This pathology case discussion focuses on a dome-shaped dermal papule in a 30-year-old female, diagnosed after a shave biopsy. The key features discussed are the tumor's location in the superficial dermis, the presence of an epithelial collarette, and dilated, "staghorn" vessels, which are noted to occur in various tumors, not just solitary fibrous tumor. Cytologically, the cells exhibit clear cytoplasmic vacuolation leading to a signet-ring appearance, and sometimes foamy or xanthomatous cytoplasm. The presenter strongly associates these features with Epithelioid Fibrous Histiocytoma (EFH), noting that this entity was previously sometimes grouped with dermatofibromas but has distinct features, including occasional EMA positivity and, crucially, the presence of an ALK gene fusion in the majority of cases. Immunohistochemistry confirms ALK positivity (though the presenter prefers the D5 F3 clone for stronger staining). The presenter emphasizes the wide morphological spectrum of benign EFH, which can mimic granular cell tumor, DFSP, or even show areas of bone formation, urging practitioners to consider ALK testing for ambiguous spindle/epithelioid/histiocytoid lesions with a collarette, as EFH typically offers a benign diagnosis.
Summary of Epithelioid Fibrous Histiocytoma Case Presentation
00:00:10 Case Introduction: Presentation of Case 2: a dome-shaped papule in a 30-year-old female, diagnosed via shave biopsy.
00:00:20 Histological Findings: Lesion fills the superficial dermis, presses against the epidermis, and exhibits an overlying epithelial collarette.
00:00:37 Vascular Pattern: Noteworthy dilated, "staghorn" vessels observed, a pattern seen in various tumors, including solitary fibrous tumor.
00:01:03 Cytological Features: Many cells show cytoplasmic clearing (vacuolation) causing a signet-ring appearance, and others show foamy/xanthomatous cytoplasm.
00:02:13 Primary Differential Diagnosis: The combination of dome-shaped papule, superficial dermal filling, collarette, and dilated vessels strongly suggests Epithelioid Fibrous Histiocytoma (EFH).
00:02:42 Distinction from Dermatofibroma: EFH differs from conventional dermatofibromas by being more superficial, having a collarette, lacking collagen trapping, and sometimes expressing EMA.
00:03:19 Key Diagnostic Marker (ALK Fusion): Studies show that the majority ($\sim90%$) of EFH cases harbor an ALK gene fusion.
00:03:37 Immunohistochemistry Result: ALK testing in this case was positive, although the staining was weak and focal.
00:04:42 Immunohistochemistry Tip: The presenter recommends using the D5 F3 ALK clone for stronger, more robust staining in both EFH and ALK-positive melanocytic lesions compared to other standard clones.
00:05:38 Benign Nature: EFH is considered a benign entity, providing reassurance when diagnosed definitively.
00:05:51 Diagnostic Clues: If a superficial, papular lesion is epithelioid/spindled/histiocytoid, has a collarette, shows ALK expression, and is SOX10 negative, EFH is highly likely.
00:06:00 Wide Morphological Spectrum: EFH can present with diverse appearances, including signet-ring change, granular cell-like features, spindled whirling growth (mimicking DFSP), and even bone/chondroid formation.
00:07:06 Clinical Advice: If uncertain about a weird lesion, perform ALK testing, as EFH is generally benign and may only require monitoring or re-excision if a local recurrence occurs.
00:07:54 Final Diagnosis: Epithelioid Fibrous Histiocytoma with signet-ring cell and xanthomatous change.