Extrapulmonary inflammatory myofibroblastic pseudotumor: a potential cytologic trap in childhood.

A previously healthy 20-mo-old infant presented with a right leg limp and was discovered to have a palpable intra-abdominal mass. Radiographic studies showed the mass arising from the right psoas muscle. Preoperative clinical diagnosis was probable sarcoma. Percutaneous fine-needle aspiration biopsy of the mass was interpreted as a sarcoma based on the abundant cellularity of spindled shaped mesenchymal cells. Exploratory laparotomy showed the mass to be unresectable. Histologic and ultrastructural examination of biopsy fragments, however, revealed an inflammatory myofibroblastic pseudotumor. Radiographic follow-up showed a gradual disappearance of the mass. The infant was clinically well at a clinic visit one year following surgery. This case serves to illustrate the aspiration cytology of an uncommon benign tumor-like proliferation that, when located in an unusual anatomic site, has the potential to mislead the cytopathologist into a malignant interpretation.