be reported as a medical event.
Nuclear medicine events reported to NRC are uncommon, but have included radioactive isotopes being spilled on patients, the wrong patient receiving radiation, and the wrong dose being injected. In the case of an infiltration, if just 10% of the injected dose for a typical PET imaging procedure infiltrates into one cubic centimeter of tissue, the radiation exposure to that tissue will be 1,500 times greater than if the dose had been injected cleanly. That would surpass the NRC reporting threshold by about six times.
Despite this risk, an NRC policy drafted in 1980 — based on the belief that infiltrations “occur frequently” and are “virtually impossible to avoid” — specifically exempts hospitals from reporting any infiltrations. In 2002, after NRC established the risk-based reporting limit, it continued to exempt infiltrations even when they exposed patients’ tissues to more than 500 millisieverts.
By retaining this loophole, the NRC creates inconsistent reporting requirements that make little sense. In 2020, radioactive isotopes that are inadvertently spilled onto a patient resulting in radiation exposure to tissue that exceeds 500 milliSieverts is reportable as a medical event while radioactivity infiltrated internally into a patient that exposes tissue to 500 millisieverts or even higher is not reportable as a medical event.
The consequence of this loophole is that the NRC — along with patients and physicians — have no idea of the true impact of infiltrations.
This problem will only become worse, as radioactive drugs are increasingly used not only for diagnosing and staging disease but also for treating it. Recent FDA approvals include radium 223 dichloride for treating certain types of prostate cancer and lutetium Lu 177 dotatate for treating certain neuroendocrine tumors.
Fortunately, the NRC is now re-evaluating the loophole that allows significant infiltrations to go unreported. Requiring reporting would be a small step for hospitals to implement but a big step toward improving patient care.
Recent clinical studies show that it is possible to dramatically reduce the incidence of infiltrations, including those that could cause patient harm. They are no longer “virtually impossible to avoid.” Given this advance, the NRC should do away with the policy that exempts infiltrations from being reported. As with all other medical events, if an infiltration exceeds the reporting threshold, it should be reported. This will drive quality improvements and improve patient care and safety.
David Townsend is a co-inventor of the PET/CT scanner and a pioneer of three-dimensional reconstruction algorithms for PET.