Basosquamous carcinoma involving the anterior skull base: a neglected tumor treated using intraoperative navigation as a guide to achieve safe resection margins.

Basosquamous carcinoma (BSC) or metatypical carcinoma is a rare and controversial form of basal cell carcinoma (BCC) that was first described by MacCormac in 1910. Numerous theories have been described in the literature regarding its origin but it is generally accepted as a variant of BCC that differentiates into squamous cell carcinoma (SCC). At present, it has an unsatisfactorily established phenotype but is considered to behave more like an SCC than a BCC. A reflection of this fact is its more aggressive nature with a significantly increased incidence of local and distant metastasis. It must be distinguished histologically from so-called "collision" tumors, which represent SCC and BCC arising as separate entities in the same area. We present a unique case of a neglected, large, and locally advanced case of BCC originating from the nose, but extending into the paranasal sinuses, orbit, and anterior skull base, that was treated with en bloc surgical resection using intraoperative navigation to assist in treatment planning and margin clearance.