Thickness of the Hyperechoic Capsule-like Rim Around Pancreatic Lesions Measured by Ultrasound for Differentiating Between Type 1 Autoimmune Pancreatitis and Pancreatic Adenocarcinoma

Background: Autoimmune pancreatitis (AIP) is often misdiagnosed as pancreatic adenocarcinoma (PAC), resulting in unnecessary surgical interventions. On computed tomography (CT) scans, the capsule-like rim is an essential radiological characteristic for differentiating AIP from PAC. It presents as a hypoattenuating halo surrounding the pancreas. However, this characteristic is infrequently observed in ultrasonography. Objectives: The aim of this study was to assess the accuracy of the thickness measurement of the capsule-like structure surrounding lesions during ultrasonography in order to distinguish between AIP and PAC. Patients and Methods: This case-control study was conducted on 19 patients with type 1 AIP (AIP1) as the case group and 37 patients with PAC as the controls. The ultrasound images of these patients were obtained from our institute's database. The thickest part of the hyperechoic capsule-like structure around lesions was identified and measured on the workstation retrospectively. The difference in the thickness of the capsule-like structure between AIP1 and PAC was compared in all lesions and mass lesions, respectively. The optimal cut-off thickness was determined by the maximum Youden index (calculated as sensitivity + specificity - 1). A P-value of < 0.05 (or < 0.05/3 after applying the Bonferroni correction) was considered statistically significant. Results: All lesions appeared hypoechoic, and there were no significant differences in gender, age, abdominal pain symptoms, jaundice, or weight loss between the case and control groups (P > 0.05). However, there was a significant difference regarding the involved pancreatic location (P = 0.008). Among the lesions, 46 were mass lesions. The hyperechoic capsule-like rim was thicker in the case group compared to the control group for all lesions (mean = 0.40 ± 0.12 vs. 0.32 ± 0.09 cm, P = 0.006) and also for mass lesions (mean = 0.41 ± 0.13 vs. 0.31 ± 0.09 cm, P = 0.006). The cut-off thickness for AIP1 was estimated at 0.41 cm, according to the maximum Youden index in both all lesions and mass lesions. The sensitivity, specificity, accuracy, positive predictive value, negative predictive value, and odds ratio for all lesions were 0.58, 0.86, 0.77, 0.69, 0.80, and 8.80 (95% confidence interval [CI]: 2.37 – 32.64), respectively. In mass lesions, the corresponding values were 0.58, 0.88, 0.80, 0.64, 0.86, and 10.50 (95% CI: 2.23 – 49.52), respectively. Conclusion: Patients with a hyperechoic capsule-like rim thickness of ≥0.41 cm during ultrasonography are more likely to have AIP1. This finding holds valuable clinical significance in differentiating between AIP1 and PAC.