MALIGNANT FIBROUS HISTIOCYTOMA PHENOTYPE IN PLEOMORPHIC SARCOMA DIFFERENTIATION IN RECURRENT DISEASE

Two cases of pleomorphic sarcoma with malignant fibrous histiocytoma p henotype that recurred following therapy with a distinctly different, more mature histologic appearance and immunohistochemical profile are reported. The first case recurred predominantly as extraosseous osteos arcoma at the same site 17 months after wide local excision. The secon d case recurred as widespread pulmonary, mediastinal, and hepatic meta static disease 5 years after wide local excision and adjuvant local ra diotherapy and chemotherapy with doxorubicin hydrochloride (Adriamycin ) and cyclophosphamide (Cytoxan). Fine-needle aspiration of the pulmon ary metastatic lesions showed predominantly spindle cells without any large, pleomorphic giant cells typical of malignant fibrous histiocyto ma. The patient was treated by radiotherapy to the lung and mediastinu m and by chemotherapy with ifosfamide and mesna. Biopsy of a metastati c scrotal skin nodule 9 months later showed a malignant spindle cell l esion with the histologic appearance and immunohistochemical phenotype of leiomyosarcoma. Retrospective immunohistochemical evaluation of th e primary tumor showed focal desmin expression, suggesting focal leiom yosarcoma differentiation. However, the large proportion of the primar y tumor had the phenotype of malignant fibrous histiocytoma. These two cases illustrate an unusual finding of ''differentiation'' rather tha n ''dedifferentiation'' in a recurrent sarcoma. The transformation to osteosarcoma and differentiated leiomyosarcoma demonstrates the potent ial for phenotypic changes in soft-tissue sarcomas and suggests that t he malignant fibrous histiocytoma phenotype and more-differentiated sa rcomas such as extraosseous osteosarcoma or leiomyosarcoma are related in a common pathway in differentiation from a primitive mesenchymal s tem cell.