S3211 Histological Mimicry of Autoimmune Pancreatitis in a Case of Pancreatic Adenocarcinoma

Introduction: Differentiating autoimmune pancreatitis (AIP) from pancreatic cancer (PC) poses significant challenges due to overlapping clinical, serological, and imaging features. Both may present with painless jaundice, weight loss, new-onset diabetes, and a pancreatic mass, making initial clinical distinction difficult. We present a case of pancreatic adenocarcinoma initially misdiagnosed as AIP based on imaging and histopathology. Case Description/Methods: A 71-year-old woman with a known history of ulcerative colitis (on mesalamine) presented with 2 months of dull epigastric pain without weight loss or jaundice. She consumed 1–2 glasses of wine daily and was a former smoker. Labs revealed elevated lipase (220 U/L), but normal complete blood count, complete metabolic profile, and IgG4. Contrast-enhanced computed tomography showed a diffusely enlarged pancreas without a mass or ductal dilation. Given the diagnostic uncertainty, Endoscopic ultrasound (EUS) was performed demonstrating diffuse hypoechogenicity with mass-like features. Fine needle biopsy (FNB) revealed lymphoplasmacytic infiltration with sclerosis and myxoid changes, suggestive of AIP. She was started on prednisone 40 mg daily for 4 weeks. Symptoms improved, but follow-up computed tomography showed pancreatic atrophy and irregular ductal dilation. CA 19-9 was elevated at 196. Repeat EUS identified a poorly defined, 30 × 24 mm hypoechoic mass in the pancreatic head. FNB confirmed pancreatic adenocarcinoma. She underwent laparoscopic pancreaticoduodenectomy with cholecystectomy. Pathology confirmed adenocarcinoma with perineural and lymphovascular invasion and metastases in 9 of 13 lymph nodes. Discussion: Initial findings including subacute pain, elevated lipase, and diffuse pancreatic changes were consistent with AIP. Despite normal IgG4, 20–25% of AIP cases show normal levels, especially in type 2 AIP, which is associated with inflammatory bowel disease. Steroid response was partial, raising suspicion for malignancy. AIP usually responds rapidly to steroids (∼97% response rate). Progression of imaging and tumor marker elevation prompted repeat evaluation and diagnosis of adenocarcinoma. AIP accounts for 1.6% of resected cases initially suspected to be PC, and up to 9.5% of PC cases may show AIP-like histology. This case highlights the diagnostic complexity and need for close follow-up in suspected AIP. Clinicians should maintain a high index of suspicion and pursue repeat evaluation if clinical or radiologic findings worsen despite therapy.