Basaloid skin tumours: Basal cell carcinoma

SummaryBasal cell carcinoma (BCC) is the prototypical basaloid tumour of the skin, but may show various patterns simulating other cutaneous tumours, particularly squamous cell carcinoma and trichoepithelioma (TE). Other challenges are presented by BCC with marked pleomorphism, glandular differentiation, neuroendocrine differentiation, clear cells and sarcomatoid change. Peripheral palisading and retraction (artefact) of the epithelium from the stroma are the most useful features to support a diagnosis of BCC, but can occasionally be seen in other benign and malignant cutaneous neoplasms and are inconspicuous or lacking in some cases of BCC. Increased ‘stromal’ mucin accompanying retraction and peripheral palisading is extremely suggestive of BCC and is rarely seen in other tumour types. Diffuse and strong BerEP4 staining and absence of epithelial membrane antigen staining is characteristic of BCC, and is an immunophenotype rarely encountered in other tumours except for TE. CD10 staining of the basaloid epithelium, in the absence of significant stromal staining, may support a diagnosis of TE-like BCC rather than TE in more organized variants.