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Herein, we report a rare case of a trabecular variant of thyroid follicular adenoma in a 40-year-old Japanese woman. The nodular tumor, 40 × 25 × 20 mm in size, showed extensive growth and sharp demarcation within the left lobe of thyroid gland, exhibiting entirely trabecular histological characteristics. Moreover, the atypical trabeculae were extraordinarily elongated and serpentine in form. The longest trabeculae visible in the sections were approximately 40 mm in length, arranged in irregular folds. The tumor cells showed homogenous distribution; however, the trabeculae showed irregular distribution with either dense or diffuse localization, in association with edematous stroma. No follicular structures were detected in the tumor, and no vascular and capsular invasions were observed. Immunohistochemically, tumor cells were positive for thyroglobulin, whereas laminin-positive staining was clearly visible in the surrounding trabeculae. Moreover, both amyloid A and calcitonin immunostainings were negative, and the tumor cells exhibited an extremely low labeling index of <1% for MIB-1 immunostaining; moreover, no cell membrane or cytoplasmic staining was observed for MIB-1. No signs of lymph node metastasis or malignancy were detected. We present a case of a purely trabecular variant of thyroid follicular adenomas, with extraordinarily long trabeculae. More cases of this tumor type should be identified in order to characterize its biological behavior.
Extraordinarily long trabeculae, Trabecular type, Follicular adenoma, Thyroid gland