When immunotherapy mimics pancreatic cancer: mass-forming immune-related pancreatitis during neoadjuvant treatment for triple-negative breast cancer.

Immune checkpoint inhibitor (ICI)-induced pancreatitis is a rare immune-related adverse event that can present as mass-forming lesions mimicking metastatic disease, posing a diagnostic challenge in patients with cancer. We report the case of a woman with early triple-negative breast cancer who, after 4 months of neoadjuvant chemoimmunotherapy, experienced progression of the primary tumour. Restaging PET-CT revealed four hypermetabolic pancreatic lesions, suggestive of metastases. Abdominal MRI confirmed the presence of pancreatic masses. Endoscopic ultrasound-guided biopsy revealed immune-related pancreatitis with no malignant cells. Laboratory workup showed elevated amylase and lipase levels despite the patient being asymptomatic. ICI was discontinued and corticosteroid therapy was initiated, with normalisation of pancreatic enzymes. After excluding metastatic spread, the patient underwent a total mastectomy for local progression. This case highlights that ICI-induced pancreatitis can mimic metastatic disease and that tissue biopsy is essential to avoid misdiagnosis and inappropriate treatment escalation.