Phosphaturic mesenchymal tumor diagnosed by fine-needle aspiration and core biopsy: a case report and review of literature.

Oncogenic (tumor-induced) osteomalacia is a rare paraneoplastic syndrome of phosphate wasting that is frequently associated with phosphaturic mesenchymal tumor (PMT). As the cytologic features of this tumor apparently have not been reported, we describe the fine-needle aspiration (FNA) findings for PMT that arose from the gluteal soft tissue in a patient with hypophosphatemia and multiple fractures secondary to osteomalacia. Smears from the computerized tomography (CT)-guided FNA showed groups of spindle cells having elongated nuclei, fine to moderately coarsely granular chromatin, inconspicuous nucleoli, and delicate cytoplasm. Marked nuclear atypia, mitotic figures, and necrosis were absent. The differential diagnosis included a variety of benign and malignant spindle cell neoplasms such as monophasic synovial sarcoma, leiomyoma, peripheral nerve sheath tumor, fibrosarcoma, and, less likely, metastatic melanoma and sarcomatoid carcinoma. The bland-appearing cytologic features of a spindle cell tumor in a patient with osteomalacia should suggest the diagnosis of PMT.