Follicular dendritic cell sarcoma (FDCS) is an uncommon neoplasm of mesenchymal stem cell origin whose clinical course displays much variability. It was first described in 1986 by Monda et al.  and was since classified in the 2008 World Health Organization classification of haematolymphoid neoplasms with tumours of histiocytes and dendritic cells, alongside other dendritic cell tumours including Langerhans cell tumours, interdigitating dendritic cell sarcoma and other rare tumours. Follicular dendritic cells (FDC) form a tight meshwork in primary and secondary lymphoid follicles and interact with T and B lymphocytes through antigen presentation and generation and regulation of the germinal centre reaction [2, 3].
FDCS was thought to be an indolent disease and in some patients with localized tumours, prognosis can be favourable with 2 year mortality only 3 % . However some patients have a more aggressive tumour with local recurrence occurring in up to 43 %, distant metastases in 21–24 % and disease-related mortality in 13–17 % [5–7]. A surveillance, epidemiology and end results (SEER) analysis of 54 patients reported a median overall survival of 48 months for those with distant disease . FDCS most commonly presents as nodal disease though extranodal disease is reported in 33–46 % of patients and can involve the bowel, oropharynx, liver, spleen, pancreas, peritoneum, pleura, lung and thyroid [2, 9, 10].
Diagnosis is based on morphological and particularly immunohistochemical appearance with one or more of the following being positive: CD21, CD23, CD35, podoplanin and CXCL-13 [5, 11, 12]. Other FDC markers such as R4/23, Ki-M4, Ki-M4p, Ki-FDC1 may also be positive as well as vimentin, CD68, S100, fascin but these may be nonspecific. Typically, neoplastic cells lack CD1a, desmin and CD45 staining. Between 18.6 and 58 % have been reported as misdiagnosed at initial diagnosis, particularly as the histological appearance may resemble other tumours and FDC markers are not always routine in immunohistochemical analysis [4, 5, 10].
Surgical excision is the standard minimum treatment and intraabdominal tumours and those with other adverse histologic factors should be considered for adjuvant treatment. Overall, better outcomes are reported in those patients whose initial treatment includes a combination of surgery, chemotherapy and radiation where appropriate [5, 6]. Chemotherapy regimens that have documented responses include lymphoma protocols with cyclophosphamide, doxorubicin, vincristine and prednisone (CHOP) and rituximab with CHOP (R-CHOP) as well as sarcoma protocols of doxorubicin and ifosfamide or gemcitabine and a taxane .
Due to its rare presentation, molecular analysis has been limited until more recently. Go et al.  have reported v-Raf murine sarcoma viral oncogene homolog B (BRAF)V600E mutations in 5 from 27 (18.5 %) cases tested. Phosphatase and tension homolog (PTEN), RET and TP53 mutations have also been documented in FDCS presenting as a thyroid mass . The PTEN and TP53 mutations were similar to those previously described in other malignancies, though the functional significance of the RET mutation was unknown. Griffin et al.  have now performed targeted genomic sequencing of 309 known cancer-associated genes in 13 cases of FDCS and found recurrent alterations in nuclear factor (NF)-κB regulatory genes. However, none of these findings have as yet translated into use or trials of molecularly targeted therapies and research in this regard is of course limited by its rarity.
We report the first case using molecularly targeted therapy in combination with chemotherapy in a case of FDCS.