Expression of Steroidogenic Factor (SF1) and Application in Cytopathologic Identification of Intrapancreatic Ectopic Splenic Tissue

Summary: Introduction: Accessory spleens result from failure of splenic fusion during development and are commonly located in the splenic hilum followed by the pancreatic tail. Intrapancreatic accessory spleens can present as solid or solid/cystic lesions with a varying differential. Fine needle aspiration (FNA) is commonly performed and specimens with a differential of accessory spleen are routinely stained with CD8 to highlight splenic sinusoidal cells, however small samples, non-representative architecture, and reactivity with CD8+ T-cells can create diagnostic challenges. Steroidogenic Factor 1 (SF1) has been reported in mRNA expression of splenic endothelial cells, however immunohistochemical staining (IHC) in human tissue has not been reported. Materials and Methods: A retrospective search of FNA samples from 2010 to 2023 was performed to collect cases signed out as or consistent with intrapancreatic accessory spleen or spenule. Comparison tissue was collected from regional lymph nodes. Tissues with no or hypocellular cell blocks were excluded from the study. IHC stains CD8 and SF1 (EP434) Rabbit Monoclonal Primary Antibody were performed on the cell blocks. Results: The intrapancreatic accessory spleens (13 cases) were universally positive for both cytoplasmic CD8 and nuclear SF-1 in an endothelial pattern. The regional lymph nodes (15 cases) were positive in background CD8+ T cells (scattered), and universally negative for SF1. Conclusions: While CD8 and SF1 are comparable in their sensitivity, SF1 has benefits in cytologic samples. As a nuclear stain, SF1 is more easily interpreted and better suited for cell transfers. SF1 also eliminates the background CD8+ T cell staining that can be present in lymphoid tissue. Overall, utilization of SF1 can be beneficial in the cytologic work up of an intrapancreatic accessory spleen.