Hybrid thyroid carcinoma with a coarse chromatin pattern and nuclear features of papillary thyroid carcinoma

Hybrid follicular carcinoma (FC) and papillary thyroid carcinoma (PTC) have not been previously well described. Consecutive cases of 29 FC, 12 Hurthle cell carcinomas (HC), 247 PTC and 13 Hurthle cell PTC (HPTC) were reviewed with special attention to the coarse (CC) and fine chromatin patterns (FIC), as well as to the presence of nuclear grooves, pseudoinclusions or optically clear appearance. Limited nuclear features of PTC (LNF-PTC) are defined as areas of tumor with FIC in addition to some other nuclear features, but insufficient for the diagnosis of PTC. Tumors with nuclei showing an admixture of CC and PTC or LNFPTC were submitted for immunostaining for cytokeratin 19, HBME and Ret/PTC. FC and HC contained areas of LNFPTC in 25 tumors and focal PTC in 3 tumors. None of these cases was associated with lymph node metastasis. Areas with CC were found in 54 PTC and 3 HPTC. The rates of vascular invasion and distant metastasis tended to be higher for PTC with areas of coarse chromatin pattern than for PTC without such areas; however, the difference was not statistically significant. Immunoreactivity for cytokeratin 19 and HBME was moderate to strong for PTC and focal areas of PTC or LNFPTC in FC without Hurthle cell changes. Ret/PTC immunostaining was positive in areas of LNFPTC or focal PTC in three FC. Focal PTC or areas of LNFPTC are frequently seen in FC. Likewise, areas of CC are often present in PTC. The presence of these focal areas does not appear to change the clinical behavior of the tumor and therefore does not warrant a change of nomenclature.