SSTR2a Is a Useful Diagnostic Marker for Follicular Dendritic Cells and Their Related Tumors

: SSTR2a, a member of the somatostatin receptor family, has been used as a diagnostic marker of meningioma. However, the expression of SSTR2a in follicular dendritic cells (FDCs) and their related tumors has been poorly characterized. This study aimed to assess the potential diagnostic utility of measuring SSTR2a immunohistochemically in FDCs and their related tumors. We evaluated whole-tissue sections from 182 cases including 83 lymphoid reactive follicular hyperplasias, 17 follicular lymphomas, 18 follicular dendritic cell sarcomas (FDCSs), 6 inflammatory pseudotumor-like FDCSs, and 58 other histologic mimics. Immunohistochemistry for SSTR2a and other FDC markers (CD21, CD23, CD35, clusterin, and podoplanin) were performed in all 182 cases. Diffuse membrane immunoreactivity for SSTR2a in FDCs was observed in 100% of follicular lymphoma and FDCS cases and in 96.4% of the reactive follicular hyperplasias cases. Notably, the positive rate of SSTR2a in FDCSs was higher than that of CD21 (88.9%), CD23 (77.8%), CD35 (94.4%), clusterin (55.6%), and podoplanin (94.4%). All inflammatory pseudotumor-like FDCSs were negative for SSTR2a. The histologic mimics were negative for SSTR2a, except for 1 leiomyosarcoma case that showed focal (~10%) positive expression for SSTR2a. Overall, our findings indicate that SSTR2a is a highly sensitive and diagnostically useful marker for FDCs and FDCSs. Furthermore, immunohistochemistry for SSTR2a may be helpful to distinguish FDCSs from inflammatory pseudotumor-like FDCSs and other histologic mimics. Moreover, our findings suggest that SSTR2a may be a potential therapeutic target for treatment of FDCSs.