Byline: Yaping Luo, Department of Nuclear Medicine, Chinese Academy of Medical Sciences and Peking Union Medical College Hospital Beijing Key Laboratory of Molecular Targeted Diagnosis and Therapy in Nuclear Medicine, Beijing, P.R. China.; Guilan Hu; Yanru Ma; Ning Guo; Fang Li Abstract RATIONALE: Pancreatic acinar cell carcinoma (ACC) is a rare malignant tumor of exocrine pancreas. It is typically a well-marginated large solid mass arising in a certain aspect of the pancreas. Diffuse involvement of ACC in the pancreas is very rare, and may simulate pancreatitis in radiological findings. We report 2 cases of ACC presenting as diffuse enlargement of the pancreas due to tumor involvement without formation of a distinct mass. PATIENT CONCERNS: The patients consisted of a 41-year-old man with weight loss and a 77-year-old man who was asymptomatic. DIAGNOSES: Computed tomography (CT) and 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT showed diffuse enlargement of the pancreas forming a sausage-like shape with homogenously increased FDG activity. INTERVENTIONS: Endoscopic ultrasound (EUS)-guided fine needle aspiration (FNA) biopsy of the pancreatic lesion was performed. OUTCOMES: Histopathology results from the pancreas confirmed the diagnosis of pancreatic ACC. LESSONS: Because diffuse enlargement of the pancreas is a common imaging feature of pancreatitis, recognition of this rare morphologic pattern of ACC is important for radiological diagnosis of this tumor.