Utility of high molecular weight cytokeratins, but not p63, in the differential diagnosis of neuroendocrine and basaloid carcinomas of the head and neck.

High-grade neuroendocrine carcinomas of the head and neck overlap significantly in morphology with both basaloid squamous and solid-type adenoid cystic carcinomas. High-grade neuroendocrine carcinomas have sheets of small cells with scant cytoplasm, granular chromatin, and inconspicuous nucleoli. Basaloid squamous and adenoid cystic carcinomas are aggressive variants of their respective tumor types which both have nests of basaloid tumor cells with round nuclei, little cytoplasm, and inconspicuous nucleoli. As the management and prognosis of these tumors are very different, it is important to differentiate them. We performed high molecular weight cytokeratin (CK) and p63 immunohistochemistry on 19 neuroendocrine carcinomas, 18 basaloid squamous carcinomas, and 11 solid-type adenoid cystic carcinomas. All tumors were immunostained for p63, CK 34betaE12, CK 5/6, synaptophysin, chromogranin-A, S-100, and smooth muscle actin. All basaloid squamous and adenoid cystic carcinomas were positive for CK 5/6 and 34betaE12. Only 4 and 5 of the 19 neuroendocrine carcinomas, respectively, were positive for these markers. Staining was focal in the neuroendocrine cases when positive, whereas almost all basaloid squamous and adenoid cystic carcinomas showed strong staining. Almost all tumors of each type were positive for p63, including neuroendocrine carcinomas, but with different staining patterns. Basaloid squamous carcinomas were diffusely positive, neuroendocrine carcinomas were diffusely positive, but with weak staining, and adenoid cystic carcinomas showed a distinct pattern with staining at the periphery of the cell nests only. We conclude that high molecular weight cytokeratin immunostaining is helpful in distinguishing high-grade neuroendocrine carcinomas from similar tumor types.