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THYROID follicular cell tumors present a unique model for the study of genetic and environmental factors predisposing to benign nodule formation, well differentiated malignant tumors, and anaplastic cancer. Although these histological changes are not necessarily sequential, there is evidence that gradation of proliferative and differentiative potential exists among cells in each thyroid follicle. Conditions conducive to rapid growth ensure that the progeny of cells with high growth potential establish local dominance, a prelude to nodule formation (2). There is also evidence that well differentiated carcinoma may progress to an anaplastic form (3). It is only speculation that benign nodules may develop into well differentiated carcinoma.

Cancer is a complex, multistep process (4). Based on the measurements of age-dependent tumor incidence, it was inferred mathematically that a succession of five or six independent …