Folliculocentric B-cell-rich follicular dendritic cells sarcoma: a hitherto unreported morphological variant mimicking lymphoproliferative disorders.

We report three cases of follicular dendritic cell sarcoma (FDCS) showing a hitherto undescribed histological pattern consisting of nodular tumor growth associated with small B lymphocytes. FDCS tumor cells consistently showed large epithelioid features and were intermingled with small lymphocytes in the nodules in two cases, whereas they formed cohesive aggregates surrounded by lymphocyte mantle in the other. These features were easily confused with lymphomatous proliferations and, in particular, subtypes of Hodgkin lymphoma, high-grade follicular lymphoma, and germinotropic large B-cell lymphomas. The diagnosis was established by the use of a broad panel of antibodies that showed a variable expression of the FDC markers CD21, CD23, CD35, clusterin, podoplanin, claudin 4, epidermal growth factor receptor, and CXCL13. The associated B lymphocytes revealed a mantle zone B phenotype, with expression of CD20 and PAX5, together with TCL1 and IgD. Of notice, in all cases, morphological features suggesting hyaline-vascular Castleman disease were recognized in the interfollicular areas, containing scattered epithelioid cells similar to those found in the nodules, thus providing a useful clue for FDCS diagnosis. Of the 3 cases, 1 presented multiple recurrences unresponsive to chemotherapy and radiotherapy and finally died of disease 14 years after diagnosis. This study further emphasizes the extreme variability of morphological presentation of FDCS and expands the spectrum of lesions showing a nodular growth pattern occurring in human lymph nodes.