Cutaneous lymphoma-simulating Merkel cell carcinoma-molecular genetic demonstration of a clonal disease with divergent immunophenotypes.

Merkel cell carcinoma and malignant lymphoma are important differential diagnoses for undifferentiated cutaneous round cell tumors and immunohistochemistry is instrumental in their evaluation. We describe a case of a 73-year-old man who had cutaneous large cell lymphoma in the right leg (immunophenotype CD45+, CD19+, CD20+ CD22+, lambda clonal, cytokeratin-, NSE-) and lymphoma in left leg simulating Merkel cell carcinoma showing absence of leukocyte antigens (CD45-, CD20-, no light chains) and focal expression of keratin and NSE. However, analysis of polymerase chain reaction amplification products of DNA extracted from both lesions showed two amplifiable sharp bands indicating clonal rearrangements of both alleles of the immunoglobulin heavy chain. Cloning and sequencing of the products from left and right leg lesions showed either 100% homology (one band), or close similarity (the other band), indicating that both tumors were derived from the same B-cell lymphoma clone. This case shows the value of polymerase chain reaction and sequencing in analyzing the ultimate nature of lymphoproliferations and illustrates the potential limitations of immunophenotyping.