Prevalence and interpretation of AFF immunostain of DEK::AFF2 fusion-associated papillary squamous cell carcinoma in a retrospective cohort of recurrent sinonasal papillomas

DEK::AFF2 fusion-associated papillary squamous cell carcinoma is a novel entity characterized by its unique translocation and malignant clinical course. In this study, AFF immunohistochemistry (IHC) was performed in recurrent sinonasal papillomas for reviewing the prevalence of undiagnosed DEK::AFF2 carcinomas and to investigate the performance of AFF IHC in diagnosis of DEK::AFF2 carcinomas. Recurrent sinonasal papillomas after surgical excision in a two-decade period were retrieved. Histologic slides were reviewed for features of DEK::AFF2 carcinoma. AFF IHC was performed, and cases with any (> 1%) nuclear positivity were validated by DEK break apart fluorescence in situ hybridization. Totally 43 cases were included, comprising 28 inverted, 6 exophytic, one oncocytic, and 8 non-specified sinonasal papillomas. Five (11.6%) cases exhibited positivity to AFF IHC. Three cases exhibited patchy weak to moderate staining intensity predominantly in a granular cytoplasmic pattern. Two cases exhibited strong and diffuse (> 90%) nuclear staining. Cases showing weak staining were negative for DEK rearrangement, while those with strong staining were positive. Both cases of DEK::AFF2 carcinoma showed aggressive behavior with extensive local invasion and nodal metastasis. Background stromal plasma cells, when present, consistently showed strong and diffuse staining. AFF IHC was further performed in plasmacytoma samples as control and showed strong and diffuse immunoreactivity. A significant minority of recurrent sinonasal papillomas represent DEK::AFF2 carcinomas. Granular, cytoplasmic, or incomplete AFF staining should be considered as negative. In view of the rarity of DEK::AFF2 carcinomas, plasma cells and plasma cell neoplasms are potential for internal and surrogate external controls.