Primary osteochondrorhabdomyosarcoma (malignant mesenchymoma) of the fibula: a rare tumour in an unusual site -- case report and review of the literature.

Malignant mesenchymoma, defined by Stout as sarcomas comprising two or more unrelated differentiated tissue elements other than a fibrosarcoma component, is rare. We report a case of primary malignant mesenchymoma of the proximal fibula in a 10-year-old female student who presented with pain and swelling of the right knee for 2 months. Initial biopsy showed features of rhabdomyosarcoma only, but the resected specimen revealed additional osteosarcomatous and chondrosarcomatous elements. The patient remained well more than 5 years after initial presentation. Including our present patient, 16 cases of primary malignant mesenchymoma of bone are found in the English literature, affecting mainly adolescents and young adults, with a slight male predominance and predilection for the metaphysis of long bones, especially around the knee. More than 60% of the patients develop metastasis, almost invariably to the lung, but occasionally to the brain. About 60% of the patients, all with metastasis, died mostly within one year of diagnosis. The clinical features of primary malignant mesenchymoma of bone thus resemble those of conventional osteosarcoma. Moreover, our case illustrates that, with combination chemotherapy targeted for individual elements, the prognosis of this rare tumour might be much improved, as in osteosarcoma.