Case 65

A 67 year old male presented with a 4.5 cm skin lesion on the upper back.

Guess the stain

1. What's your diagnosis?

   A: trichoepithelioma

   B: Basal cell carcinoma, nodular

   C: Squamatization basal cell carcinoma

   D: Basosquamous carcinoma

   E: Basal cell carcinoma with squamous differentiation

Answer

There are various ways to report this case, and per WHO, the correct answer is D. The stain is BerEP4. Basosquamous carcinoma exhibits distinct histologic and immunophenotypic zones of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) in varying proportions. The two components may transition gradually or be closely intertwined.

The BCC areas display typical basaloid cell growth, often with an infiltrative pattern, while SCC regions show histologic atypia and keratinization. Perineural invasion occurs in 5.5% of cases. Differential diagnosis: Nodular BCC with squamous differentiation: Shows focal keratinization within basaloid nests but lacks high-grade atypia.

Metatypical BCC or basal cell carcinoma with squamous differentiation are not recommended.

References: https://tumourclassification.iarc.who.int/chaptercontent/64/28

Case credit: UCSD Pathology

Author: Wangpan Shi, MD