Abstract:

An 81-year-old male presented with a rapidly growing nodule on the right cheek. Biopsy revealed a dermal infiltrate of large atypical cells, some exhibiting a horseshoe-shaped nucleus. The epidermis was uninvolved. Immunohistochemistry revealed positivity for CD4, CD3, CD45, and CD30 (>95%). Melanocytic markers were negative, as well as CD8, CD20, CD56, ALK1, synaptophysin, CD1a, and ERG. Notably, there was weak but diffuse expression of pan-cytokeratin (AE1/AE3) and Oscar keratin in a membrane and cytoplasmic staining pattern. There was also weak expression of EMA. CAM 5.2, p40, TIA-1, granzyme B, perforin, and IRF4/DUSP22 rearrangement studies were negative. Further staging revealed skin-limited disease without nodal involvement. A diagnosis of primary cutaneous anaplastic large cell lymphoma (PC-ALCL) was rendered. We present a rare case of cytokeratin positivity in PC-ALCL, a finding never reported in the literature. Both PC-ALCL and systemic ALCL (S-ALCL) evoke a broad differential. In cases with lymph node involvement, CD45, EMA, and cytokeratin stains can help differentiate from metastatic carcinoma. Most lymphomas will be negative for EMA and cytokeratin, as opposed to carcinomas. There have been rare prior reports of cytokeratin expression in S-ALCL, which tend to show an unusual pattern of cytoplasmic and membranous expression like our case, have variable co-expression of EMA, and have null T-cell phenotypes. These demonstrate the significant diagnostic challenges that can arise in differentiation from metastatic or primary skin carcinomas. Awareness of the issue, careful attention to morphology (e.g. hallmark cells), and considering routine CD30 can help lead the pathologist to the correct diagnosis.

Financial Disclosure:
No current or relevant financial relationships exist.

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Jack Lee, MD