Case-control study of thyroid cancer in Northern Italy: attributable risk.

BACKGROUND The percentage of thyroid cancer cases attributable to specific risk factors can be calculated to focus preventive strategies. The per cent population attributable risks (PAR) for thyroid cancer were estimated in relation to history of benign thyroid diseases, history of radiotherapy, residence in endemic goitre areas and selected indicators of a poor diet, using data from a case-control study conducted between 1986 and 1992 in Northern Italy. METHODS Cases were 399 histologically confirmed incident thyroid cancers and controls were 617 patients, admitted to hospital for a wide range of acute, non-neoplastic, non-hormone-related diseases. The PAR were computed on the basis of multivariate odds ratios (OR) and on the distribution of risk exposure among cases, assuming they are representative of the general population of cases. RESULTS A history of benign thyroid disease accounted for 18.9% of cases, radiotherapy for 1.2%, residence for > or =20 years in endemic goitre areas for 2.4% of cases, and their combination for 21.7% of thyroid cancer cases; selected indicators of a poor diet accounted for 40.9% of thyroid cancer cases in this population. The combination of all factors considered explained over 57% of thyroid cancer cases in both sexes. The estimates for thyroid-related conditions were higher in women than men, whereas the opposite was true for dietary indicators. The overall PAR were somewhat higher in people aged > or =45 years (63.8%) than in younger subjects, and for follicular (69.1%) rather than papillary (53.7%) cancers. CONCLUSIONS Exposure to a few simply identified and potentially modifiable risk factors or indicators (benign thyroid disease, residence in endemic goitre area and a poor diet) explained about 60% of thyroid cancer cases in this Italian population, indicating the theoretical scope for prevention.