Histiocytic/Dendritic Cell Neoplasms: Primary and Transdifferentiated

Abstract

Histiocytic/dendritic cell neoplasms are derived from bone marrow monocytic or soft tissue mesenchymal precursors. The term histiocyte has been used as an umbrella term for all extramedullary macrophage and dendritic cells. According to the updated 2017 WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues, there are at least eight distinct entities under this category. The paucity and morphologic similarities of these tumors make it difficult to reach a definitive diagnosis and also have hindered the ability to study their clinical and biologic properties. As a pathologist, it is important to recognize these rare tumors and to be familiar with their microscopic and immunophenotypic characteristics and the differential diagnoses. This chapter will focus on providing the key microscopic features and immunophenotypes of the relatively frequently encountered histiocytic sarcoma, Langerhans cell histiocytosis/sarcoma, follicular dendritic cell sarcoma, and interdigitating dendritic cell sarcoma. Please refer to the books WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues (2017) and Hematopathology (second edition) for other rarer or benign entities. As some of the histiocytic/dendritic cell tumors have clonal immunoglobulin gene rearrangement either related or unrelated to previously diagnosed lymphoma, we will take histiocytic sarcoma as an example to discuss the pathogenesis of histiocytic sarcoma clonally related to the previously diagnosed lymphoma. A better understanding of the causative molecular and cytogenetic changes will provide new opportunities to effectively treat these rare tumors.