Mycobacterial spindle cell pseudotumor of skin

Spindle cell pseudotumors may occur due to mycobacterial infection in immunocompromised hosts, particularly those with acquired immunodeficiency syndrome (AIDS). Most of the reported mycobacterial spindle cell pseudotumors were found in the lymph nodes. We report a case of spindle cell pseudotumor in a 37-year-old man with AIDS who presented with a firm nodule over his right arm. Histologically, the tumor was composed of proliferative spindle cells admixed with histiocytes and inflammatory cells. Ziehl-Neelsen stain revealed many acid-fast bacilli in the spindle cells and histiocytes. The acid-fast bacilli were shown to be Mycobacterium avium intracellulare by culture and sequencing of the polymerase chain reaction product of mycobacterial 65-kDa heat shock protein gene. Immunohistochemically, the spindle cells were reactive to CD68, suggesting macrophage differentiation of these cells. It is important for pathologists to recognize this unusual manifestation of mycobacterial infection in immunocompromised patients and avoid mistaking the lesion for a mesenchymal neoplasm.