Increased risk for nonmedullary thyroid cancer in the first degree relatives of prevalent cases of nonmedullary thyroid cancer: A hospital-based study

The genetic basis for nonmedullary forms of thyroid cancer (NMTC) is less w ell established than that of medullary thyroid cancer. However, epidemiolog ical and family studies suggest that a proportion of NMTC may be due to inh erited predisposition. To estimate the familial risk of thyroid cancer, we conducted a hospital-based case-control study at the Princess Margaret Hosp ital in Toronto, Ontario, Canada, and at 2 university hospitals in Montreal , Quebec, Canada. We obtained pedigrees from 339 unselected patients diagno sed with NMTC and from 319 unaffected ethnically matched controls. Family h istories of cancer were obtained from the cases and controls for 3292 first degree relatives of cases and controls. Seventeen cases (5.0%) and 2 contr ols (0.6%) reported at least one first degree relative with thyroid cancer. In relatives of patients with thyroid cancer, the incidence of any type of cancer (including NMTC) was 38% higher than in relatives of controls (incid ence rate ratio, 1.4; 95% confidence interval, 1.1-1.7). The relative risk for thyroid cancer was 10-fold higher in relatives of cancer patients than in controls (incidence rate ratio, 10.3; 95% confidence interval, 2.2-47.6) . Our findings suggest that hereditary or other familial factors are importan t in a small proportion of NMTC. Molecular studies are needed to determine the genetic basis of cancer susceptibility in these families.