• Yong Jin Cho, Song Iy Han, and Sung-Chul Lim.
• Department of Orthopedic Surgery, College of Medicine, Chosun University, Gwangju, Republic of Korea.
• Medicine (Baltimore). 2021 Sep 10; 100 (36): e27209e27209.

RationaleFollicular dendritic cell sarcoma (FDCS) is an intermediate-grade malignancy originating from follicular dendritic cells. Nodal FDCS is the most common type, meaning that the extranodal type may not be recognized and could be easily misdiagnosed. Reported extranodal sites include the head and neck, retroperitoneum, spleen, liver, and gastrointestinal tract. FDCS in the soft tissue is extremely rare.Patient ConcernsA 75-year-old male presented with complaints of a localized swelling and intra-muscular soft tissue mass in the left upper thigh.DiagnosisThe present tumor consisted of fascicular or vague storiform-arranged spindle cells with less pleomorphism and many lymphoid aggregates. Tumor cells were positive for CD21, CD35, CD68, vimentin, and EGFR. Intra-muscular FDCS was confirmed by immunohistochemical studies.InterventionsThe patient received a wide marginal excision, followed by adjuvant radiotherapy.OutcomesSymptomatic improvements were achieved and no subsequent relapses were observed.LessonsIf the tumor arises in the extranodal sites, especially in the soft tissue, it is difficult to include FDCS in the differential diagnosis. When the immunoprofile is not consistent with that of common spindle cell tumors, immunostaining for follicular dendritic cell markers such as CD21, CD23, and CD35, as well as further immunohistochemistry for D2-40, CD68, EGFR, Epstein-Barr virus, and BRAF can be helpful for the diagnosis and subtyping of FDCS. To the best of our knowledge, the present case is the first case of intramuscular FDCS.Copyright © 2021 the Author(s). Published by Wolters Kluwer Health, Inc.

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