ALK Gene Fusions in Epithelioid Fibrous Histiocytoma: A Study of 14 Cases, With New Histopathological Findings.

Previous studies showed that ALK is often positive in epithelioid fibrous histiocytoma (EFH). Two cases of EFH with ALK gene fusions have been recorded. Our objective was to study a series of EFH to present histopathological variations of EFH, identify novel ALK gene fusions, and determine whether there is a correlation between histopathological features and particular gene. We investigated 14 cases of EFH, all ALK immunopositive. The cases were assessed histopathologically as well as for ALK and TFE-3 rearrangements using FISH and ALK gene fusions using next-generation sequencing. The analysis of the sequencing results was performed using the Archer Analysis software (v5; ArcherDX Inc). The study group consisted of 8 female and 6 male patients, ranging in age from 18 to 79 years (mean 42 years; median 37.5 years). All presented with a solitary lesion. Microscopically, most lesions were polypoid and composed of epithelioid cells with ample cytoplasm. In addition, a variable number of bi-, tri-, or multinucleated, spindled, multilobated, cells with eccentric nuclei, cells with nuclear pseudoinclusions, mucinous, and grooved cells were admixed. In 5 cases, the predominant epithelioid cell component consisted of rather small cells, whereas spindled cells dominated in 3 cases. Of these, 2 lesions were composed rather of pale eosinophilic to clear cells, occasioning a resemblance to PEComa or leiomyoma. Immunohistochemically, all cases expressed ALK and 11 were positive for TFE-3. The break apart test for ALK was positive in 11 cases, whereas specimens from the remaining 3 cases were not analyzable. ALK genes fusions were found in all but 3 cases and included SQSTM1-ALK (3), VCL-ALK (3), TMP3-ALK (2), PRKAR2A-ALK (1), MLPH-ALK (1), and EML4-ALK (1). No correlation between histological features and type of ALK fusion was found. TFE-3 break apart test was negative. It is concluded that ALK-immunopositive EFH shows ALK gene fusions that involve various protein-coding genes, implicated in a variety of biological processes. Rare variants of EFH rather consist of spindled "non-epithelioid" cells.