Case: AP114
Contributor's Comment: The bone marrow shows patchy but extensive infiltration by abnormal medium-sized cells. Residual hematopoietic elements can still be identiifed in many areas. The medium-sized cells are remarkable for their monotony and for the wide spacing between nuclei. The latter indicates the presence of abundant lightly eosinophilic to clear cytoplasm. The nuclei show indentation.

With this picture, the main differential diagnoses are between mastocytosis and hairy cell leukemia. The former in particular has to be considered because of the spindling of the cells and focal peritrabecular accentuation. Cytoplasmic granules can be very difficult to appreciate in some cases of mastocytosis.

Immunohistochemical staining is required for the distinction. In this case, the cells are strongly positive for CD20, but negative for the mast cell marker CD117 (c-kit). Thus the diagnosis is HAIRY CELL LEUKEMIA. This case illustrates that the hairy cells can sometimes show striking spindling.

Because of the atypical morphology, this case is not scored, but instead put out for education purposes.