Histiocytic Sarcoma Arising in a Background of Nodular Lymphocyte Predominant Hodgkin Lymphoma

Histiocytic sarcoma is a very rare aggressive neoplasm of mature histiocytes which may present as a primary malignancy or transforming from a primary B-cell lymphoma that includes chronic lymphocytic leukemia/small lymphocytic lymphoma, follicular lymphoma, and extra nodal marginal zone lymphoma. A 69-year-old female presented with lymphadenopathy, and CT scan of chest, abdomen, and pelvis revealed extensive lymphadenopathy. Left axillary lymph node excision was performed. Histologic sections showed enlarged lymph nodes with architectural effacement by nodular and diffuse infiltrate comprising a mixture of small lymphocytes, histiocytes, occasional plasma cells, and scattered large atypical lymphocytes with irregular nuclear contours, vesicular chromatin, and prominent nucleoli. In addition, there were a few nodules of atypical histolytic cells including epithelioid and spindled forms and scattered large multinucleate forms. Immunohistochemical (IHC) stains showed that the large atypical B-cells were positive with variable intensity for CD20, PAX5, BCL6, BOB1 (weak), OCT2, MUM1, PU.1, CD45 (subset), CD19 (weak), CD79A (weak), and CD30 (subset, weak). They were negative for CD3, BCL2, CD15, ALK, CD10, IgD, HHV8, CAM5.2, EBER, GMS, AFB, SOX10, MART1, and HMB45. T-lymphocytes positive for CD3 showed rosette formation around scattered negative atypical large B-cells. CD21 and CD23 highlighted mild expansion of the follicular dendritic cell meshwork in a few areas of nodular infiltration by atypical cells. The nodules of atypical histiocytes were positive for CD68, CD163, BCL6, PU.1 (partial), cyclinD1, and S100 (partial) while negative for CD20, CD3, ALK, CD1A, HHV8, langerin, CAM5.2, HMB45, and MART1. The case was diagnosed as "Histiocytic sarcoma arising in a background of nodular lymphocyte predominant Hodgkin lymphoma." Histiocytic sarcoma is a rare hematopoietic neoplasm, with limited cases of secondary histiocytic sarcoma transforming from a B-cell lymphoma reported in the English literature. Some of these case reports show the same clonal origin of histiocytic sarcoma and B-cell lymphoma. The diagnosis of the transformation is made based on the morphological, immunophenotypic, and genotypic features.