CT Scans, Nuclear Medicine, and Radiation Risk to Patients

Introduction

Modern medicine makes extensive use of nuclear technology in numerous diagnostic and therapeutic applications. This has led to growing concern about the potential detrimental effects on the human body of radiation exposure connected to these tools. In principle, nuclear imaging and the nuclear bombardment of cancer cells present possible risks of radiation-induced illness that must be factored into any reasoned patient decision to undergo nuclear diagnostic imaging or radiation therapy. On one hand, nuclear imaging and radiation therapy are valuable tools that extend life by enabling the earliest detection and most effective treatment of many human cancers. On the other hand, the undisciplined overuse of nuclear medicine — in situations where its advantages may not outweigh the known or suspected potential harms — represents a legitimate and addressable clinical concern.

The weight of the empirical evidence suggests that more research is necessary to determine precisely the degree to which nuclear radiation exposure that appears harmless in the short term may be more dangerous over a lifetime. It seems that low doses of radiation may contribute small but non-negligible amounts of cellular damage that are cumulative in their detrimental health effects. Accordingly, physicians should be educated in the relative risk-to-benefit analysis of the various clinical tools upon which they are increasingly relying.

How Radiation Damages the Human Body

Certain naturally occurring elements (and manmade compounds) differ from other elements in that they undergo spontaneous nuclear decay, a process moderated by the so-called weak nuclear force (Bleise, Danesi, & Burkart, 2003). During this decay process, particles are emitted that, although microscopic in size, travel at such great velocities that they are highly energetic and capable of passing through both organic and inorganic matter (Bleise, Danesi, & Burkart, 2003). Human beings are exposed to many sources of benign background radiation that fall well below the threshold below which isolated radiation exposures are not considered harmful to human health (Brenner & Hall, 2007). Exposure to more intense radiation, however, is known to cause acute illness, as described in the clinical literature on radiation poisoning and radiation disease (Bleise, Danesi, & Burkart, 2003).

There are principally two mechanisms by which radioactivity damages living organisms: cellular destruction and cellular mutation (Schanz, Schuler, Lorat, Fan, Kaestner, Wennemuth, & Rube, 2012). Cellular destruction results from the microscopic damage carved by radiation particles as they pass through the body; cellular mutation results from spontaneous changes to DNA molecules caused by bombardment from radioactive particles (Harbron, 2012). The energy associated with the near-light-speed velocity of particles released during nuclear decay causes specific types of damage to cellular DNA — some of which, such as double-strand breaks, may be incapable of complete repair (Schanz, Schuler, Lorat, et al., 2012).

Brenner and Hall (2007) describe the ionization process in detail:

"Ionizing radiation, such as x-rays, is uniquely energetic enough to overcome the binding energy of the electrons orbiting atoms and molecules; thus, these radiations can knock electrons out of their orbits, thereby creating ions. In biologic material exposed to x-rays, the most common scenario is the creation of hydroxyl radicals from x-ray interactions with water molecules; these radicals in turn interact with nearby DNA to cause strand breaks or base damage. X-rays can also ionize DNA directly."

Cellular Mutation and Cancer Risk

The second principal mechanism by which radiation exposure causes disease is through the triggering of DNA mutation. Cancer generally occurs when the DNA within organic cells is damaged in ways that interfere with the ordinary physiological regulation of tissue cell growth (Brenner & Hall, 2007). There is evidence suggesting that radiation exposure either directly causes these mutations or initiates one of several mutations that, in combination within the same cell, result in uncontrolled growth (Brenner & Hall, 2007).

Conclusion

There can be no doubt that nuclear medicine — in the form of diagnostic imaging and radiation therapy for cancer — provides valuable tools that enable contemporary physicians to identify and treat human cancers more successfully than could have been imagined less than a century ago. However, there is mounting empirical evidence that clinical use of nuclear medicine modalities must be weighed against potential risks owing to the well-documented connection between radiation exposure and cancer-causing cellular mutations attributable to the irradiation of living organic tissue.

The risks associated with nuclear medicine may be well worth the benefits in some situations, such as in the treatment of cancer through targeted irradiation. However, a more careful analysis may reveal that the risks are not justified in other situations — particularly in diagnostic contexts where alternative modalities provide comparable results at lower radiation-related risk. This calculus is especially important for patient populations identified as more susceptible to radiation-related harm. Ultimately, improving physician education about radiation dosages, long-term risks, and the availability of lower-risk alternatives is essential to responsible clinical practice in the era of modern nuclear medicine.

References

Bleise, A., Danesi, P.R., and Burkart, W. (2003). Depleted uranium: Properties, use and health effects of depleted uranium (DU): A general overview. Journal of Environmental Radioactivity, 64(2/3), 93–112.

Brenner, D.J., and Hall, E.J. (2007). Computed tomography — An increasing source of radiation exposure. New England Journal of Medicine, 357, 2277–2284.

Cirincione, J. (2007). Bomb scare: The history and future of nuclear weapons. Columbia University Press: New York.

Einstein, A.J., Henzlova, M.J., and Rajagopalan, S. (2007). Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA, 298(3), 317–323.

Harbron, W.R. (2012). Cancer risks from low dose exposure to ionising radiation — Is the linear no-threshold model still relevant? Radiography, 18(1), 28–33.

Schanz, S., Schuler, N., Lorat, Y., Fan, L., Kaestner, L., Wennemuth, G., and Rube, C.E. (2012). Accumulation of DNA damage in complex normal tissues after protracted low-dose radiation. DNA Repair, 11(10), 823–832.