CT spurs concern over thyroid cancer: Should it?

An article in the December 2006 edition ("CT spurs concern over thyroid cancer," page 25) pointed to an association between increased incidence of CT utilization and thyroid cancer. It conveyed a strong impression that this association was destined to take us to a radiological pandemic if no preventive measures were taken.

While we all can agree with most of the radiation safety considerations mentioned in that article, its authors border on the edge of hysteria in their implications.

We must remember that association does not imply causation. First of all, looking at the authors' own reference data on a single plot, the association is visually obvious. However, there is no indication on their graph of a latent period between the rise in CT utilization and the rise in incidence rates of thyroid cancer. A similar graph might have been drawn with the increased use of ultrasonography and fine-needle aspiration for earlier detection of thyroid cancer (perhaps found more often given the increased detection offered by chest CT), which the authors briefly acknowledge may have explained their graph.

A latent period anywhere from five to 20 years is generally accepted to exist for the diagnosis of thyroid cancer following radiation exposure.1-4 This is clear from the literature on children and others treated with radiotherapy for Hodgkin's disease, radiotherapy used for treating ringworm in the scalp, and data from nuclear reactor accidents.

If, indeed, the causal relationship of the improved early detection and more frequent fine-needle aspiration being performed in recent years is the dominant factor, we should soon see a leveling off of the incidence rates with the likely practice adjustment to reduce overdiagnosis and avoid the impending radiologic pandemic predicted by the authors.

Similar trends in thyroid cancer incidence are seen in Canada, yet one can surely assume that the "misapplication of CT resources and the overuse of CT utilization" have not been as dramatic in Canada, given its strict single-payer system and utilization controls as compared with the U.S. The increased incidence of thyroid cancer and overall stable thyroid cancer mortality rates suggests increased detection of subclinical disease, not an increase in true occurrence of thyroid cancer.5 The 2.4-fold increase in thyroid cancer detected since 1973 on the authors' graph is mostly due to papillary cancer, a relatively benign form of cancer, which is also evident in the surveillance data of Hodgkin's disease patients who receive high doses of therapeutic radiation to the thyroid. On the other hand, the effects of the dramatic increase in CT procedures between 1980 and 2005 (close to a 35-fold increase), if significant, should have been apparent a decade or two later and not have directly paralleled the trends in thyroid cancer incidence.

Articles such as the one published appropriately fuel the increasing concern with overutilization of CT, a very valid concern. On the other hand, using nonscientific scare tactics to combat such unfortunate overutilization misleads our colleagues and the public and ultimately detracts from the credibility of the authors and the publications that foster them.

-Khachig Jerjian, Ph.D.
Michael Brant-Zawadzki, M.D., FACR
Newport Beach, CA

References

• Metzger ML et al. Natural history of thyroid nodules in survivors of pediatric Hodgkin's lymphoma. Ped Blood Cancer 2006 Mar;46(3):314-319.

• Sklar C et al. Abnormalities of the thyroid in survivors of Hodgkin's Disease: Data from the Childhood Cancer Survivor Study. J Clin Endocr Metab 2000 Sep;85(9):3227-3232.

• Acharya S et al. Thyroid neoplasms after therapeutic radiation for malignancies during childhood or adolescence. Cancer 2003;97(10):2397-2403.

• Kikuchi S et al. Latency period of thyroid neoplasia after radiation exposure. Ann Surg 2004;239(4):536-543.

• Davies L, Welch HG. Increasing incidence of thyroid cancer in the United States, 1973-2002. JAMA 2006;295:2164-2167.

Reply from Drs. Baker and Tilak

Drs. Jerjian and Brant-Zawadzki protest that we have projected the possibility, not the certainty, of a causal relationship between increasing CT use and the increasing incidence of thyroid cancer. Raising a hypothesis based upon associations in both space and time is a valid scientific exercise even if troubling in its implications for defenders of the status quo.

The claim that a latent period of five to 20 years exists between radiation exposure and the appearance of thyroid cancer is confounded by information recently reported in a review of the clinical consequences of the Chernobyl disaster. The only lasting radiation-induced effect on the regional population was an increase in thyroid cancer, noted even four years after the event, rising 100-fold in downwind Belarus and sevenfold upwind in nearby Ukraine.1,2

Surely this temporal relationship must now be considered as advancing the notion of a putative close temporal relationship between the radiation from CT scanning and thyroid cancer incidence. It is reassuring that most of the cancers accounting for the rise of incidence of this disease are of the papillary type. But the label "more benign" cannot be applied to a carcinoma. That is a contradiction in terms. More indolent, perhaps, but still a malignancy. Moreover, an increase in incidence along with stability in survival may also reflect better treatment, with new techniques and medications enabling those individuals who otherwise would have succumbed to be cured.

Furthermore, the reference cited in defense of a lack of support for a radiation-related etiology for an increase in thyroid cancer is invalid because the authors of that citation somehow have forgotten that diagnostic imaging exists.

They state that radiation is the major risk factor for the disease but then incredibly claim, "If anything, radiation exposure is less common today than it was in the past. The last major nuclear atmosphere testing in the U.S. was conducted in 1961."3 Do the authors not know that in the past 46 years of medical imaging, particularly recently, CT imparts radiation to individuals?

Drs. Jerjian and Brant-Zawadzki are quick to label data and projections that do not comport with their preconceptions as being "unscientific." They also raise an alarm that the implications derived from incontestable geographic and historical information should be considered a scare tactic. I submit that it is only considered a scare tactic by those who find the possibility we offered as uncongenial to their presumptions.

-Stephen R. Baker, M.D.
Gauri S. Tilak, Ph.D.
Piscataway, NJ

References

• Davis S, Stepanenko V, Rivkind N, et al. Risk of thyroid cancer in the Bryansk Oblast of the Russian Federation after the Chernobyl Power Station Accident. Radiat Res 2004;162(3):241-248.

• Chernobyl's legacy: Health, environmental and socio-economic impacts. Accessed at http://www.iaea.org/NewsCenter/Focus/Chernobyl/index.shtml.

• Davies L, Welch HG. Increasing incidence of thyroid cancer in the United States, 1973-2002. JAMA 2006;295:2164-2167.