Retroperitoneal Soft Tissue Xanthogranulomatous Pseudocyst: A Report of a Rare Case and Literature Review.

Xanthogranulomatous conditions are rare entities. Xanthogranulomatous inflammation, primarily involving the retroperitoneal soft tissue without visceral organ involvement, is very rare. Pseudocyst formation as a sequela of this condition is even rarer and has been barely reported in the literature. In the literature review, only a few cases of retroperitoneal xanthogranulomatous pancreatic (visceral) pseudocyst were reported, with no reports of retroperitoneal primary soft tissue origin. We report a very rare case of a large, benign, primary soft tissue retroperitoneal xanthogranulomatous pseudocyst in a 33-year-old woman who presented with abdominal pain and significant weight loss. Complete excision of the mass was possible due to the lack of infiltration. Histopathologic examination showed xanthogranulomatous inflammation, consisting of sheets of cluster of differentiation 68 (CD68)-positive, foamy, lipid-laden histiocytes (xanthoma cells) surrounding areas of necrosis with pseudocyst formation. Other immunostains were negative, including CD1a, S100, and B-Raf proto-oncogene (BRAF) V600E, supporting the sporadic nature without multisystem involvement. Although the histology and immunostains for vascular markers (D2/40 and CD31) did not identify a vascular tumor, the possibility that this xanthogranulomatous inflammation originated from a ruptured and infected retroperitoneal lymphangioma cannot be entirely excluded. Retroperitoneal soft tissue origin, extensive adherence to multiple organs without their involvement, absence of adjacent tissue infiltration despite being large, and significant weight loss, despite being benign, are unique features in the present case. Retroperitoneal xanthogranuloma can become a critical condition, given the adequate space in the retroperitoneum, allowing it to grow until large enough to manifest clinically, and its potential for adjacent tissue infiltration even when small. In addition, the clinical presentation is often non-specific, and radiologic diagnosis is difficult. Considering all these factors, it is prudent to include it in the differential diagnosis, as early removal of the mass is the only way to prevent local tissue infiltration and halt the aggressive nature of this disease.