Retroperitoneal fibrosis mimicking benign disease - a paraneoplastic clue to small bowel adenocarcinoma: A case report

Abstract

BACKGROUND

Retroperitoneal fibrosis (RPF) is a rare fibro-inflammatory disorder, classically affecting middle-aged Caucasian men, and is characterized by the development of dense fibrotic tissue surrounding the abdominal aorta and adjacent retroperitoneal structures. Most cases are idiopathic or secondary to medications, infections, immunoglobulin G4 (IgG4)-related disease or malignancies, mostly breast or gastric. We report a case of RPF secondary to metastatic small bowel adenocarcinoma, which was missed on initial biopsy.

CASE SUMMARY

A 73-year-old woman presented with recurrent urinary tract infections and progressive abdominal pain. Imaging revealed extensive retroperitoneal soft-tissue infiltration causing right-sided ureteropelvic junction obstruction with hydroureteronephrosis, left renal atrophy and hypodensities in the liver. Biopsy of the retroperitoneum showed reactive tissue with concerns for RPF without evidence of malignancy. Autoimmune workup, including IgG4 testing, was unrevealing. Given the extent of disease and atypical features, biopsy of the liver was pursued which revealed metastatic adenocarcinoma of small bowel origin. The patient underwent urinary decompression followed by systemic chemotherapy with folinic acid, fluorouracil, and oxaliplatin, resulting in radiographic improvement of metastatic disease and stabilization of RPF.

CONCLUSION

This case highlights the need to consider occult malignancy in patients with extensive or refractory RPF. Timely imaging and histopathologic evaluation are essential to avoid diagnostic delay and ensure appropriate management, particularly when clinical or radiologic features are atypical.

Keywords: Retroperitoneal fibrosis; Small bowel adenocarcinoma; Metastasis; Urinary tract infection; Malignancy; Case report

Core Tip: Retroperitoneal fibrosis is a rare fibroinflammatory disorder that typically involves the infrarenal aorta and ureters, leading to insidious symptoms such as abdominal or back pain, constitutional features, and obstructive uropathy. Laboratory findings are often nonspecific, with elevated inflammatory markers and occasional autoimmune serologies. Cross-sectional imaging with computed tomography or magnetic resonance imaging is essential for diagnosis and for guiding biopsy to exclude malignancy. Glucocorticoids remain the cornerstone of therapy, with immunomodulatory agents reserved for refractory or relapsing disease. Long-term follow-up is critical due to high relapse rates despite treatment.