Answer: Squamous cell carcinoma, basaloid type (basaloid squamous cell carcinoma)

Histology: There is high-grade dysplasia of the surface epithelium. The tumor is growing as large and expansive lobules within the submucosa. The centers of many of the lobules display prominent comedo-type necrosis. In some of these lobules, cells at the edges show nuclear pallisading. The cells have a high N:C ratio and exhibit pleomorphism, a high mitotic rate, and individual cell necrosis. Within some of the lobules, the basaloid cells show an abrupt transition with cellular nests showing overt squamous differentiation consisting of abundant eosinophilic cytoplasm and cellular bridges.

Discussion: Basaloid squamous cell carcinoma (BSCC) is a histologic variant of squamous cell carcinoma of the head and neck. It is clinically characterized by rapid growth, a propensity for metastatic spread, and a tendency to present at an advanced stage. Although it can arise anywhere in the upper respiratory tract, it has a strong predilection for the pyriform sinus, base of tongue, supraglottic larynx, and the palatine tonsil.

BSCC is histologically characterized by expanding nests of basaloid cells with a high nuclear to cytoplasmic ratio. Given its prominent basaloid morphology, it is easily confused with other basaloid neoplasms including the solid variant of adenoid cystic carcinoma and small cell undifferentiated carcinoma. Distinction from adenoid cystic carcinoma relies heavily on the demonstration of clear-cut squamous differentiation. This may take the form of dysplasia of the overlying surface epithelium, abrupt squamous eddies within the basaloid nests, or a separate component of conventional squamous cell carcinoma. Due to the focal nature of these findings, distinction from adenoid cystic carcinoma may be particularly problematic on biopsy. Immunohistochemistry may be of some use if the distinction between small cell carcinoma and BSCC cannot be reliably made on morphologic grounds. Although BSCC may show positive staining for NSE, it lacks immunoreactivity for more specific markers of neuroendocrine differentiation including chromogranin and synaptophysin.