Malignant Ossifying Fibromyxoid Tumor With Lung Metastasis: A Case Report and Literature Review

Ossifying fibromyxoid tumor (OFMT) is a rare mesenchymal neoplasm of uncertain differentiation, typically exhibiting shell-like ossification and an indolent clinical course. However, some cases demonstrate aggressive behavior with local recurrence or metastasis. A 48-year-old Japanese man presented with a painless right thigh mass. He had been aware of it for 20 years and it had been enlarging slowly for the past year. Pathologic examination revealed that the removed 55-mm tumor chiefly consisted of spindle tumor cells proliferating, in the central area in a hypocellular fashion with scattered ossifications, and peripherally in a more cellular, plexiform-like perivascular fashion with nuclear pleomorphism and frequent mitosis (10 per 50 high-power fields). Tumor cells were positive for keratin, S-100 protein, α-smooth muscle actin, and MUC4, and the Ki67 labeling index was about 40%. S-100 protein immunoreactivity was decreased in the peripheral hypercellular areas. Two months after the surgery, a solitary lung metastasis was evident and was confirmed histologically. Additional fluorescence in situ hybridization examination of the primary tumor cells demonstrated PHF-1 rearrangement. We concluded that the present case is a rare malignant OFMT. The presence of dense perivascular proliferation and vascular permeation were considered histological indicators for lung metastasis in this case.