Analyzing the morphological spectrum of epithelioid fibrous histiocytoma and the immunohistochemical performance of the ALK D5F3 and ALK1 clones.

Epithelioid fibrous histiocytoma (EFH) is a cutaneous neoplasm driven by translocations of the anaplastic lymphoma kinase (ALK) gene, which can be demonstrated by immunohistochemical (IHC) analysis. We analyzed the performance of two ALK clones, D5F3 and ALK1, in a cohort of EFHs and described the range of architectural variation of these lesions. TFE3 IHC was performed in ALK-negative EFHs. We identified 21 cases of EFH, 76.2% of which showed an exophytic appearance and 19% displayed flat architecture. A well-developed epidermal collarette was present in 48% of all cases with just more than a third of all the exophytic lesions presenting as dermal-based nodules. ALK D5F3 expression was identified in 76.2% (16/21) of all cases, but only 68.8% were concordantly positive with the ALK1 clone, indicative of a false-negative stain with ALK1 in 31.2% of the cases. For the subset of cases showing positivity for the ALK1 clone, a marked decrease in the percentage of immunolabelled cells was identified when compared with D5F3 (5-50% vs. 100%, respectively). Five cases (23.8%) did not demonstrate ALK expression for either clone, with 3 of those cases showing nuclear positivity for TFE3 IHC and the remaining 2 cases being double negative (ALK-/TFE3-). In summary, we identified that the prototypically described exophytic appearance with epidermal collarette is present in only less than half of the cases. We also demonstrated that the ALK1 antibody is suboptimal in EFH and should not be utilized in this setting. A subset of ALK-negative cases express TFE3, but double-negative cases occur.