Differential Diagnosis

The histologic differential diagnosis of osteomyelitis is usually not problematic when dealing with the acute (suppurative) form. Chronic osteomyelitis, with its prevalence of lymphocytes,

FIGURE 1 Acute osteomyelitis. Acute inflammatory cells (neutrophils) within marrow space adjacent to nonviable trabecular bone (hematoxylin and eosin, original magnification x 40).
FIGURE 2 Chronic osteomyelitis. Prominent plasma cell focus with background lymphocytes. Focal fibrosis and nonviable bone fragment (hematoxylin and eosin, original magnification x 40).

can simulate lymphoma of bone. If a prominent plasma cell component is biopsied, the possibility of multiple myeloma/plasmacytoma may be entertained. A diagnosis of Langerhans cell histiocytosis (eosinophilic granuloma) may be considered if a biopsy involves a heterogeneous mixture of plasma cells, lymphocytes, macrophages, scattered neutrophils, and eosinophils. Immunohistochemical stains (T and B-cell markers, and/or immunoglobulin class) are utilized to demonstrate clonality in the case of suspected lymphoma or plasma cell dyscrasia. Langerhans cell histiocytosis is characterized by expression of the Langerhans cell phenotype (S-100+, CD 1a+). Another small round blue cell tumor to consider is Ewing's sarcoma/peripheral neuroectodermal tumor (PNET). Immunohistochemical stains and cytogenetic studies may aid in the diagnosis by demonstrating the MIC2 gene product (CD99) and reciprocal translocation of chromosome 11 and 22, which are associated with Ewing's sarcoma/PNET (19).

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