Rapidly enlarging pancreatic acinar cell carcinoma with discordant multimodality treatment response

Pancreatic acinar cell carcinoma (ACC) is a rare malignant epithelial tumor whose imaging appearance can overlap with pancreatic ductal adenocarcinoma (PDAC) and other solid pancreatic neoplasms. We report a man in his 50’s with a pancreatic head mass causing biliary and pancreatic ductal obstruction. A noncontrast computed tomography (CT) performed approximately 10 months earlier, retrospectively reviewed, showed a small pancreatic head lesion without secondary signs of malignancy, which subsequently enlarged to approximately 8 cm. Contrast-enhanced CT demonstrated a large expansile heterogeneous mass with prominent enhancement and extensive central necrosis, initially raising concern for PDAC but prompting consideration of ACC, which was confirmed by endoscopic biopsy with immunohistochemistry. Surgical resection was initially considered, but the patient repeatedly declined surgery, and systemic chemotherapy was given after biliary decompression. Follow-up CT showed near-complete morphological resolution of the mass, whereas FDG-PET/CT showed persistent focal uptake (SUV max 6.2) and MRI showed a residual diffusion-restricting lesion. The patient was subsequently persuaded to proceed to surgery and underwent a Whipple procedure; histopathology confirmed residual acinar cell carcinoma with clear margins and no nodal metastases, indicating that the CT response had underestimated persistent disease. This case highlights the value of reviewing prior imaging for interval growth, recognizing atypical imaging features of a pancreatic head mass, and integrating CT, PET/CT, magnetic resonance imaging and, when feasible, pathological confirmation when assessing treatment response.