Mycobacterial pseudotumor of the skin.

Inflammatory pseudotumors have a diverse etiology, mycobacterial pseudotumor (MP) being one of them. MP is a rare entity; it has been reported infrequently in various organs and is extremely rare in the skin. We report a cutaneous MP in an immunosuppressed liver transplant recipient. The lesion consisted mostly of spindle cells, with small numbers of lymphocytes. Conventional acid-fast bacilli (AFB) stain revealed a large number of acid-fast bacilli within spindled histiocytes and the presence of Mycobacterium avium was determined by polymerase chain reaction. Given that the patient had a prior history of cutaneous squamous cell carcinoma resected and reconstructed in the same area, establishing the diagnosis was challenging. Immunohistochemical staining for lysosome-associated membrane protein was strongly positive, suggesting the presence of numerous mature lysosomes within infected spindle cells. Mycobacterial spindle cell pseudotumors can mimic malignant or benign neoplasms and should be considered in differential diagnosis of spindle cell lesions, especially in immunocompromised patients. Further studies are needed to determine mechanisms that permit the survival of mycobacteria within the lesions and that cause this unusual manifestation of infection.