Malignant teratomas of the thyroid gland: clinico-radiologic and cytomorphologic features of a rare entity

Introduction: Primary thyroid gland malignant teratomas are extremely rare and can pose diagnostic challenges on fine needle aspiration (FNA) due to their cytomorphologic heterogeneity. Recent next generation sequencing studies have identified recurrent DICER1 hotspot mutations in these tumors, suggesting that malignant teratomas of the thyroid should be considered a distinct pathological entity. Herein, we review the clinico-radiologic and FNA findings in a series of DICER1 mutated malignant teratomas. Methods: We performed a retrospective case review of 9 FNAs from 5 patients with a histologically confirmed malignant teratoma of the thyroid gland from 2 large tertiary care pathology practices. Results: The patients included 4 females and 1 male, with an average age of 43 years (22-65 years). The nodules were centered within the thyroid gland and ranged from 1.7 to 10 cm in diameter. FNAs of primary thyroid teratomas demonstrate marked cellularity, epithelial proliferations, an absence of colloid, and a predominance of immature spindled cells, representing the mesenchymal and neural ectodermal components of these tumors. The FNA interpretations ranged from atypia of undetermined significance to overtly malignant. Three patients died of their disease and 2 are alive with no evidence of disease. Conclusions: Malignant thyroid teratoma is a rare entity with cytomorphologic overlap with other high-grade neoplasms of the thyroid. Recent molecular studies have defined recurrent DICER1 mutations in malignant thyroid teratomas and propose these as a distinct clinicopathological entity. The features described here may be helpful in providing a correct prospective interpretation.