Individual cells were spindly to polygonal, having round to oval vesicular nuclei, conspicuous nucleoli and indistinct cell borders (Figure-3).



The lesion was infiltrating muscularis propria. Nuclear atypia was mild to moderate and mitosis were 2-3/10hpf. No evidence of necrosis was seen. Resection margins were clear.
Immunohistochemically, tumour cells were strongly positive for dendritic cell markers CD21 and CD23 (Figure-4 and 5).

CD68 and S-100 were focally expressed. Tumour cells were negative for CD117, DOG1, CD1a, SMA and ALK. On the basis of morphology and immunohistochemistry, it was diagnosed as FDCS-conventional type.
Patient was treated by surgical excision only and was found free of recurrence after 5 months till our writing this report.

Discussion

Follicular dendritic cells (FDC), also known as dendritic  reticulum cells are the cells of immune system which function in antigen presentation and germinal center regulation.^1,2 Follicular dendritic cell sarcoma (FDCS), first reported by Monda et al in 1986, is an uncommon intermediate grade sarcoma derived from FDC.^4,5
Histologically a conventional/classical type and inflammatory pseudo tumour type has been recognized. The conventional type is reported in all age groups with equal sex distribution, while the inflammatory pseudotumour like variant shows female predilection.^5,6 One study showed that FDCS could occur even in the first decade, with male predominance. It involves a variety of nodal sites, especially the cervical and axillary The extra nodal sites (>50% cases) include GI tract, retroperitoneum, mesentery, liver, spleen, pancreas, mediastinum, thyroid, pharynx, and tonsils.^5,6Both the morphology and immunohistochemistry are critical in the diagnosis of FDCS. The typical morphologic features of FDCS include proliferation of spindly to rounded/ovoid cells forming fascicles or whorls with sprinkling of lymphocytes. The nuclei are oval and exhibit vesicular /granular chromatin, distinct nucleoli and smooth nuclear contours. Tumour cells may form sheets, follicle like structures, trabeculae and pseudo-vascular spaces as well. More than one growth pattern can be observed in a single tumour and cuffing of lymphocytes around blood vessels is commonly seen. Occasionally multinucleate giant cells may be present. Mitotic rates can be variable and upto 10 mitoses/10hpf can be seen. FDCS have a broad morphologic pattern overlapping with ectopic meningioma, myoepithelial tumours, inflammatory myofibroblastic tumours, thymoma and gastrointestinal stromal tumour, while osteosarcoma like pattern has been described in one study.^7,8 All these tumours have unique histological features and have characteristic Immunoprofile.
Post radiation morphologic changes in FDCS include nuclear pleomorphism, with nuclear grooving, pseudo-inclusions along with sheets of foamy histiocytes with surrounding fibrous septa. It should be noted that in inflammatory pseudotumour like variant (which selectively occurs in liver, spleen and a single case reported in colon), the tumour cells are dispersed among prominent lymphoplasmacytic cells. It has female predominance, strong association with Epstein Barr Virus (EBV) and indolent behaviour as compared to conventional type.
Immunohistochemically, FDCS show strong positivity for CD21, CD23 and CD35 and occasional positivity for S-100, CD68, leukocyte common antigen and muscle specific actin. They also frequently express vimentin, desmoplakin, human leukocyte antigen-DR (HLA-DR) and epithelial membrane antigen (EMA). FDCS lacks the expression of CD1a, desmin, CD45, cytokeratin and vascular markers.⁹ In our case, tumour cells showed strong positivity for CD21 and CD23 while S-100 and CD68 were focally positive. CD117, DOG1, CD1a, and ALK were negative, hence helping us to exclude gastrointestinal stromal tumour, Langerhans cell tumours and lymphoid neoplasms.
FDCS involving lymph nodes, patients usually presents with a well circumscribed painless mass or can be asymptomatic in abdominal FDCS.⁹ The nonspecific constitutional symptoms include low grade fever, abdominal fullness and weight loss. On CT scan, abdominal FDCS can be single or multiple, having ovoid, lobulated or irregular shape with well defined or ill defined margins. Necrosis and focal areas of calcification can be seen and tumour may invade surrounding structures, with or without regional lymphadenopathy.^9,10 Abdominal FDCS have larger size than FDCS at other sites with an average size of 10.2 cm (range 3-22cm).¹¹
Aetiology is unclear, however there is an association with Castleman\\\'s disease, suggesting hyperplasia, dysplasia and neoplasia sequence with malignant transformation. FDCS (inflammatory pseudotumour like variant) shows strong association with Epstein-Barr Virus (EBV), which may be due to the expression of CD21. Through CD 21 these cells can be experimentally infected by EBV. The role of p53 and LMP-1 (latent membrane protein, a gene of EBV) in the transformation of FDCS has also been proposed.^11,12 However additional studies will be required to better define their relationship with FDCS.

There is variation in therapeutic options for FDCS; however, complete surgical excision is almost always included. Only surgical treatment was opted for our patient and was found free of recurrence after five months. The role of adjuvant chemotherapy and radiotherapy in improving the survival is not clearly demonstrated, as metastasis may occur even after postoperative radiotherapy, but adjuvant therapy should be considered for tumours having high-grade histologic features or intra-abdominal location. According to Soriano et al, those patients who were treated with surgical excision along with adjuvant chemo and radiotherapy had longer disease free interval. ¹³ Spontaneous regression of FDCS of lung has also been reported in the literature.^14,15
It is established that the epidermal growth factor receptor (EGFR) is expressed in a significant number of FDC sarcoma, which can be demonstrated by Immunohistochemistry. There is possible up regulation of EGFR in FDC. These are strongly associated with Castleman\\\'s disease. These findings identify a target for possible therapy in un-resectable or refractory cases. This has not been widely used.¹⁶ Role of chemotherapy especially in relapse is questionable.

Disclaimer: None to declare.
Conflict of Interest: None to declare.
Funding Disclosure: None to declare.

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