Answer: Hurthle cell adenoma

Histology: On cut section, the tumor measured 6 cm, and it was round, solitary and surrounded by a thin (1 mm) capsule. Grossly there was no evidence of disruption of the capsule by the tumor. By light microscopy, the tumor demonstrated a highly cellular growth pattern including a sheet-like arrangements, trabecular formations and microfollicles. Some areas of the tumor exhibited papillary-like structures. The tumor cells were characterized by abundant pink granular cytoplasm and round to oval central nuclei with prominent nucleoli. Cells with large hyperchromatic and misshapen nuclei were scattered throughout the tumor. Careful inspection of the entire tumor/capsule interface failed to disclose any areas of capsular penetration or vascular invasion.

Discussion: Hurthle cell adenoma is defined as a benign tumor of follicular epithelial origin composed exclusively or predominantly (>75%) of oncocytes. Although this definition appears conceptually quite straightforward, confusion regarding the diagnosis, nature and behavior of Hurthle cell tumors abound. Part of the problem is conceptual and related to an old but persistent notion that all Hurthle cell tumors (particularly large ones) should be uniformly regarded as malignant. This notion has been dispelled by a number of studies that have conclusively shown that Hurthle cell tumors should be evaluated by using the standard criteria which are applied to all well-differentiated thyroid neoplasms (Although Hurthle cell carcinomas as a group tend to be larger than their benign counterparts, size is not a malignant criteria.). Other problems contributing to confusion over Hurthle cell adenomas are related to certain troubling morphologic features. For example, the presence of scattered cells with enlarged hyperchromatic nuclei are commonly encountered in Hurthle cell adenomas. Although these cells may have an alarming cytologic appearance, there presence alone is not diagnostic of malignancy and they – by themselves – are of no prognostic significance. Another feature that can cause alarm is the apparent presence of papillae. These probably do not represent true papillae. Instead, when the long and thin septae separating adjacent follicles are cut tangentially, one can get the impression of a papillary lesion. Importantly, the cells lining these papillae do not exhibit the hallmark nuclear atypia of papillary carcinoma.