CT and MR evaluation of main pancreatic duct dilation: A radiologist's guide to differentiating benign from malignant causes.

Main pancreatic duct (MPD) dilation is an imaging finding that encompasses a broad spectrum of benign and malignant etiologies. Differentiating between these conditions is critical for appropriate patient management, and radiologists play a central role in this diagnostic process. CT and MR, particularly with MR cholangiopancreatography (MRCP), are crucial for assessing ductal morphology, parenchymal changes, and associated lesions. Thin-slice dual-phase CT provides excellent spatial resolution and is particularly effective for evaluating pancreatic ductal adenocarcinoma (PDAC), vascular invasion, and metastatic disease. Conversely, MR offers superior soft-tissue contrast and ductal delineation, allowing detailed assessment of subtle strictures, cystic lesions, and intraductal abnormalities. Benign causes of MPD dilation include chronic pancreatitis with ductal calculi, post-inflammatory strictures, low-grade main-duct intraductal papillary mucinous neoplasms (IPMNs), and focal autoimmune pancreatitis. These entities often show gradual ductal narrowing, multiple strictures, or smooth contour irregularities without abrupt cutoff. In contrast, malignant etiologies - such as PDAC, high-grade IPMN, ampullary carcinoma, pancreatic neuroendocrine carcinoma (NET), and metastases - typically present with abrupt ductal truncation, associated mass effect, and upstream atrophy. Recognition of imaging patterns such as the "double duct sign," enhancing mural nodules, or restricted diffusion improves diagnostic confidence. Technical optimization of CT and MR protocols, awareness of artifacts, and correlation with clinical data are essential to avoid misinterpretation. Radiologists must integrate morphological, functional, and clinical information to ensure accurate characterization of MPD dilation and guide optimal management.