Basosquamous Carcinoma — SCE Dermatology MCQ

Correct answer: B — Basosquamous carcinoma (metatypical BCC)

The correct answer is B, basosquamous carcinoma (metatypical BCC). This tumour shows histological overlap between basal cell carcinoma and squamous cell carcinoma, with basaloid cells displaying peripheral palisading alongside areas of squamous differentiation and keratinisation intermingled within a single lesion. Current BAD guidance classifies this as a distinct high-risk BCC subtype rather than either parent tumour alone, and notes that the basosquamous variant is the most aggressive type, with a tendency for lymphovascular or perineural invasion. This behaviour, intermediate between BCC and SCC with genuine metastatic potential, mandates wider excision margins or Mohs surgery and closer follow-up compared with low-risk BCC subtypes. Why the other options are wrong: C. Collision tumour: this describes two histologically and biologically distinct tumours (for example BCC and SCC) arising adjacent to each other by coincidence, with clear separation between the two components rather than true intermingling of basaloid and squamous elements within one tumour. D. Nodular BCC: this is the commonest BCC subtype, composed purely of basaloid cells with peripheral palisading and no squamous differentiation or keratinisation, and behaves as a low-risk lesion. A. Trichoblastoma: this is a benign follicular tumour composed of basaloid cells resembling BCC but lacking cytological atypia, stromal retraction and any squamous or keratinising component, and does not behave malignantly. E. Keratoacanthoma: this is a rapidly growing, often self-resolving squamoproliferative lesion with a central keratin plug and glassy squamous cells, but it lacks a basaloid palisading component and is not a hybrid BCC/SCC tumour. Key point: intermingling of basaloid and keratinising squamous elements within the same tumour defines basosquamous carcinoma, a high-risk BCC subtype requiring wider margins, not a coincidental collision tumour.

Reference: British Association of Dermatologists guidelines for the management of adults with basal cell carcinoma 2021, British Journal of Dermatology, https://onlinelibrary.wiley.com/doi/10.1111/bjd.20524