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Chondroblastoma of the rib presenting as an intrathoracic mass. Report of a case with fine needle aspiration biopsy, immunocytochemistry and electron microscopy.

We describe an unusual case of chondroblastoma of the rib, initially presenting as a mediastinal mass eroding a vertebra, in which the preoperative diagnosis was made by fine needle aspiration (FNA) cytology and confirmed by histology and electron microscopy of the surgical specimen. Cytologic study of the smears revealed osteoclastlike giant cells and dishesive, mononucleate tumor cells; sections of the paraffin-embedded, aspirated material showed the chondroid matrix and typical chicken wire calcific deposits. Supporting diagnostic evidence was provided by immunohistochemical demonstration of S-100 protein. Unusual features were the presence of intranuclear pseudoinclusions and cytoplasmic granular deposits, which proved to contain iron on histochemical staining, ultrastructural morphology and x-ray analysis. This case emphasizes the value of FNA cytology in providing a correct diagnosis of chondroblastoma as well as the utility of embedding the aspirated material for histologic, immunohistochemical and ultrastructural studies.