Benign Hürthle cell adenoma with papillary architecture: a benign lesion mimicking oncocytic papillary carcinoma

We studied the significance of encapsulated Hürthle cell thyroid nodules with papillary structures lacking the nuclear features of papillary thyroid carcinoma (PTC); 19 cases fulfilling these criteria were encountered The patients' ages ranged from 22 to 40 years (32+/-6), and the F:M ratio was 3:1 The tumors measured from 0.5-5 cm (2+/-1.1). The diameter of the tumor cell nuclei ranged from 5.6 to 7.2 microns. Many nodules had nuclei displaying a fine chromatin pattern somewhat resembling those of PTC, but these were present in <20% of the tumor cells. Immunohistochemically, there was reactivity for MIB-1 in the papillary structures, negativity to focally weak reactivity for HBME and galectin-3, and negativity to moderate diffuse reactivity for CK19. Clinical follow-up from 1 to 19 years revealed no evidence of metastases in any of the cases. It is unlikely that the papillary structures in the study cases represent degenerative changes in view of the proliferative activity we have demonstrated in them. In view of (1) the encapsulation and the uniformity of the constituent cells, (2) the negative or weak immunoreactivity for galectin-3 and HBME and negative to moderate immunoreactivity for CK19, and (3) the absence or paucity of nuclear criteria for the diagnosis of PTC and the absence of lymph node metastasis in all study cases, we believe that these lesions represent the papillary variant of oncocytic follicular adenoma (Hürthle cell adenoma). Recognition of this entity is important to avoid an overdiagnosis of oncocytic PTC.