Unusual thyroid carcinoma with excessive extracellular hyaline globules: a case of "hyalinizing papillary carcinoma"

We present an unusual case of papillary thyroid carcinoma in a 47-year-old Japanese woman. The tumor, 0.8 cm in diameter, was located in the upper left lobe of the thyroid. Histologically, we observed a microfollicular-like and trabecular arrangement of the tumor cells with marked hyalinized stroma and hyaline globules. Immunohistochemically, tumor cells were positive for thyroglobulin and thyroid transcription factor 1. Hyaline stroma and globular bodies were immunopositive for laminin and type IV collagen. MIB-1 index was approximately 1% without membranous immunoreactivity. Under the electron microscope, hyaline stroma and globules showed electron-dense, complex meshwork structures composed of granular and fibrous elements similar to the structure of the lamina densa. Genetic analysis demonstrated a BRAF(V600E) mutation. Based on these findings, we diagnosed the present tumor as a rare morphological variation of papillary thyroid carcinoma with excessive hyaline globules consisting of basal membrane materials.