Increased incidence of differentiated thyroid carcinoma and detection of subclinical disease

Background: Recent reports from North America and Europe have documented an annual increase in the incidence of differentiated thyroid carcinoma. We sought to investigate the relation between rates of detection, tumour size, age and sex. Methods: Using the Ontario Cancer Registry, we identified 7422 cases of differentiated thyroid carcinoma diagnosed from Jan. 1, 1990, to Dec. 31, 2001. We obtained pathology reports for a random 10% of the 7422 patients for each year of the study period. The sample represented all Cancer Care Ontario regions. We compared the size of the patients' tumours by year, sex and age. Results: As expected, the incidence of differentiated thyroid carcinoma increased over the 12-year period. A significantly higher number of small (<= 2 cm), nonpalpable tumours were resected in 2001 than in 1990 ( p= 0.001). The incidence of tumours 2-4 cm in diameter remained stable. When we examined differences in tumour detection rates by age and sex, we observed a disproportionate increase in the number of small tumours detected among women and among patients older than 45 years. Interpretation: Our findings suggest that more frequent use of medical imaging has led to an increased detection rate of small, subclinical tumours, which in turn accounts for the higher incidence of differentiated thyroid carcinoma. This suggests that we need to re-evaluate our understanding of the trends in thyroid cancer incidence.