Epithelioid fibrous histiocytoma: three diagnostically challenging cases with novel ALK gene fusions, unusual storiform growth pattern, and a prominent spindled morphology

We report three EFHs with unusual histological presentation represented by a prominent/predominant spindle cell proliferation arranged in a variably storiform/whirling architectural pattern with or without stromal sclerosis. One of the cases closely resembled cellular fibrous histiocytoma. All three cases were immunohistochemically ALK-positive and were analyzed forALK gene rearrangements using a next-generation sequencing –based assay (FusionPlex Sarcoma Kit, ArcherDx). Three novel fusions, namelyAP3D1::ALK,COL1A::ALK, andLRRFIP2::ALK, were detected and further confirmed by FISH in all 3 cases and RT-PCR in 1 case. All patients were elderly (62 –63 years) and presented with a solitary polypoid lesion on the extremities. The awareness of these morphological variants is important since it entertains a wide and slightly different differential diagnosis than conventional EFH. We also presented evidence that a clear separation of EFH from BF H in all cases may not be as straightforward as previously thought. The consistent ALK immunoexpression and the continually expanding scale of ALK gene rearrangements provide a useful tool to distinguish EFH from its histologic mimics.
Source: Virchows Archiv - Category: Pathology Source Type: research