Fibrosarcoma mimicking plasmacytoma or carcinoma: An ultrastructural studyof 4 cases

Because the fibroblast has a remarkable capability of phenotypic modulation s, reflected in both morphologic and immunohistochemical (IHC) changes, ult rastructural studies are mandatory to identify the variants of fibroblasts. Myofibroblasts or histiofibroblasts are such examples, demonstrating chime ric ultrastructural features of fibroblastic cells in common with smooth mu scle cells or with histiocytes, respectively. The presence of epithelioid f ibroblastic cells sharing morphologic features with epithelial or plasma ce lls has not been yet characterized. The authors identified 4 cases of fibro sarcomas (FS) characterized by an unusual phenotype and associated with pec uliar ultrastructural findings. The electron microscopic (EM) findings were correlated with the histologic appearance and immunoprofile. All tumors we re located in the extremities, 3 in soft tissues and 1 in the bone. By ligh t microscopy 2 cases were composed predominantly by round uniform cells wit h a striking plasmacytoid appearance. One case mimicked carcinoma, composed predominantly by epithelioid cells and scattered giant tumor cells. The fo urth case showed a mixture of plasmacytoid-like and epithelioid cells. By I HC, tumor cells were positive for vimentin and in 2 cases also for epitheli al membrane antigen. Kappa/lambda light chain and cytokeratins markers were negative. By EM all 4 tumors showed in addition to classic features of fib roblasts, unusual epithelial-type features, such as secretory granules of " neurosecrerory-type" (3 cases), rudimentary cell junctions (3 cases), micro villi (2 cases), and lumen-like structures (1 case). One plasmacytoid-type tumor showed finely granular extracellular deposits. The study describe 4 e xamples of fibrosarcomas with unusual features at the ultrastructural level , which are associated microscopically with a peculiar phenotype, mimicking plasmacytoma or carcinoma. These findings broaden the spectrum of fibrobla stic cell variants in neoplasia.