[Special histopathological variants and potential diagnostic traps of classical follicular dendritic cell sarcoma]

Objective: To investigate the clinicopathological features, special morphologic variants and potential diagnostic traps of classical follicular dendritic cell sarcoma (FDCS). Methods: A total of 25 cases of classical FDCS diagnosed in the First Hospital Affiliated to Army Medical University from 2006 to 2018 were examined by hematoxylin-eosin staining, immunohistochemistry and in situ hybridization for Epstein-Barr virus-encoded mRNA (EBER). Meanwhile, the types and characteristics of the special variants of FDCS were summarized along with those reported in the literature. Results: The age of patients ranged from 23 to 77 years (mean 52 years), the male to female ratio was 1.5, and the maximum diameter of tumor was 1.5 to 20 cm (mean 7.4 cm). Twelve cases (48%) were misdiagnosed at the initial evaluation. Follow-up information was available for 17 patients, and the follow-up time was 5 to 96 months. The propotion of patients having recurrence, metastasis and mortality was 3/17, 5/17 and 2/17, respectively. Microscopically, besides the typical morphology, 10 cases of FDCS showed special histomorphologies and/or structures, including those mimicking lymphoepithelioma-like carcinoma, desmoplastic infiltrating carcinoma, classical Hodgkin's lymphoma (CHL), anaplastic large cell lymphoma (ALCL) and hemangiopericytoma. These morphologic variants were potential diagnostic pitfalls and warranted attention. Immunohistochemistry showed that more than two markers of follicular dendritic cells (such as CD21, CD23, CD35, etc.) were expressed in cases showing typical morphology and the special variants. All 25 cases were all negative for EBER by in situ hybridization. Conclusions: Classical FDCS is rare, besides the typical morphologic features, there are many special variants. In particular, these may be confused with lymphoepithelioma-like carcinoma in the nasopharynx, CHL or ALCL in the mediastinum/lymph node. Awareness of these variants is essential for accurate diagnosis.