An exponential growth in incidence of thyroid cancer: trends and impact of CT imaging

Abstract

Background and purpose: Workup of incidental thyroid nodules detected on CT imaging could be contributing to the increased diagnosis of small thyroid cancers. The purpose of this study was to evaluate recent trends in the incidence of thyroid cancer, and to determine the relationship between annual CT imaging volume and rate of thyroid cancer diagnosis.

Materials and methods: This retrospective cohort study used data bases for thyroid cancer and CT imaging volume. Thyroid cancer data from 1983-2009 were obtained from the Surveillance, Epidemiology, and End Results data base. National Council of Radiation Protection and Measurements Report No. 160 provided data on hospital and nonhospital CT imaging volume for 1993-2006. Trends in thyroid cancer were modeled for overall incidence on the basis of patient age, tumor histologic features, and tumor size and stage. Linear regression analysis was performed to evaluate the strength of the relationship between annual CT scan volume and the incidence of thyroid cancer by tumor size and histologic type.

Results: In 2009, the incidence of thyroid cancer was 14 per 100,000, which represented a 1.9-fold increase compared with 2000. The growth in incidence was exponential compared with a minimal linear increase in thyroid cancer mortality rate. The subgroup with the greatest change was subcentimeter papillary carcinoma, with doubling in incidence approximately every 6.2 years. The linear relationship between annual CT scan volume and the incidence of subcentimeter papillary carcinoma was very strong (R(2) = 0.98; P < .0001).

Conclusions: The incidence of subcentimeter papillary carcinoma is growing at an exponential rate without significant change in mortality rate. The strong linear relationship between new cases of subcentimeter papillary carcinomas and the number of CT scans per year suggests that an increase in CT scans may increase the detection of incidental thyroid cancers.

Fig 1.

Trends in incidence and mortality of thyroid carcinoma. The rate of increase in incidence (%) per year is shown for thyroid cancer, papillary cancer, thyroid cancer deaths, and the size of the population. The base population is expressed in units of persons × 10⁻⁴. All of the trend lines were fitted to an exponential function, but the low growth rates for base population and thyroid deaths could be approximated by linear growth curves. Dashed vertical lines indicate the period of 1993–2006 for which annual CT volume data were also available.

Fig 2.

Trends in the number of new cases per year of papillary carcinoma by size. Symbols show observed values. Lines show fit from multivariate log-linear model for incidence (On-line Appendix B). Dashed vertical lines indicate the period of 1993–2006 for which annual CT volume data were also available.

Fig 3.

Relationship between annual CT scans and incidence of thyroid cancer for (A) papillary carcinoma and (B) nonpapillary carcinoma. The CT cases represents million cases per year.