Merkel cell carcinoma with divergent differentiation

Abstract

Merkel cell carcinoma (MCC) is a rare primary neuroendocrine carcinoma of the skin, usually occurring at sun-exposed sites in elderly people. Divergent differentiation in MCC, although rare, has been reported in previous case series. We describe two new cases of MCC with divergent differentiation. Patient 1 was a 96-year-old man with a scalp lesion; on biopsy, the morphology and immunoprofile suggested MCC with divergent squamous differentiation. Patient 2 was an 87-year-old woman with a lesion on her leg, originally reported as squamous cell carcinoma, later showing extensive local recurrence. On review, primary histology showed an MCC with divergent differentiation, most likely trichilemmal carcinoma; the recurrence showed only MCC. These cases illustrate that MCC is capable of divergent differentiation, including squamous and adnexal morphologies. Correct diagnosis is essential for appropriate prognosis and management, as later recurrence or metastases may only show the Merkel cell component.