Occult carcinoma of the thyroid. A systematic autopsy study from Spain of two series performed with two different methods

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Abstract
Background. Occult carcinoma of the thyroid (OCT) seems to be present in a significant proportion of the general population. Previous studies have shown large variations in the prevalence rate of OCT, which may be due to differences in the prevalence rates between different geographic areas, but also to the lack of standardized diagnostic criteria and methods of examination. The epidemiologic features of OCT at autopsy in Spain are reported for the first time. Methods. To investigate the influence of methodology in the results, two series were studied, each one using a different method. In Series A, 625 cases were studied, and sections were taken only from grossly visible lesions. In Series B, the whole thyroid glands of 100 autopsies were cut into blocks and all blocks were histologically studied. In addition, immunocytochemical stainings were performed in Series B for calcitonin, thyroglobulin, and epidermal keratin. Results. Series A found 33 OCTs (5.28%), consisting of 29 occult papillary carcinomas (OPC), 2 occult follicular carcinomas, 1 occult oxyphilic carcinoma, and 1 occult medullary carcinoma. Series B found 22 OPC cases (22%) containing a total of 53 tumor foci. Tumor diameter varied from 0.07 to 1.8 mm. The epithelial cells of all the OPC were negative for calcitonin, weakly positive for thyroglobulin, and intensely positive for epidermal keratin. One case had metastasis in a regional lymph node. Conclusions. This study shows that the differences in the incidence of OCT found in numerous studies are due not only to actual different geographic incidences but also to the method of study.