Reappraising basosquamous carcinoma: a summary of histologic features, diagnosis, and treatment.

Basosquamous carcinoma (BSC) is a malignant and aggressive neoplasm with unclear molecular etiology. It is often misdiagnosed as basal cell carcinoma (BCC) or squamous cell carcinoma (SCC) on biopsy as there are unclear histologic criteria for this neoplasm. It has been interchangeably referred to as metatypical BCC and collision tumor, although these entities are different. On histology, BSC consists of basal cells with areas with nests of squamous cells and an intermediate transition zone. The nature of this transition zone is not clearly defined in literature; however, Ber-EP4 staining is diagnostic for BSC. A gradation of Ber-EP4 staining from strongly positive in basaloid areas to weakly positive in an intermediate zone is demonstrated (no staining of squamous areas). Treatment with an array of modalities including wide local excision, Mohs surgery, radiotherapy, and palliative chemotherapy has been performed. We recommend further molecular studies in understanding the genetic mechanisms leading to BSC. For the purpose of good clinical practice, multiple biopsies and immunohistochemical studies should be performed to avoid sampling error that can lead to a misdiagnosis of BSC.