Small round cell lesions of the bone: Diagnostic approach, differential diagnoses and impact on treatment.

Small round cell lesions of the bone encompass a heterogeneous group of tumors and tumor-like lesions, including Ewing sarcoma, small cell osteosarcoma, mesenchymal chondrosarcoma, neuroblastoma, non-Hodgkin's lymphoma (NHL), "Ewing-like" undifferentiated round cell sarcomas, metastasizing small cell carcinoma, along with plasma cell dyscrasia and Langerhan's cell histiocytosis. At the same time, there are tumor mimics, for example, chronic osteomyelitis, which has overlapping radiologic features with Ewing sarcoma and a primary intraosseous NHL. An exact diagnosis necessitates integration of clinical, radiologic, pathologic, and ancillary test results, including immunohistochemical and molecular results. Currently, there are several immunohistochemical markers and specific molecular signatures, driving most of these tumors, available, for an exact diagnosis. This review focuses on a pragmatic approach towards uncovering specific small round cell lesions of the bone, emphasizing upon integration of traditional morphology with ancillary techniques, including immunohistochemical markers and molecular techniques, the latter, especially in cases of Ewing sarcoma, Ewing-like undifferentiated round cell sarcoma, mesenchymal chondrosarcoma, and neuroblastoma. Subsequent to the diagnostic approach, including an impact on treatment, individual intraosseous round cell lesions have been described in detail. The references include updated articles from PUBMED.