Anaplastic thyroid carcinoma with rhabdoid features.

Anaplastic thyroid carcinoma (ATC) is a rare, highly aggressive neoplasm, characterized by complete or partial composition by undifferentiated cells. We report a case of ATC with rhabdoid features in a 68-year-old male, who presented with a rapidly enlarging neck mass. Fine-needle aspiration (FNA) of the thyroid mass showed discohesive, pleomorphic round to polygonal rhabdoid cells with one to multiple eccentric, large, rounded nuclei with a prominent nucleolus, moderate to abundant, globoid cytoplasm which oftentimes harbor a pale para-nuclear inclusion. The cytoplasm of some cells contained variously sized, eosinophilic granules. Rare cells contained neutrophils in their cytoplasm. Mitoses including atypical mitotic figures and necrosis were readily seen. Histologic examination of needle core biopsy (NCB) revealed individual dispersed and sheets of pleomorphic neoplastic cells with similar cytomorphologic features as described above. The tumor extensively infiltrated a myxocollagenous stroma containing lymphocytes and neutrophils, and demonstrated foci of necrosis. Tumor cells were immunoreactive for keratins AE1/AE3, CAM5.2, and CK19; PAX-8, and p63, but negative for S-100, HMB-45, calcitonin, TTF-1, thyroglobulin, CD56, HBME-1, glypican-3, PAX-5, myogenin, CD31, and INI-1. The differential diagnosis of this malignant rhabdoid tumor is discussed.