Quantitative classification of pancreatic atrophy: the İstanbul morphometric pancreatic atrophy classification based on CT and MRI measurements

Background: Pancreatic atrophy (PA) is a progressive reduction in pancreatic parenchymal volume, accompanied by acinar cell loss, fibrosis, and fatty infiltration. It is an important radiological marker of glandular damage associated with physiological aging, chronic inflammatory diseases, and metabolic disturbances, such as diabetes mellitus and obesity. However, PA is still reported mainly using subjective terms, which leads to inter-observer variability and limits reliable clinical correlation and longitudinal follow-up. Purpose: This study aimed to introduce and validate the İstanbul Morphometric Pancreatic Atrophy Classification (IM-PAC), a four-tier morphometric grading system based on CT and MRI measurements, and to evaluate its association with graded CP severity features (CP). Methods: In this retrospective single-center study, 280 abdominal CT and MRI examinations performed in 2023 were analyzed. Pancreatic head, body, and tail thicknesses were measured at standardized anatomical landmarks. Chronic pancreatitis (CP)-related imaging features were graded using predefined ordinal severity scales for ductal abnormalities and MRI-defined fibrosis. Analyses used true modality-specific denominators (CT n = 168; MRI n = 112). Inter-observer reproducibility was assessed using intra-class correlation coefficients (ICC) and quadratic weighted Cohen's kappa. Results: IM-PAC grades were distributed as Grade 0 (n = 54, 19.3%), Grade 1 (n = 119, 42.5%), Grade 2 (n = 93, 33.2%), and Grade 3 (n = 14, 5.0%). Increasing IM-PAC grade was significantly and monotonically associated with ductal abnormality severity (Spearman ρ = 0.52, p < 0.001) and MRI-defined fibrosis severity (ρ = 0.48, p < 0.001). Calcifications assessed in the CT subgroup (n = 168) showed progressive enrichment across grades (p < 0.001). Inter-observer agreement was excellent for thickness measurements (ICC = 0.89; 95% CI: 0.84-0.92) and substantial for ordinal ductal and fibrosis grading (weighted κ = 0.81 and 0.78, respectively). Conclusion: By integrating severity-based CP grading, IM-PAC provides a reproducible and clinically relevant morphometric framework for quantifying pancreatic atrophy.