Mycobacterial Spindle Cell Pseudotumor of the Nasal Cavity

Mycobacterial spindle cell pseudotumor (MSCP) represents an uncommon tumor-like proliferation associated with nontuberculous mycobacterial infection, i.e., M. avium intracellulare, affecting primarily the lymph nodes of immunocompromised men in their 5th decade. Involvement of the nasal cavity by MSCP is exceedingly rare with only 3 well-documented examples in the literature.

A 74-year-old, HIV-negative, man presented with a 0.5-cm nodule of the left nasal cavity clinically presenting as a "nasal polyp." His medical history was significant for colonic adenocarcinoma, cutaneous basal cell carcinoma, and chronic lymphocytic leukemia (CLL) transforming to B-cell prolymphocytic leukemia, responsive to chemotherapy. The patient was diagnosed with prostatic adenocarcinoma treated with radiotherapy two months before the nasal lesion was detected. No lymph node enlargement, pulmonary involvement or hepatosplenomegaly were noticed. The nasal nodule was surgically excised and histopathologically examined to rule out metastatic disease or CLL relapse.

Microscopically, the lesion comprised a well-circumscribed, monotonous, spindle cell population in a vaguely storiform arrangement mixed with a heavy infiltrate of neutrophils and sparse lymphocytes. The spindle cells featured finely granular rich eosinophilic cytoplasm with rounded, oval to epithelioid, or elongated nuclei with vesicular chromatin and one or two distinct nucleoli. The lesional cells lacked overt cytologic atypia and showed occasional regular mitoses. The surface epithelium was intact or focally ulcerated. By immunohistochemistry, the spindle cell population stained strongly and diffusely for CD68 and was negative for AE1/AE3, SMA, CD34, and PSA. CD3 highlighted scattered lymphocytes. Ziehl-Neelsen stain disclosed numerous intracytoplasmic acid-fast bacilli. A diagnosis of MSCP was rendered. No recurrences were observed during a 24-month follow-up period.

Although exceptionally rare, MSCP should be considered in the differential diagnosis of nodular lesions of the nasal cavity that are characterized microscopically by marked spindle cell proliferation in a vague storiform pattern, admixed with a lymphocytic or mixed inflammatory infiltrate. A negative medical history for HIV infection and medication-induced immunosuppression should not preclude a diagnosis of MSCP, particularly in extranodal sites. Once the diagnosis is established, prognosis appears to be excellent for nasal MSCP following conservative surgical excision.