Infectious pseudotumors: red herrings in head and neck pathology.

Infectious pseudotumors are tumor-like growths caused by specific microbial organisms. Proliferating histiocytes in these lesions may show atypical cytology with nuclear pleomorphism and increased mitotic activity, erroneously suggestive of malignancies including carcinoma, sarcoma, and lymphoma. Specific and nonspecific immunohistochemical staining profiles may lead to the consideration of a wide range of benign and malignant neoplastic processes. Two such cases are reported. The first is an obstructive endotracheal mass in an AIDS patient caused by Rhodococcus equi infection. The proliferating histiocyes were cytologically atypical with deeply eosinophlic granular cytoplasm leading to the consideration of oncocytic carcinoma, Hurthle cell carcinoma, and pleomorphic rhabdomyosarcoma. The second case is a nasal mass with a microscopic and immunohistochemical profile suggestive of chordoma and paraganglioma. Special microbial stains revealed intracellular bacilli consistent with Klebsiella rhinoscleromatis. In both cases, microbial virulence factors affecting phagocytosis prolonged their intracellular survival and resulted in active histiocytic proliferation. It is of importance that the surgical pathologist be conscious that some infectious processes can clinically and microscopically mimic malignant neoplasms. Accurately identifying these lesions and the specific causative agent is of particular significance since they can be successfully treated with antibiotics.