Thyroid cancer in an iodide rich area. A histopathological study

Abstract

A comparison of the incidence of the different histological types of thyroid carcinoma in an area of high dietary iodide and an area of normal iodide intake was made. The areas chosen were Iceland and the region of NE Scotland centered on Aberdeen; both areas have clearly defined populations served by a single pathology laboratory. All definite and dubious thyroid carcinomas from both regions were examined and classified by the same 2 pathologists. The age-specific incidence rates for papillary carcinoma in surgical specimens in both areas rose with age; they were 5 times higher in Iceland (high iodide area) than in NE Scotland. The numbers of follicular carcinomas were small, and this tumor was relatively less frequent in Iceland than Aberdeen. These findings, together with the known high relative frequency of follicular carcinoma and low frequency of papillary carcinoma in areas of endemic goiter, lead to the suggestion that the incidence of papillary carcinoma and follicular carcinoma are separately influenced by dietary iodide, papillary carcinoma being high in areas of high iodide intake and low in areas with low dietary iodide. No evidence to implicate lymphocytic thyroiditis, radiation or genetic factors in the genesis of thyroid carcinoma in Iceland or NE Scotland was found in this study. Undifferentiated carcinoma was 3 times as common in Iceland as in NE Scotland. Malignant lymphoma of the thyroid was suprisingly common in NE Scotland, possibly related to the high frequency of thyroiditis found in this region. These studies suggest that the incidence of different histological types of thyroid malignancy is influenced by different etiological factors. They also provide support for the subdivision of thyroid malignancy into these different types, and for the general importance of accurate histological typing in cancer epidemiology.