Answer: Metastatic basaloid squamous cell carcinoma

Histology: The specimen consists of a 3 cm circumscribed nodule. Microscopically, the tumor is composed of basaloid cells in a solid lobular arrangement. The cells are small and crowded with scant cytoplasm and hyperchromatic nuclei. Many of the solid nests exhibit central comedo-type necrosis. Within the solid nests are small cystic spaces filled with eosinophilic stromal material. Special studies were performed. The tumor cells are immunoreactive for CK7 and p16; and they are not immunoreactive for CK20 or various neuroendorcrine markers. An HPV16 hybridization study demonstrates a punctate hybridization signal in the tumor cell nuclei.

Discussion: Basaloid squamous cell carcinoma (BSCC) is a histologic variant of squamous cell carcinoma first described by Wain in 1986. BSCC differs from conventional squamous cell carcinoma histologically and clinically. Microscopically, BSCC is a biphasic carcinoma with a prominent basaloid component and a minor component showing more overt squamous differentiation in the form of surface dysplasia and/or focal keratinization.

Clinically, it is recognized as a particularly aggressive form of squamous carcinoma. Patients typically present with more advanced disease including metastatic spread to regional lymph nodes. These tumors have a predilection for the hypopharynx, supraglottic larynx, and oropharynx.

The main differential diagnosis includes solid variant of adenoid cystic carcinoma and small cell carcinoma. Negativity for neuroendocrine markers can be useful in excluding small cell carcinoma. Distinction from solid variant of adenoid cystic carcinoma can be particularly problematic in cases like this where overt squamous differentiation is not appreciated. It is noteworthy that areas exhibiting classic cribriform growth can almost always be uncovered even in solid variants of adenoid cystic carcinoma with careful histologic evaluation. Correlation with the clinical and radiographic features is also important. Adenoid cystic carcinoma rarely if ever metastasize to cervical lymph nodes in the absence of a conspicuous salivary gland mass. The same is not always true for BSCCs. These can often present as large cervical masses with occult primary tumors, particularly those arising from the tonsillar crypts.

The vast majority of basaloid carcinomas arising from the oropharynx are caused by HPV16. Accordingly, the documentation of HPV16 in a metastatic implant strongly points to the oropharynx as the primary site of tumor origin. In the present case, the ipsilateral tonsil was removed and found to harbor a small BSCC.