This video discusses the histopathology of a shoulder papule in a 30-year-old female, diagnosed via shave biopsy, which is identified as a variant of Epithelioid Fibrous Histiocytoma (EFH), alternatively called Epithelioid Cell Histiocytoma. The lesion is characterized morphologically by a dome-shaped proliferation in the superficial dermis with an overlying epithelial colorette and the presence of dilated, "staghorn" vessels, a pattern seen in other tumors like solitary fibrous tumors. Cytologically, the cells exhibit cytoplasmic vacuolation giving a signet-ring appearance, or a foamy/xanthomatous cytoplasm. The speaker emphasizes that EFH classically presents this way, differs from dermatofibroma (which it was previously considered a variant of), and often expresses EMA. Crucially, the diagnosis is strongly supported by immunohistochemistry, as the majority of EFHs carry an ALK gene fusion, which was confirmed positive in this case. The speaker recommends using the D5F3 ALK clone for enhanced staining robustness compared to older clones, especially in ALK-positive melanocytic lesions and EFH. The presentation highlights the wide morphological spectrum of benign EFH, which can mimic other entities like granular cell tumor or DFSP, urging clinicians to consider ALK testing for ambiguous superficial dermal spindle/epithelioid proliferations.
Epithelioid Fibrous Histiocytoma: Histopathology and Diagnostic Clues
00:00:10 Case Presentation: Examination of a dome-shaped shoulder papule from a 30-year-old female, confirmed via shave biopsy.
00:00:20 Key Morphological Features: The lesion is situated in the superficial dermis, pressed against the epidermis, exhibiting an epithelial colorette peripherally and containing dilated, staghorn-pattern vessels, mimicking solitary fibrous tumor patterns.
00:01:03 Cellular Features: Cells show significant cytoplasmic vacuolation leading to a signet-ring appearance, or a foamy/xanthomatous cytoplasm.
00:02:40 Distinction from Dermatofibroma (DF): EFH is distinguished from conventional DF because it pushes against the surface, has a colorette, lacks collagen trapping, and importantly, about half the time expresses EMA, which is unusual for DF.
00:03:19 Molecular Hallmark: The majority (~90%) of EFH cases harbor an $ALK$ gene fusion and express $ALK$ protein via IHC. The case showed positive $ALK$ staining.
00:04:22 Immunohistochemistry Tip: The speaker highly recommends the D5F3 $ALK$ clone for stronger, more robust staining in EFH and ALK-positive melanocytic lesions compared to older clones.
00:05:38 Benign Nature: EFH is considered a benign entity, providing a solid diagnosis when $ALK$ is positive and SOX10 is negative, ruling out many melanocytic mimics.
00:06:00 Wide Morphological Spectrum: EFH can present with diverse appearances, including signet-ring change, granular cell-like features, spindled morphology (mimicking DFSP), or even chondroblastoma-like areas (bone formation).
00:07:09 Diagnostic Advice: If an ambiguous dermal lesion has epithelioid or spindle cell features, check for $ALK$ expression.
00:07:36 Management: The speaker views EFH as benign and suggests watchful waiting, although acknowledging the possibility of non-destructive local recurrence requiring re-excision.