Dendritic fibromyxolipoma: clinicopathologic study of a distinctive benign soft tissue lesion that may be mistaken for a sarcoma.

Twelve cases are described of a distinctive benign soft tissue lesion that may be mistaken for a sarcoma. The tumors occurred in 11 men and a woman aged 33 to 81 years (mean, 64 years), and measured from 2 to 11 cm in greatest diameter (mean, 6 cm). They were grossly described as soft, well-circumscribed, yellow-gray, with a mucoid cut surface. All cases were superficially located in the subcutis or muscular fascia of the head and neck region or the chest and back. Histologically, the tumors were characterized by a proliferation of spindle or stellate fibroblastic cells variably admixed with mature adipose tissue embedded in an abundant myxoid and collagenized stroma. The spindle and stellate fibroblastic cells were characterized by slender dendritic prolongations of their cytoplasm, which appeared to extend for short distances along connective tissue planes. Electron microscopy in two cases confirmed the dendritic nature of the fibroblastic cells, which showed elongated cytoplasmic processes lacking external lamina and displaying foci of pinocytotic activity. Immunohistochemical studies in 11 cases showed strong positivity of the spindle cells with vimentin, CD34 and bcl-2, and negative staining for smooth muscle actin, muscle-specific actin (HHF35), desmin, S-100 protein, keratin, and EMA. Because of their prominent myxoid stroma and relatively large size, some of these tumors were initially misinterpreted as low-grade sarcomas. Clinical follow-up in five cases, however, showed that the patients were alive and well without evidence of recurrence between 5 and 13 years (mean follow-up, 8 years) after simple local excision. The present cases appear to represent a distinctive form of benign soft tissue neoplasm that should be distinguished from myxoid liposarcoma and other benign and malignant myxoid tumors of superficial soft tissues.