Hurthle cell adenoma of the mediastinum: intraoperative cytology and differential diagnosis with correlative gross, histology, and ancillary studies

A 66-year-old man was found to have a 7.5 cm mediastinal mass detected on routine chest X-rays as part of his preoperative work up for an inguinal hernia repair. An orthotopic (normally located) nongoitrous thyroid gland without evidence of connection to the mediastinal mass was also identified. The clinical differential diagnoses included lymphoma, thymoma, and germ cell tumor. Fine-needle aspiration (FNA) biopsy smears and touch imprints of the mediastinal mass showed a loosely cohesive, highly cellular population of relatively uniform cells with abundant granular cytoplasm, low nuclear to cytoplasmic (N/C) ratios, and prominent nucleoli consistent with a Hurthle cell (HC) neoplasm. Subsequently, the diagnosis of HC adenoma was confirmed on the surgically excised mediastinal mass. To the best of our knowledge, the surgical pathology and cytologic features of an HC adenoma of the mediastinum have not been reported in the literature. The gross, histologic, immunohistochemical, and electron microscopic (EM) findings, in addition to the cytologic features, are presented along with a differential diagnosis of this mediastinal neoplasm.