Interdigitating dendritic cell sarcoma presenting as retroperitoneal mass and obstructive jaundice. An uncommon entity of complex diagnosis.

A 60-year-old male was admitted for obstructive jaundice secondary to a 2 cm retroperitoneal lesion. Ultrasound endoscopy (UES) with fine needle biopsy (FNB) was performed, as well as endoscopic retrograde cholangiopancreatography (ERCP) with placement of a metal stent for bile duct drainage. Initially IgG4-related disease was suspected from FNB. After 8 months of treatment the patient attended with a progression evidenced in computed tomography (CT) with retroperitoneal adenopathies and liver metastasis, with involvement of duodenum and fistulation towards hepatic angle of colon. Upper endoscopy and colonoscopy was performed. The patient underwent a cephalic duodenopancreatectomy with antrectomy and right hemicolectomy. The histological study of the specimen revealed cells of fascicular distribution, fusiform aspect, with ovoid nuclei, accompanied by a polymorphic inflammatory infiltrate with mononuclear cells. Immunohistochemistry (IHQ) was positive for S100, CD21, CD23, D2-40 and Bcl-2 and negative for CD1a, CD2, CD3, CD4, CD5, CD8, CD20, CD79a, PD1, desmine, langherin, c-kit, PDGFR. These findings were suggestive of interdigitating dendritic cell sarcoma (IDCS). Systemic chemotherapy with R-CHOP was initiated and after the first 4 cycles of treatment, retroperitoneal adenopathies regressed, with stability of metastatic liver disease.