Mycobacterial Spindle Cell Pseudotumor Presenteing as an Aggressive Lytic Lesion in the Tibia in an Immunocompromised Patient

Mycobacterial spindle cell pseudotumor is a rare manifestation of atypical mycobacterial infection. The process consists of a tumor-like proliferation of bland, spindle-shaped histiocytes that contain acid-fast-positive mycobacteria. Most patients are immunocompromised, and the lymph nodes are the most commonly involved site. The presentation of this process as an isolated aggressive osseous lesion is rare and conjures up a wide differential diagnosis, including neoplastic conditions. Accurate diagnosis of this entity is essential for proper management. We report a 79-year-old man with hypogammaglobulinemia who presented with an aggressively appearing lytic tibial lesion after sustaining a fall. Biopsy of the lesion revealed a mycobacterium-laden spindle cell pseudotumor without granuloma formation. The mycobacteria were visible on acid-fast Ziehl-Neelsen (AFB) stain and modified acid-fast (Fite) stain as well as PAS and GMS stains. PCR of the microbiology culture identified <i>M</i><i>ycobacterium</i> <i>kansasii</i>. Our report demonstrates that, although rare, mycobacterial spindle cell pseudotumor is a diagnostic consideration when encountering a lytic bone lesion in immunocompromised patients composed of cytologically benign, spindled histiocytes without characteristic caseating granuloma formation. Histochemical stains, including PAS, GMS, AFP, or Fit, are imperative to identify intracellular organisms.