Follicular adenoma with papillary architecture: a lesion mimicking papillary thyroid carcinoma

Aims: The purpose of this study was to investigate the significance of 'ben ign' encapsulated follicular thyroid nodules with papillary structures. Methods and results: Twenty-one cases of encapsulated neoplastic thyroid no dules with papillary structures and nuclear features not diagnostic of papi llary thyroid carcinoma (PTC) were obtained, All cases were reviewed with p articular attention to nuclear features (fine chromatin pattern, optical cl earing, grooves and inclusions), Representative sections were submitted for measurement of the maximum diameter of,OO round or nearly round nuclei and for immunostaining for MIB1, CK19, HBME and Ret oncogene protein, Nine cas es displayed scattered optically clear nuclei or nuclear grooves in less th an 30% of total neoplastic cells. They were grouped in the category of thyr oid nodules with limited nuclear features of papillary thyroid carcinoma (P TC), but not diagnostic of PTC. The other 12 cases had fine or coarse chrom atin, but lacked other features of nuclei in PTC. The diameter of the nucle i ranged from 5.6 to 7.2 mum and were smaller than those of PTC (6.3-10.0 m um). Immunostaining revealed positive reactivity for MIB1 in the papillary structures. Immunostaining for CK19 and HBME varied from negative or focall y weak to diffusely moderate reactivity. Ret oncogene protein immunostainin g showed focal and weak reactivity in one case and was negative in other ca ses of the study, Clinical follow-up from 6 months to 15 years revealed no evidence of metastasis. Conclusions: The papillary structures in the study cases are unlikely to re present degenerative changes due to their proliferative activity, In view o f (i) the encapsulation and the uniformity of the constituent cells, (ii) t he varying degrees of immunoreactivity for CK19 and HBME and negative immun oreactivity for Ret oncogene protein, and (iii) the absence or insufficienc y of nuclear criteria for the diagnosis of PTC and the absence of lymph nod e metastasis in all study cases, we believe that these lesions represent th e papillary variant of follicular adenoma. Recognition of this pathological entity is important to avoid an over-diagnosis of PTC.