Fig. 1

Imaging and histologic findings of the hyalinizing trabecular tumor. Ultrasonography revealed a well-defined, hypoechoic mass with microcalcification measuring 8 × 7 × 7 mm in the left lobe of the thyroid gland (a). CT scan revealed solid mass exhibiting slight homogeneous enhancement in the lower pole of the thyroid gland (b, c). FNAC demonstrated various clusters of follicular cells with enlarged and polygonal nuclei, fine granular chromatin, and intranuclear pseudoinclusions. Nuclear atypia was mild. The tumor was diagnosed as atypia of undetermined significance (AUS) (d). The specimen appeared as a well-circumscribed, tan-white, and homogeneous nodule (e). Histologically, the nodule consisted of wide trabeculae or small nests delimited by stromal bundles. Within the trabeculae, a variably abundant eosinophilic hyaline material was present, enveloping tumor cells. Tumor cells were large or medium-sized and polygonal or elongated, oriented perpendicular to the trabeculae (f). Tumor cell nuclei were convoluted with prominent membrane irregularities that formed intranuclear pseudoinclusions (g). Weak membranous and cytoplasmic staining was found by MIB1 (Ki-67) immunostaining (h). Scale bars: 50 μm