LAUSR

The phenomenon of histiocytic/dendritic cell sarcomas arising through transformation of a pre‐existed lymphoproliferative disease is called transdifferentiation. Langerhans cell sarcoma transdifferentiating from chronic lymphocytic leukemia/small lymphocytic lymphoma is extremely rare and all the reported cases were localized in lymph nodes. We present a case of concurrent cutaneous localization of Langerhans cell sarcoma and chronic lymphocytic leukemia/small lymphocytic lymphoma, in which the chronic lymphocytic leukemia/small lymphocytic lymphoma preceded the development of the Langerhans cell sarcoma. A cutaneous lesion from a 63‐year‐old patient with a history of chronic lymphocytic leukemia/small lymphocytic lymphoma was biopsied. The histologic examination revealed a mixture of two cell populations infiltrating diffusely the dermis. The first was composed of small lymphoid cells with somewhat monotonous appearance and mild nuclear atypia positive for PAX5, CD79a, CD20, CD23, CD5, and LEF1. The second was composed of large cells with abundant cytoplasm and pleomorphic nuclei. These cells were positive for CD1a, CD207, and S100 protein and exhibited a high mitotic rate and a high MIB‐1 immunostaining index. Therefore, two different entities, chronic lymphocytic leukemia/small lymphocytic lymphoma and Langerhans cell sarcoma, were detected in the same skin fragment. The patient died 3 years after initial diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma.