Heterologous osteosarcomatous and rhabdomyosarcomatous elements in dedifferentiated solitary fibrous tumor: further support for the concept of dedifferentiation in solitary fibrous tumor

Dedifferentiation within solitary fibrous tumor is a rare and only recently characterized phenomenon. It differs from malignant solitary fibrous tumor in that there is abrupt transition between classical solitary fibrous tumor and the dedifferentiated component. The latter is a high-grade sarcoma, which can exhibit a number of morphologies, but heterologous differentiation is exceptionally rare. We report a case of dedifferentiated solitary fibrous tumor, with heterologous osteosarcomatous and rhabdomyosarcomatous elements, arising in the deep soft tissue of the thigh of a 59-year-old man. This comprised morphologically and immunohistochemically typical solitary fibrous tumor, juxtaposed to pleomorphic, high-grade malignant neoplasm of 2 distinct lineages. The sharp demarcation between well-differentiated and dedifferentiated components is typical of the dedifferentiation seen in other mesenchymal neoplasms. This expands the range of histopathology of this rare, newly characterized type of malignant progression in solitary fibrous tumor.