Epithelioid Fibrous Histiocytoma With Dot-Like Perinuclear ALK Expression and PRKAR2A-ALK Fusion

Epithelioid fibrous histiocytoma (EFH) is a rare, benign, cutaneous neoplasm. This fibrohistiocytic tumor was once believed to be a variant of fibrous histiocytoma, but EFH is now known to be a distinct entity based on the presence of ALK gene rearrangements in most cases. The pattern of immunohistochemical expression of ALK in EFH in the literature thus far describes both granular cytoplasmic staining and nuclear staining. We present a case of EFH with dot-like Golgi pattern perinuclear ALK expression, a previously undescribed staining pattern. We surmised this unique staining pattern could be due to a novel fusion partner, and using FISH, we confirmed a rearrangement of the ALK (2p23) locus. Further investigation with whole transcriptome sequencing led to the discovery of PRKAR2A-ALK fusion, and the function of this fusion partner reflects a Golgi-predominant localization of the protein. Attention to the distinct immunohistochemical pattern of ALK expression may provide clues to the function of the fusion partner.