CT spurs concern over thyroid cancer

A truism about any rapidly adopted technology, no matter what its specifics, is that the benefits are readily apparent soon after its introduction, and the untoward effects are inevitable but delayed. The initial wave of enthusiasm about a new test or a new technique's virtues drowns out, at least for a while, any discussion about its putative ill effects. This truism is especially apparent when the innovation inspires a metamorphosis in practice and perception that alters the allocation of resources, the focus of training, and the nature of work.

Advances in CT over the past decade meet these criteria. Even into the early 1990s, CT was for many conditions a procedural adjunct, only one element in the diagnostic mix supplementing history, physical examination, and serum chemistry tests. Now, with the advent of faster and faster scans, even patients who cannot give a history because they are too young or enfeebled can have their organs assessed in one breath-hold.

Until about 10 years ago, a particular lab value or physical finding could determine whether a CT study was indicated. CT may now in many medical facilities precede the questioning of patients, the laying on of hands, and the drawing of blood. As this technology has progressed, however, medical education has been transformed, or dumbed down. CT has also contributed greatly to the acceleration of expenditures for imaging in general, which by some accounts now exceeds both the rate of rise and the aggregate costs of pharmaceuticals in the U.S.¹

The data are clear (Table 1). In 1980, approximately two million CT scans were done in the U.S. That number grew to 20 million by 1995, to 40 million by 2000, to 65 million by 2004.² It is estimated that in 2010, 100 million scans will be performed on 300 million U.S. residents. That means that while many will need no such exam each year, an increasing segment of the population will have more than one CT study annually.

And the U.S. is by no means the utilization leader. As long ago as 1990, the CT rate in Australia was double that of the U.S. It was triple that of the U.S. in Belgium and nearly 10 times greater in Japan.

The explosive growth of CT has induced, in medicine at least, a change in mores with respect to the public's valuation of the role of sophisticated imaging modalities. Rapid scans have stimulated a sharp increase in CT studies of children.² CT has become the centerpiece of diagnosis for many conditions, and the condition need not even be physical. Recently, anxiety or mere curiosity emerged as an engine of demand, as CT became an avidly sought-after screening test for the worried well. Only by regulatory fiat did such a misapplication of resources cause CT screening to lose its luster as an investment opportunity.

Given this increase in use, we can no longer neglect the fact that CT imparts radiation.^3,4 The toxic potential of absorbed radiation in the dose range of both single and multiple CT studies must be reckoned with. That concern, if pursued by radiologists, should at least temper the unbridled growth of this technique. It is encouraging that the issue of dose is getting more attention in discussions of CT utilization. Among the regulations governing pediatric radiology fellowship programs, for example, attention must now be paid to how dose reduction methodology is taught and implemented.5

Other responses seem contradictory. Somewhat belatedly, manufacturers are devising schemes to lower dose per slice, but they continue to promote increased utilization and to introduce new machines that multiply the number of slices per examination. At the other end of the spectrum, apprehension that spurs investigation is one response, but fear mongering is another. Some commentators have become so convinced of the dangers of CT that careful assessment of data is neglected amid scenarios of doom.

THYROID CANCER ASSOCIATION

Information from both U.S. and foreign sources makes a compelling, if not yet conclusive, case that CT use and cancer of the thyroid gland are related. CT provides by far the bulk of all radiation applied for medical purposes. In a study from the U.K., where CT studies make up only 4% of all radiology exams, it provided 40% of total dose.⁶ Our facility does 45,000 CT studies per annum and 250,000 exams overall. It is safe to say that 75% of the radiation received by our patients comes from CT. Many of these studies include the thyroid gland in the field-of-view.

The temporal relation between the unprecedented increase in CT utilization and the expanding roster of thyroid cancer cases in many countries is coincidental but probably not merely coincidental. Thyroid malignancies account for 1% to 2% of cancers worldwide.³ In general, women are three times more likely to be affected than men, and incidence occurs at an earlier age in women. In the U.S. in the past 10 years, thyroid cancer increased in incidence by 4.3% per annum, five times more rapidly than any other malignant neoplasm (Table 2).^7,8 A recent accretion of cases has been noted as well in many European countries and in Australia and in New Zealand.⁹

A survey of incidence data directed by the National Institutes of Health covered nine locations in five states and four metropolitan areas, encompassing both coastal and interior regions and including rural and urban populations. It revealed a slight but steady increase in new cases of thyroid cancer in men over the past decade at each location and an increasingly steep increase in the number of cases in women.^7,8

Hence, two phenomena are occurring in nearly the same time period. The rise of CT use has been accompanied by a slightly later but similar increase in the incidence of thyroid cancer. The Chernobyl disaster of 1986 brought few lasting radiation effects except for a sustained rise in thyroid malignancy cases, many of them first noticed within five years of the reactor meltdown.³ Until recently, most discussions about the causes of thyroid cancer concerned atmospheric releases from nuclear explosions or nuclear power plant leaks.^10-13

Some investigators have proposed that the supposed expansion of the case pool of thyroid cancer is an artifact occasioned primarily by an increase in ultrasound exams and fine-needle biopsies, which reveal small foci of disease that could remain clinically innocuous indefinitely.¹⁴ Yet in such deliberations, the contributions of medical radiation, predominantly a consequence of CT, remain the elephant in the room, obviously there but largely unacknowledged.

If CT does induce thyroid cancer, it will take a generation to confirm that assertion or to totally disprove it. In the interim, therefore, we should carefully examine how we perform CT studies when the thyroid gland is interrogated by the radiographic beam.

Many questions arise: Should we routinely shield the thyroid? Should we be more fastidious about the length of a cervical CT scan when followed by a thorax CT in trauma patients? If we routinely overlap the lower border of the neck study with the upper border of the chest study, are we are unwittingly giving the thyroid gland a double dose of radiation? Should the criteria be more restrictive for women and for children in light of the likelihood of heightened risk to both?^15,16 Should we more diligently prescreen patients to determine those more susceptible to radiation, such as individuals recessive for ataxia-telangiectasia? Should we be wary of giving intravenous contrast material just before obtaining CT sections of the neck, since iodine is avid for the thyroid gland and, once iodinated molecules are situated within the confines of the thyroid, they could more effectively block the passage of photons, causing more radiant energy to be deposited within the gland instead of passing through without perturbing nucleotide bonds?

Each of these questions lacks a definitive answer, but all deserve study. If such a causal relationship is real and we do little over the next few years here and abroad to reduce or restrict exposure, we as the proprietors of imaging might have to accept credit for being the perpetrators of the first radiological pandemic. The time to address this concern in earnest is now.

Dr. Baker is chair of diagnostic radiology and Dr. Tilak is a medical student and research fellow in diagnostic radiology, both at New Jersey Medical School of the University of Medicine and Dentistry of New Jersey.

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