Extrapancreatic Findings and Retrospective Diagnostic Pitfalls in Type 1 Autoimmune Pancreatitis: A Descriptive CT and MRI Study

Background/Objectives: Type 1 autoimmune pancreatitis (AIP) can resemble pancreatobiliary malignancy. We described pancreatic and extrapancreatic CT/MRI findings and explored retrospective interpretive patterns, including unrecognized diffuse disease. Methods: This retrospective single-center study included 40 selected patients with archived definite/probable JPS 2018 classifications. CT was available for 31 patients and MRI/MRCP for 36. Two radiologists reviewed features independently and reached consensus; access to diagnostic information during these assessments could not be reconstructed. Six operated patients formed an illustrative case series reviewed post hoc with final diagnoses known. Results: Diffuse disease occurred in 28/40 (70.0%), a capsule-like rim in 33/40 (82.5%), homogeneous portal venous and/or delayed enhancement in 40/40, and qualitative diffusion restriction in 36/36 MRI examinations. Extrapancreatic abnormalities compatible with IgG4-related involvement were recorded in 32/40 patients (80.0%): biliary abnormalities in 30/40 and renal abnormalities in 11/40. Two extrapancreatic presentations initiated evaluation. Three of six operated patients had diffuse disease. The narratives illustrate biliary-dominant findings, temporal change, and discordant multimodality evidence. These are selected-cohort frequencies, not population prevalence or sensitivity estimates. Imaging also contributed to diagnostic adjudication, creating incorporation bias; malignant controls were absent. Conclusions: The findings describe an imaging spectrum and generate educational hypotheses. The diagnostic-role framework has not been validated as a diagnostic algorithm or shown to reduce misdiagnosis or unnecessary surgery. Independent comparative validation is required.