Small, Round Cell Lesions

Abstract

Small, round cell lesions encompass a broad spectrum of entities, including reactive and inflammatory processes, benign neoplasms, and malignant tumors. Osteomyelitis is among the most common nonneoplastic conditions and denotes an inflammation of bone and bone marrow, commonly caused by bacteria (pyogenic osteomyelitis), although it may be produced by any type of organism. The incidence of small, round cell tumors of bone is tightly linked to the age at presentation, with Ewing sarcoma/primitive neuroectodermal tumor (PNET) and Langerhans cell histiocytosis (LCH) being the most common entities in young individuals. In contrast, hematologic neoplasms (myeloma and lymphoma) constitute approximately 40 % of all bone tumors but occur almost exclusively in adults, especially elderly patients. Most entities included in this chapter are those of traditional small, blue, round cell tumors that produce minimal extracellular matrix, such as Ewing sarcoma/PNET and hematologic neoplasms, whereas others, such as LCH and Rosai-Dorfman disease, tend to be composed of mixed-cell populations. Several other tumors with small, round cell histomorphology, either those that occur primarily in bone (e.g., small cell osteosarcoma and mesenchymal chondrosarcoma) or metastases from other organs (e.g., neuroblastoma and most rhabdomyosarcomas), are discussed in other chapters.