S3667 Ileocecal Mass: A Rare Gastrointestinal Manifestation of IgG4-Related Disease

Introduction: IgG4-related disease (IgG4-RD) is a rare, multisystemic autoimmune disease that presents a unique diagnostic challenge due to its shared similarities with malignancy, infectious and inflammatory disorders. Diagnosis is primarily based on histology and supported by raised serum IgG4 levels. While the gastrointestinal (GI) manifestations commonly include autoimmune pancreatitis and retroperitoneal fibrosis, ileocecal involvement is rare. . Case Description/Methods: A 65-year-old woman presented with complaints of sub-acute, and radiating right flank and lumbar pain, relieved by analgesics and accompanied by 2 episodes of non-bilious vomiting. She reported similar complaints 30 years ago which spontaneously resolved and had a family history of an unspecified malignancy. Examination revealed a palpable lump in the right hypochondrium and lumbar region. Initial investigations included a complete blood count, liver function test, RFT, and tests for malignancy (stool occult blood examination and CEA), which were all unremarkable. Abdominal ultrasound demonstrated the presence of an appendiceal lump. Computed tomography (CT) abdomen revealed an ill-defined enhancing retroperitoneal mass encasing the right ureter, causing moderate hydroureteronephrosis and loss of function in the right kidney along with associated lymphadenopathy and an embedded appendiceal tip. Exploratory laparotomy with right hemicolectomy and ureteral reconstruction was performed. A 5 cm fibrotic mass localized to the ileocecal region was identified and resected (R0 margins), and a frozen section biopsy suggested a fibroinflammatory process. Final histopathology of the resected mass showed storiform fibrosis, dense lymphoplasmacytic infiltrates, and an IgG4+ and IgG ratio of greater than 40%, confirming IgG4-related disease. Postoperatively, the serum IgG4 levels were normal, and positron emission tomography-computed tomography (PET)-CT did not show any evidence of multi-organ involvement. She was stable at discharge and is under outpatient surveillance. Discussion: This case highlights the need to consider IgG4-related disease in the differential diagnosis of atypical GI masses. Tissue biopsy remains the gold standard for diagnosis, as serum IgG4 levels may often be normal, as in this patient. Thus, while PET-CT and serology are helpful in staging and follow-up, they have poor sensitivity. Due to the rarity of ileocecal involvement even among GI manifestations, a high index of suspicion is necessary to prevent misdiagnosis and unnecessary surgery, as it is frequently confused with malignancy.