Nasopharyngeal alveolar rhabdomyosarcoma expressing CD56: a mimicker of extranodal natural killer/T-cell lymphoma.

Alveolar rhabdomyosarcoma (ARMS) is remarkably rare in adults older than 45 years. Histologically, the tumor is composed of blue round cells with frequent expression of CD56 in addition to myogenic markers. Recent studies of ARMS have shown two specific recurrent translocations: PAX3-FKHR [t(2;13)(q35;q14)] or PAX7-FKHR [t(1;13)(p36;q14)]. Extranodal natural killer (NK)/T-cell lymphoma (ENKTL) occurs most frequently in the upper aerodigestive tract with a male preference in East Asia and Central and South Americas with neoplastic cells frequently expressing CD56. We report a 53-year-old Taiwanese man presenting with a nasopharyngeal mass, cervical lymphadenopathy, and multiple bone metastases. Histologically, the nasopharyngeal biopsy revealed diffuse sheets of small blue round tumor cells without obvious alveolar pattern, angioinvasion or tumor necrosis. An initial erroneous diagnosis of ENKTL was made due to CD56 expression using fresh tumor tissue with flow cytometric analysis and the patient was treated accordingly. Retrospective study showed that the tumor cells expressed CD56, desmin, and myogenin. Fluorescence in situ hybridization revealed that the tumor cells were positive for FKHR gene rearrangement, confirming the diagnosis of ARMS. Our case illustrates that a diagnosis of ENKTL based solely on CD56 expression can be misleading for a nasopharyngeal small blue round cell tumor. ARMS should be included as a differential diagnosis, and a correct diagnosis can be reached only after a high index of suspicion and a thorough histological examination with the aid of ancillary studies.