Histiocytic Sarcoma: Review, Discussion of Transformation From B-Cell Lymphoma, and Differential Diagnosis

Context.— Histiocytic sarcoma is a rare neoplasm of mature histiocytes with an aggressive clinical course that can arise de novo or from a low-grade B-cell lymphoma. In particular, chronic lymphocytic leukemia/small lymphocytic lymphoma is a very common malignancy in the Western hemisphere, and most cases of chronic lymphocytic leukemia/small lymphocytic lymphoma have an indolent course and behavior. However, 2% to 8% of chronic lymphocytic leukemia/small lymphocytic lymphoma cases transform. Histiocytic sarcomatous transformation is rare and portends poor prognosis. Objective.— To review the clinical features, morphology, and key points related to the differential diagnosis for histiocytic sarcoma. We discuss recent understanding of the biology underlying transformation. Data Sources.— University of Michigan case and review of pertinent literature about histiocytic sarcoma and morphologic differential diagnosis. Conclusions.— Histiocytic sarcoma is a rare histiocytic neoplasm that can arise as a result of transdifferentiation from low-grade B-cell lymphomas, and has a wide differential diagnosis including other histiocytic/dendritic cell neoplasms, myeloid neoplasms, lymphomas, melanoma, and carcinoma. However, some key morphologic and immunohistochemical features allow for accurate classification.