ALK Expression in Angiomatoid Fibrous Histiocytoma: A Potential Diagnostic Pitfall

We recently encountered a case of primary pulmonary angiomatoid fibrous histiocytoma (AFH), which was initially misdiagnosed as inflammatory myofibroblastic tumor (IMT) based in part on anaplastic lymphoma kinase (ALK) expression by immunohistochemistry (IHC). Prompted by this experience, we evaluated ALK expression in 11 AFH, 15 IMT, and 11 follicular dendritic cell sarcomas using 3 different antibody clones: D5F3, 5A4, and ALK1. ALK IHC positive cases were analyzed with fluorescence in situ hybridization (FISH) using dual color ALK break-apart probe kit. The majority of AFH cases studied were positive for ALK IHC with at least 1 antibody (9/11 D5F3, 6/9 5A4, 1/9 ALK1), most demonstrating moderate to strong cytoplasmic staining. AFH with positive ALK IHC showed no ALK gene rearrangement by FISH (0/8) with ALK copy number ranging from 1.6 to 2.1. Sixty-seven percent of IMT were ALK positive by IHC (10/15 D5F3, 8/15 5A4, 7/15 ALK1), and 9 of the 10 cases were positive for ALK gene rearrangement by FISH. All follicular dendritic cell sarcomas were negative for ALK by IHC (D5F3 and 5A4). Our results indicate that ALK expression in AFH is common, particularly with the highly sensitive D5F3 and 5A4 antibodies and enhanced detection systems, and may be a potential source of diagnostic confusion with IMT. The underlying mechanism of ALK expression in AFH is unclear, although it does not appear to be from ALK rearrangement or amplification.