Answer

Fibrous Histiocytoma

Discussion

Fibrous histiocytomas are uncommon mesenchymal tumors of the soft tissue in head and neck and lower limb regions. They are believed to be of mixed fibroblastic and histiocytic origin. CT and MRI have been widely used in the diagnosis and staging of mesenchymal fibrous histiocytoma (MFH). Based on location, benign fibrous histiocytomas (BFHs) are classified into cutaneous and deep tissue subtypes. Cutaneous BFHs are common, while deep-seated BFHs are rare and painful. When BFH is deep seated and associated with pain, the diagnosis may be very difficult because of its clinical features. As such, it is frequently confirmed histopathologically following local excision. Deep-seated BFH is uncommon and accounts for less than 1% of all benign fibrohistiocytomas. Both cutaneous and deep-seated subtypes are usually asymptomatic. Approximately 5% of deep-seated BFH are associated with pain or tenderness. Although cutaneous BFH poses little diagnostic difficulty, lesions arising in subcutaneous or deep soft tissues are often poorly recognized and diagnosed. It remains challenging to diagnose deep BFH clinically, especially when pain coexists, because most BFHs are asymptomatic. As such, the diagnosis of deep fibrous histiocytoma is frequently confirmed intraoperatively or on histopathologic examination after excision. The most important differential diagnoses of BFH are the aggressive forms of fibrohistiocytic neoplasms, including dermatofibrosarcoma protuberans (DFSP), malignant fibrous histiocytoma, and leiomyosarcoma. DFSP especially has very similar histopathologic features with BFH. DFSP is generally characterized by more uniform spindle cells and more prominent storiform pattern. The epidermis overlying DFSP is usually attenuated or ulcerated, and the tumor has a more infiltrative pattern of growth, showing diffusely positive CD34 staining. Malignant fibrous histiocytoma and leiomyosarcoma exhibit cellular atypia and bizarre giant cells, which are not features of BFH. Further differential diagnoses in our case included dermatomyofibroma. But dermatomyofibroma usually reveals skin appendages, and prominent storiform pattern of tumor cells and thickened collagen bundles are not features of BFH. Specifically, dermatomyofibroma usually does not express factor XIIIa.

The pain of deep-seated dermatofibroma may be due to tumoral nerve invasion or mass effect on the nerve fibers. In our case, pain was thought to be secondary to the latter because the mass was separated from the underlying fascia and the pain gradually subsided after surgery.

Optimal treatment for BFH includes wide excision and continuous follow-up, because local recurrence or metastasis can occur in deep fibrous histiocytoma. Spindle-shaped cells, plump nuclei in storiform pattern, bands of collagen, foamy macrophages, absence of nuclear atypia, and positive CD 68 stain confirm fibrohistiocytic nature of this lesion.