Follicular thyroid neoplasm: clinicopathologic features suggesting malignancy

Thyroid follicular neoplasms are the most common tumors of the thyroid. The criterion for their malignancy is evidence of capsular or vascular invasion, which makes preoperative diagnosis difficult. The poorly differentiated thyroid carcinoma entity was introduced by World Health Organization in its 2004 classification with an incidence still poorly known. We found 356 follicular neoplasms treated between 1990 and 2006. Among these tumor patients, adenomas were more common in women than in men (3.6:1), but carcinomas differed little with respect to gender (1.2:1). All follicular carcinomas (n = 39), atypical adenomas (n = 6), and oxyphilic adenomas (n = 15) were included in the study, as well as 30 consecutive conventional follicular adenomas. Five tumors were reclassified as poorly differentiated follicular thyroid carcinomas, representing 13% of carcinomas in this unselected material. Predictors of malignancy were high proliferation index (PI) by MIB-1 (p < 0.001), large tumor size (p < 0.001), and old age (p = 0.006). High PI was also a marker of worse prognosis in malignant tumors. Oxyphilic tumor cells were more frequent in carcinomas than in adenomas; however, among carcinomas, they were non-prognostic. Probability for malignancy is thus greater in a male patient with a large oxyphilic follicular neoplasm. The PI requires evaluation in all follicular thyroid carcinomas to identify poorly differentiated tumors with worse prognosis.