Health Threat of Medical Ionizing Radiation Impact of Nuclear Medicine Exposures

In October 2009, the U.S. Food and Drug Administration (FDA) alerted all healthcare facilities involved in performing brain perfusion computed tomography (CT) to ensure their patients were not being overexposed to ionizing radiation (Samson, 2009). This notice was in response to the discovery that 206 patients subjected to this procedure at the Cedars-Sinai Medical Center in Los Angeles had been exposed up to eight times the recommended dose. The immediate effects to some patients were skin reddening and hair loss, but the long-term effects could be an increased risk of cancer. Since the error was in the programmed settings for the CT equipment, none of the medical personnel bothered to check the actual radiation doses the patients were receiving. The absence of exposure monitoring allowed the equipment to perform at this setting for over a year and a half.

This event, together with other radiation overexposure scandals, led to publication of a series of investigative articles on the risk of medical radiation in the New York Times. Based on the article by Bogdanich (2010), there are a number of ways a patient can become exposed to unnecessary ionizing radiation. Should a dose miscalculation become part of a standard treatment procedure at a busy clinic or major medical center, dozens of patients could be overexposed repeatedly during treatment. Overworked medical professionals may overlook standard safety procedures and hospital budget constraints may degrade patient safety oversight protocols. According to some radiologists, advances in medical radiation equipment have outstripped the development of appropriate patient safety procedures. The relevant laws are lax in some states and rarely enforced in others. Reporting guidelines are frequently ignored, as are safety rules in some hospitals.

Even in clinics where the above problems are absent, patients are still at risk for being exposed to ionizing radiation unnecessarily or at dosages exceeding that required to produce an image of sufficient diagnostic quality (FDA, 2010). This is especially true for children seeking care at healthcare facilities without dedicated pediatric radiology equipment and personnel. This essay will examine the nature and extent of the health risk that medical radiation poses to patients, with an emphasis on pediatric care.

The Dramatic Growth of Medical Imaging in the United States

Medical imaging began in the late 19th century and almost immediately problems began to appear (reviewed by Linet et al., 2012). Medical imaging workers developed skin and blood cancer, in addition to other deleterious conditions, before realizing that both patients and radiologists needed protection from the ionizing radiation these devices produced. Radiation doses were reduced and protection devices developed by the 1950s that effectively eliminated the immediate health risks for workers and adult patients.

Following implementation of safety devices and procedures in the 1950s, the greatest danger recognized by the medical community was the cumulative effect of repetitive exposures (reviewed by Linet et al., 2012). However, studies examining the effects of radiation exposure on young children (reviewed by Laack et al., 2011) and fetuses in utero (reviewed by Linet et al., 2012) revealed that the health threat of medical radiation was much more complex than previously assumed. Given the dramatic revolution in medical imaging technologies over the past 30 years and the resulting increases in exposure frequency and dosage, the health threat this technology represents to the young is assumed to be considerable. Prior to the 1980s, medical radiation represented just 15% of the total radiation from all sources that the average U.S. citizen was exposed to each year. Current estimates suggest medical radiation now represents 50% of the ionizing radiation exposure each year.

The large increase in exposure to ionizing radiation is caused primarily by the increased use of CT scans (reviewed by Laack et al., 2011). From 1980 to 2007, the number of CT scans performed in the U.S. increased from about 3 million to almost 70 million per year. Not only were patients being exposed to ionizing radiation more often, the dose received was also higher. Compared to a standard chest X-ray, which exposes the patient to an average of 0.1 mSv (millisievert, a unit of measure representing the biological effects of ionizing radiation) of ionizing radiation, a standard CT scan of the chest will expose the patient to 7.0 mSv (ASRT, 2012). By comparison, ionizing radiation from natural and other manmade sources amounts to only 3.1 mSv per year.

Whole-body CT scans are particularly concerning, because the estimated average dose is 20.9 mSv. Since children are often trauma victims, the risk/benefit ratio for this demographic may favor limiting the use of whole-body scans to avoid short-term and long-term increases in cancer risk. One estimate suggests that the prevalent use of CT scans could be responsible for up to 2% of all cancers in the U.S. today.
Carcinogenic Effects of Ionizing Radiation

The radiation dose determines the level of risk for an adverse outcome (reviewed by Mettler, 2012). Below 0.05 to 0.1 Gy (Gray) the amount of harm caused is hard to quantitate, but it is believed to exist. Compared to Sv, the unit Gy is a measure of absorbed dose rather than the biological effect of a radiation dose. Between 0.05 and 1.0 Gy, leukemia and other cancers are believed to be the primary health risk. The risk of cancer was estimated to increase by 5% for every Sv of radiation exposure. If the route of exposure is through the skin, a single acute dose between 2 and 5 Gy can cause transient reddening of the skin and hair loss. Prolonged or permanent reddening of the skin can occur at doses between 10 and 15 Gy, in addition to permanent tissue damage. However, survival becomes an issue at these higher doses. For example, survival is unlikely at doses above 5 Gy without medical intervention and unlikely above 10 Gy with treatment.

Lower dosages can still pose a health hazard if inhaled or ingested, and some tissues are more radiation sensitive than others (reviewed by Mettler, 2012). Bone marrow, breast, thyroid, and salivary glands are highly susceptible to radiation exposure, while brain, kidney, and connective tissue have a lower cancer risk. Sterility is a common effect of radiation exposure, but does not occur in males until an acute dose exceeds 0.5 Gy. However, reduced fertility can result when exposed chronically to lower doses. For women, an acute dose of 10 Gy is required to cause sterility in females who have not reached puberty and 2 to 3 Gy for women over 40 years of age. Radiation of sufficient dosages can also cause cataracts and suppress the immune system.

Such high dosages rarely occur during diagnostic medical imaging procedures, but accumulated exposures over a period of years can become significant enough that the health risk must be considered (reviewed by Linet et al., 2012). As mentioned above, the per capita yearly dose of ionizing radiation from medical sources has increased from 0.53 mSv in 1980 to 3.0 mSv in 2006. Assuming a 5% increase in cancer risk for every Sv of exposure, this translates into an increase of cancer risk from 0.00265% to 0.015% per year. While this increase in risk seems negligible, over the course of a lifetime (80 years) the total increase is from 0.2% to 1.2%, or 6-fold. In addition, this risk is an average and therefore does not represent the actual increase in cancer risk for radiation sensitive tissues like the bone marrow or thyroid.

Radiation experts recently concluded that the adverse biological effects of ionizing radiation occur at all levels of exposure, even if they cannot be measured (reviewed by Linet et al., 2012). This linear, no threshold model implies that no exposure is the only exposure level that can eliminate the cancer risk associated with medical imaging. This conclusion is based on research findings, which revealed extremely low radiation exposures can induce base changes in DNA. These mutations may or may not represent a deleterious change in an important tumor suppressor gene, but over a period of years, the chance that a tumor-inducing mutation will occur is predicted to increases significantly under the linear, no-threshold model.

The carcinogenic effects of radiation exposure due to diagnostic imaging procedures are typically not realized immediately (reviewed by Linet et al., 2012). The latency period for the development of leukemias is between 2 to 5 years, although the latency period is shorter when the patient is a child. The type of leukemia that develops is affected by the amount of time that has passed since exposure, such that the risk of chronic myeloid leukemia decreases faster than the risk for acute myeloid leukemia. The latency period for solid tumors is at least 10 years; however, in contrast to the risk of leukemia, the risk for solid tumors continues to increase as the patient gets older. The radiation-induced solid tumors also tend to develop at the same age as tumors caused by other factors.

Risk vs. Benefit Calculations

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