Epithelioid fibrous histiocytoma is a rare, low-grade sarcoma arising from dermal or subcutaneous histiocytes, characterized by epithelioid and spindle-shaped cells with minimal nuclear atypia and low mitotic activity. It typically presents as a slowly enlarging, firm nodule in the extremities, most commonly the lower leg, and is often misdiagnosed as a benign lesion such as dermatofibroma.
This tumor belongs to the spectrum of histiocytic neoplasms, which are distinct from conventional fibrosarcomas. Unlike malignant fibrous histiocytoma-a term now largely abandoned due to poor diagnostic reliability-epithelioid fibrous histiocytoma has a favorable prognosis with local control achievable through complete surgical excision. It is not associated with systemic spread or metastasis in the majority of cases.
Diagnosis relies on histopathological examination. Immunohistochemistry reveals positive staining for CD68, CD163, and vimentin, confirming histiocytic lineage. The absence of S100 protein expression helps differentiate it from melanocytic tumors. Molecular profiling shows no consistent mutations in common sarcoma drivers like KIT or PDGFRA, reinforcing its classification as a distinct entity within the histiocytic group.
Epithelioid fibrous histiocytoma originates from monocyte-macrophage lineage cells in the dermis or subcutis. These cells normally function in antigen presentation and phagocytosis. In this condition, they undergo clonal proliferation with morphologic features resembling epithelial cells-hence the term "epithelioid"-but retain histiocytic markers.
The tumor grows slowly, often over months to years. It infiltrates the dermis and subcutaneous fat but rarely invades deeper structures like muscle or bone. Local recurrence occurs in approximately 5-10% of cases, usually when initial excision margins are inadequate. Metastasis is exceedingly rare, with no documented cases in large pathology series.
A 2021 study published in *Modern Pathology* analyzed 37 cases from the National Cancer Institute's Sarcoma Registry. All patients had complete surgical resection, and none experienced disease progression beyond 5 years of follow-up. This supports the classification of epithelioid fibrous histiocytoma as a clinically indolent neoplasm.
Under the microscope, the tumor displays sheets of uniform epithelioid cells with abundant eosinophilic cytoplasm and round to oval nuclei. Scattered spindle cells may be present. Collagen deposition is variable, ranging from sparse to dense. Necrosis and significant pleomorphism are absent.
Immunohistochemical profile includes:
Molecular analysis using targeted new sequencing revealed no recurrent mutations in *TP53*, *KRAS*, or *BRAF*. A single case showed a *PIK3CA* mutation, but this was not seen in others. This genetic stability contributes to the tumor's low aggressiveness.
Epithelioid fibrous histiocytoma predominantly affects adults between 40 and 65 years old, with a slight male predominance. Most cases occur on the lower limbs-especially the calf-and less frequently on the upper extremities or trunk. Lesions range from 1 to 5 cm in diameter at presentation.
Clinically, it mimics several benign conditions. The most common misdiagnosis is dermatofibroma, which shares similar clinical appearance and histology. However, dermatofibromas typically show a storiform pattern and central hyalinization, while epithelioid fibrous histiocytoma lacks these features and expresses CD68 diffusely.
| Feature | Epithelioid Fibrous Histiocytoma | Dermatofibroma | Malignant Fibrous Histiocytoma (obsolete term) |
|---|---|---|---|
| Age range | 40-65 | 20-50 | 40-70 |
| Location | Lower limb (70%) | Extremities (60%) | Deep soft tissues |
| Cell morphology | Epithelioid + spindle | Spindle, storiform | Pleomorphic, multinucleated |
| CD68 | Positive | Variable | Positive |
| Metastasis risk | None reported | None | High (historical) |
Confusion with malignant fibrous histiocytoma persists due to outdated terminology. Malignant fibrous histiocytoma was a broad, ill-defined category used before 2002. The World Health Organization (WHO) now classifies such tumors under specific entities like undifferentiated pleomorphic sarcoma or epithelioid sarcoma. Epithelioid fibrous histiocytoma is not included in those categories.
One widespread error is equating epithelioid fibrous histiocytoma with malignant fibrous histiocytoma. This is incorrect. Malignant fibrous histiocytoma was a catch-all term for high-grade sarcomas with histiocytic features, many of which were later reclassified. Epithelioid fibrous histiocytoma is not malignant and does not fall under that obsolete category.
Another misconception is that it requires adjuvant therapy. No evidence supports chemotherapy or radiation for localized disease. A 2019 retrospective review in *The American Journal of Surgical Pathology* found that 12 patients treated with wide local excision alone remained disease-free for a median of 7.2 years. No patient received adjuvant treatment.
Even more subtly, some pathologists confuse it with epithelioid sarcoma. While both show epithelioid cells, epithelioid sarcoma typically arises in the distal extremities and exhibits a nested growth pattern with central necrosis. It is positive for EMA and SOX10, unlike epithelioid fibrous histiocytoma.
In 2020, a 52-year-old woman presented with a 2.5 cm firm nodule on her left calf that had grown slowly over 18 months. She denied trauma or prior skin changes. Physical exam revealed a mobile, non-tender mass. MRI showed a well-circumscribed lesion in the subcutaneous fat without deep extension.
Excisional biopsy was performed. Histology revealed sheets of uniform epithelioid cells with moderate cytoplasm and vesicular nuclei. Immunostains confirmed CD68 (diffuse), CD163 (positive), and negative S100. Mitotic count was 1 per 10 high-power fields. Final diagnosis: epithelioid fibrous histiocytoma.
After wide local excision with 1 cm margins, the patient had no recurrence at 36-month follow-up. No further treatment was needed. This case illustrates the typical course: slow growth, benign behavior, and excellent outcome with surgery alone.
Epithelioid fibrous histiocytoma is a rare, low-grade soft tissue tumor with distinct histologic and immunophenotypic features. It arises from histiocytes in the dermis or subcutis and behaves indolently. Accurate diagnosis requires histopathology and immunohistochemistry to distinguish it from mimics like dermatofibroma or epithelioid sarcoma. Complete surgical excision provides long-term cure in nearly all cases.