Hyalinizing trabecular
tumor (HTT) of the thyroid gland is rare and benign, and it neither recurs nor metastasizes. In this lesion,
tumor cells are arranged in trabeculae, in association with hyalinizing mass in the stroma. The origin and nature of the hyalinizing mass are still controversial. We report here a case of HTT with cytological, immunohistochemical, and ultrastructural findings, focused in particular on the hyalinizing mass. Cytologically,
tumor cells exhibiting many intranuclear cytoplasmic inclusions and nuclear grooves were found in association with light green-positive, irregular, fluffy membranous structures on touch smear. Staining with antibody to
collagen type IV was positive in these membranous structures. Histopathologically,
tumor cells exhibited many intranuclear cytoplasmic inclusions, and were positive for staining with
antibodies to
S100 protein,
neuron-specific enolase,
thyroglobulin, and
vimentin. The hyalinizing eosinophilic mass, which was positive for PAS reaction, and for staining by antibody to
collagen type IV, gradually increased in the areas surrounding
tumor cells. This mass then appeared to replace the
tumor cells, and exhibited a peculiar filiform pattern. We demonstrated ultrastructurally that this pattern was composed of long, irregular, fine cytoplasmic processes of
tumor cells and basal lamina-like substance in the hyalinizing mass. In fact, the homogeneous hyalinizing mass, similar to basal lamina-like substance, contained many degenerated cytoplasmic processes at the ultrastructural level. These results suggested that the key cytological finding in differentiating HTT from
papillary carcinoma is the fluffy membranous structure, although nuclear pseudoinclusions are important as well. The filiform pattern noted at light microscopic level consisted of long cytoplasmic processes of
tumor cells and hyalinized mass at the ultrastructural level.