Observer Variation in Histologic Classification of Thyroid Cancer

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Abstract
Histologic slides of 696 cases of thyroid cancer reported to the national cancer registries of Finland, Iceland, Norway and Sweden were reviewed by 5 Nordic pathologists to determine the observer variation between the pathologists and the reproducibility of the WHO classification of thyroid tumors. In 58% of the cases all observers agreed on the diagnosis and in 82% at least 3 of them agreed. The observer disagreement was lowest for papillary carcinoma (7%) and highest for follicular carcinoma (27%). The corresponding figures for anaplastic and medullary carcinomas were 18% and 23%, respectively. The most common diverging diagnosis for cases finally interpreted as papillary carcinoma was follicular carcinoma, and for cases finally interpreted as follicular carcinoma, a benign thyroid lesion. The necessity of having all cases reviewed by the same pathologist or group of pathologists to obtain reliable results for comparative studies is indicated.