Inflammatory lobular hemangioma (T-cell-rich angiomatoid polypoid pseudolymphoma): assessment of FOS/FOSB and lymphoid markers and comparison with epithelioid hemangioma

Background: T-cell-rich angiomatoid polypoid pseudolymphoma (TRAPP) and inflammatory lobular hemangioma (ILH) encompass a spectrum of cutaneous vascular lesions in which a prominent lymphoplasmacytic component may impart a pattern highly reminiscent of low-grade cutaneous lymphoma (pseudolymphoma). Epithelioid hemangioma, including its most common variant angiolymphoid hyperplasia with eosinophilia (ALHE), is a distinct entity associated with FOS and/or FOSB expression detected by immunohistochemistry (IHC). These entities can show significant morphological overlap. Methods: We performed IHC for FOSB, FOS, and lymphoid markers in a series of TRAPP/ILH and ALHE. Results: We identified 13 cases of ILH/TRAPP, which showed a predominance in CD8⁺ T-cells (CD8>CD4: 11/13) while FOSB and FOS were expressed in 36% (4/11) and 27% (3/11) of cases, respectively. ALHE (n = 9) showed a predominance in CD4⁺ T-cell (67%) with FOSB and FOS co-expression in 78% (seven of nine) of the cases. Conclusion: We showed, based on FOS and/or FOSB immunohistochemical expression, that there is a possible link between ILH/TRAPP and epithelioid hemangioma/ALHE. The use of FOS and FOSB IHC in the routine diagnostic setting of cutaneous vascular lesions will help to redefine cases of ILH/TRAPP as a subset of these may represent inflammatory variants of epithelioid hemangioma.