Fig. 9.1
Basal cell carcinoma with matrical differentiation. This punch biopsy demonstrates a basaloid tumor with epidermal connection. There is peripheral palisading of basaloid nests and focal tumor–stroma clefting. Islands of tumor demonstrate deeply eosinophilic “shadow cells”
Fig. 9.2
Basal cell carcinoma with matrical differentiation. High magnification shows a nest of basal cell carcinoma that abruptly transitions to shadow cells. The shadow cells are characterized by eosinophilic cytoplasm, distinct cell borders, but no nuclear staining
Pilomatrical tumors (namely, pilomatricoma and pilomatrical carcinoma) have been described to have mutations in CTNNB1 (the gene encoding β-catenin). A mutation in this gene has been variably detected (in 2 of 5 tumors tested) in basal cell carcinomas with matrical differentiation. Interestingly, the two tumors with mutations harbored the identical mutation in exon 3.
Differential Diagnosis
Pilomatricoma (pilomatrixoma) is a benign adnexal tumor that recapitulates the hair matrix and demonstrates variable mixtures of basaloid and shadow cells, but is usually histologically distinct from basal cell carcinoma. Pilomatrical carcinoma (pilomatrix carcinoma) is the malignant counterpart of pilomatricoma and classically exhibits extreme cytologic pleomorphism, atypical mitotic figures, and an infiltrative border. It typically lacks the tumor–stroma retraction and nuclear palisading of tumor islands that is seen in basal cell carcinoma.
Both pilomatrical carcinoma and pilomatricoma have been shown to demonstrate diffuse and intense nuclear positivity for β-catenin. This is in opposition to the membranous and sometimes cytoplasmic pattern of immunoreactivity described in cases of basal cell carcinoma with matrical differentiation and the usual pattern of immunoreactivity in conventional nodular basal cell carcinomas.
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