Myofibroblastic differentiation in malignant fibrous histiocytoma (pleomorphic myofibrosarcoma): a clinicopathological study

Myofibroblastic differentiation in malignant fibrous histiocytoma (pleomorphic myofibrosarcoma): a clinicopathological study Aims: We compared the clinical and pathological features of pleomorphic malignant fibrous histiocytoma (MFH)‐like soft tissue sarcomas with and without myofibroblastic differentiation on electron microscopy. Methods and Results: Fifty‐three soft tissue tumours designated as MFH by light and electron microscopy were reassessed. Eighteen were specifically diagnosed and excluded, and follow‐up (FU) information obtained for 24 of the other 35 cases. Myofibroblastic ultrastructure was seen in 7/24 (29%). Seventeen of 24 (71%) lacked myofibroblasts on electron microscopy, which showed fibroblastic or undifferentiated cells. Histologically, all tumours but one had storiform‐pleomorphic areas; one myofibroblastic neoplasm was fascicular and myxoid. No other morphological differences were seen. In seven myofibroblastic cases, smooth muscle in four cases and muscle‐specific actin in two cases, desmin in three cases and S100 in one case were present. In 15 other tumours, smooth muscle in five cases and muscle‐specific actin in one case, and desmin in one case were present; none of these cases expressed S100. CD34 was found in the myxoid areas of one myofibrosarcoma and 3/15 other tumours. Positivity for bcl‐2 was seen only in non‐myofibroblastic sarcomas (4/14). On follow‐up (median 41 months), 2/7 (29%) myofibroblastic tumours recurred, 5/7 (71%) metastasized, and 3/7 (43%) patients died of disease. Among the non‐myofibroblastic sarcomas, with a median follow‐up of 47 months, 6/17 cases (35%) recurred, 10/17 (59%) metastasized, and 7/17 patients (41%) died of disease. Conclusions: Pleomorphic sarcomas with and without myofibroblastic differentiation on electron microscopy are clinically and histologically similar. The former display myoid immunohistochemical markers more frequently.