Splenosis involved by in situ follicular B-cell neoplasm

We report the first well-documented case of in situ follicular B-cell neoplasm (ISFN) arising in splenosis in a patient with a remote history of splenectomy. A 60-year-old man undergoing evaluation for hydrocele was incidentally found to have multiple mesenteric masses on imaging. His past surgical history was notable for splenectomy 40 years earlier following a motor vehicle accident. Biopsy of the largest mass revealed fragments of splenic parenchyma containing two small follicles infiltrated by atypical lymphocytes. Follicular architecture was preserved without expansion or disruption. Immunohistochemistry demonstrated that the neoplastic cells were positive for CD20, PAX5, CD10, BCL2, and BCL6 and negative for CD3 and CD5. Flow cytometry performed on a concurrent specimen identified a clonal CD10-positive aberrant B-cell population. Fluorescence in situ hybridization (FISH) revealed a chromosomal translocation, t(14;18). These findings supported a diagnosis of ISFN.