Smooth Muscle Myosin Heavy Chain Expression in Nodal and Extranodal Follicular Dendritic Cell Sarcoma.

Follicular dendritic cell sarcoma (FDCS) is a rare neoplasm requiring a high index of suspicion, especially on small biopsies. Smooth muscle myosin heavy chain (SMMHC) is a common immunohistochemical (IHC) stain that has been reported to mark normal nodal follicular dendritic cells (FDCs). We hypothesize that SMMHC can be a sensitive marker for FDCS and aim to compare its performance with established markers of FDCS. The archive of a large academic center was searched for cases of FDCS. Clinical features, including age, sex, and site at diagnosis, were reviewed. A hematoxylin and eosin-stained slide was evaluated to assess for morphology and presence of hyaline vascular Castleman disease. The established FDC markers CD21, CD23, fascin, clusterin, and D2-40 were reviewed and compared with SMMHC for all cases. The staining pattern was classified as positive (strong or weak) or negative. Seven unique cases of nodal and extranodal FDCS were collected. SMMHC was positive in most cases (n=5) and performed similarly to established FDC markers, including clusterin, CD21, and CD23 (n=7 each), and fascin and D2-40 (n=4 each). SMMHC was also negative in all 14 examined cases among 5 common differential diagnoses. We demonstrate that the common IHC marker SMMHC has high specificity and similar sensitivity as established FDC markers in nodal and extranodal FDCS, and is useful in the evaluation of common neoplastic mimics.