Metatypical basosquamous cell carcinoma is a subset of cutaneous cancers that poses an increased risk of metastases to the patient due to its aggressive behavior. The classic histology of metatypical basosquamous cell carcinoma includes features of both basal cell carcinoma and squamous cell carcinoma, as well as areas of intermediate differentiation. The cells are larger and more squamoid, there is no palisading, and the stroma is fibrous rather than myxoid. The key to the diagnosis of basosquamous carcinoma is the absence of a transition zone between the basal cell and the squamous cell types. Basosquamous carcinoma is not a collision carcinoma of basal cell and squamous cell carcinoma. It appears that basal cell carcinoma undergoes squamous cell differentiation, which in turn alters its normal biologic behavior. Thus, these intermediate lesions have a greater tendency to recur and metastasize. Metatypical basosquamous cell carcinoma, in and of itself, is an indication for Mohs surgery due to its aggressive biological nature.
This needs to be differentiated from basal cell carcinoma with keratinization. Many basal cell carcinomas ulcerate; the resultant inflammation at the periphery induces reactive keritinization. The fibrous pattern can also be found in non-metatypical basal cell carcinomas. There, physical trauma, such as patient picking the lesion, may have been present overlying the basal cell carcinoma ulceration. Again, this is not equivalent to the more aggressive metatypical basosquamous cell carcinoma, but indeed represents variants of basal cell carcinoma behavior.
Often, the use of the term basosquamous cell carcinoma does not adequately describe the nature of the cutaneous cancer and thus requires further definition of the term. True metatypical basosquamous cell carcinomas must be treated as highly aggressive tumors with metastatic potential. As clinicians, we must rely on our pathology colleagues to make that differentiation, and we must question each diagnosis of basosquamous cell carcinoma.