Bone metastasis in follicular dendritic cell sarcoma-a rare site in a rare disease: case report and review of literature.

Follicular dendritic cell sarcoma (FDCS) is a rare sarcoma subtype, presenting as a relatively indolent disease in most cases. Given its rarity, clinicopathological characteristics and behavior as well as treatment, are reported in the literature through case reports and case series. Bone metastasis in FDCS is extremely rare and the outcome of the disease in this group of patients is unknown. We report one case of FDCS with bone involvement as the first site of metastasis. We present the progression of the disease over a period of almost a decade in a detailed manner, particularly the therapies used including immunotherapy with checkpoint inhibitors. In parallel, we provide a comparison with other cases of FDCS metastatic to the bone (total of eight cases) through a systematic review of the literature on the clinical and pathological manifestations as well as the outcomes of all such cases reported to date, to the best of our knowledge. These cases highlight the challenges associated with setting the correct diagnosis at presentation, the lack of evidence to support the role of adjuvant therapy following primary surgery but also the role and/or sequence of systemic options in the advanced setting.