Follicular dendritic cell sarcoma (FDCS) is a rare mesenchymal neoplasm arising from the follicular dendritic cells (FDC) of lymphoid follicles. FDCS can be found in nodal and extranodal sites.

In this report, we present an unusual case of pleura-based FDCS with epithelioid cytology and aberrant expression of cytokeratins, including discussion of differential diagnosis.

A comprehensive panel of immunohistochemical (IHC) stains was performed. Additionally, fluorescence in situ hybridization, allele-specific quantitative polymerase chain reaction, and next-generation sequencing assays were utilized to detect somatic gene mutations and potential fusions.

This case of FDCS reveals predominantly epithelioid cytology with a focal spindle cell component. Upon IHC staining, both the epithelioid and spindle cells are positive for FDC markers, including CD21, CD23, CD35, clusterin, CXCL13, and podoplanin. Particularly, both components show aberrant expression of cytokeratin (CK) AE1/3 and CAM5.2. Molecular genetic studies detected a splice site alteration of RB1 gene without other significant changes.

FDCS is a rare neoplasm with variable morphologic and staining patterns. To the best of our knowledge, this is the first reported instance of pleura-based FDCS with epithelioid morphology and aberrant expression of cytokeratins. Diagnosis of such cases can be challenging, which has to be separated from the morphologic mimicries, particularly carcinoma and mesothelioma.