Head and neck fibromyxoid sarcoma: clinicopathological correlation with emphasis on peculiar ultrastructural features related to collagen processing.

A fibromyxoid sarcoma of the jaw of a young patient is described. The tumor recurred and was locally aggressive but displayed mostly a low-grade morphology with only a few areas showing undifferentiated (high-grade) appearance. An isolated lung metastasis was identified 15 years after the original presentation. Ultrastructurally, the tumor cells showed features of partial fibroblastic differentiation, with abundant and often markedly dilated rough endoplastic reticulum cisternae, but also interrupted segments of basal lamina. The cytoplasm contained numerous large, spindle-shaped, membrane-bound inclusions of collagen that showed a complex triple periodicity. A distinct cell polarity was seen, with the nucleus at one end of the cell and the collagenous inclusions at the other. All of these features indicate a tumor with significant cytodifferentiation, correlating with the relatively protracted clinical course. The intercellular space contained mature collagen fibers and scattered aggregates of delicate, slender fibers embedded in moderately electron-dense granular matrix (fibrillogranular aggregates). The results suggest that based on its morphology and clinical presentation this tumor represents an unusual variant in the spectrum of fibromyxoid sarcomas.