Malignant fibrous histiocytoma within a recurrent chordoma. A light microscopic, electron microscopic, and immunohistochemical study.

A histologically typical chordoma and its four recurrences (3.5, 4, 7, and 7.5 years later) were operated from the spinal canal and I-III lumbar vertebrae of a 49-year-old woman. In addition to the typical areas of chordoma, pleomorphic areas resembling malignant fibrous histiocytoma also were seen in the fourth recurrence. The chordoma-like areas of the fourth recurrence displayed typical epithelial features: clusters of epithelial-like cells, electron microscopically demonstrable vacuoles lined by microvilli and desmosomes, and cytokeratin positivity in immunostaining. The areas in the fourth recurrence that resembled malignant fibrous histiocytoma lacked epithelial features as judged by electron microscopy. They also were negative for cytokeratin but contained, instead, vimentin type of intermediate filaments. The results show that the sarcomatous areas as seen in the recurrent chordoma lack epithelial cell features of chordoma and suggest the possibility of altered differentiation pathway of the tumor stem cell or emergence of a new malignant cell population within the recurrent tumor.