This educational segment focuses on the diagnosis and differentiation of Epithelioid Fibrous Histiocytoma (EFH), a benign cutaneous tumor often initially confused with a variant of Dermatofibroma (DF). The speaker describes the morphological characteristics of EFH, noting its plump, spindled, or epithelioid cells set within dense collagen, typically presenting as a nodule in the papillary dermis. The key diagnostic distinction relies on immunohistochemistry: EFH is strongly positive for ALK (Anaplastic Lymphoma Kinase) due to recurrent gene fusions, whereas DF is negative. The segment also briefly discusses the wide range of EFH morphology and potential pitfalls in the differential diagnosis, specifically the need to exclude melanocytic lesions and Epithelioid Sarcoma (especially when EFH also expresses EMA).
Diagnosis and Differentiation of Epithelioid Fibrous Histiocytoma (EFH)
0:11 Real-Life Case: The presentation begins by examining a real-life case example, noting it is not a "best example," reflecting practical diagnostic challenges.
0:28 Initial Differential/Diagnosis: The initial thought is often Dermatofibroma (DF). The tumor is formally identified as Epithelioid Fibrous Histiocytoma (EFH) (also called epithelioid fibrocystiocytoma).
0:31 EFH vs. DF: While some consider EFH a variant of DF, the speaker notes EFH stains differently and has distinct molecular findings (ALK fusions).
0:55 Morphology: The tumor cells are described as plump, spindled, or epithelioid with pale cytoplasm, separated by dense collagenous stroma.
1:39 Location and Appearance: EFH typically presents as a nodule located in the papillary dermis, often pushing up. They can have more circumscribed boundaries and sometimes exhibit a collarette (though not in this specific case).
2:00 Key Diagnostic Marker (Takeaway): The critical distinguishing feature is that EFH shows ALK positivity (due to ALK gene fusions), unlike common DF/benign fibrous cystiocytoma.
2:24 IHC Recommendation: The speaker recommends using the D5 F3 clone of ALK for immunohistochemical staining, noting that some other clones used in Hematopathology may be too weak for EFH or Spitz lesions.
2:59 Clinical Importance (Takeaway): EFH is totally benign. If a "weird looking DF" has circumscribed features, a collarette, or plump cells, consider EFH and perform an ALK stain.
Melanocytic lesions must be ruled out (S100 and Sox10 should be negative).
EFH can express EMA (Epithelial Membrane Antigen) in about half of cases. This finding, combined with epithelial morphology, may prompt a differential diagnosis of Epithelioid Sarcoma.
3:37 Nomenclature and Range: The older name for this entity was Epithelioid Cell Histiocytoma. EFH has a wide and expanding range of morphological features, sometimes appearing spindled or swirly (mimicking DFSP).