Pancreaticobiliary involvement in treated type 1 autoimmune pancreatitis: Imaging pattern and risk factors for disease relapse.

Purpose: To evaluate the imaging pattern of pancreaticobiliary lesions in patients with treated type 1AIP, to determine the incidence of disease relapse and malignancy, and to identify the risk factors.

Method: The institutional review board approval was acquired. All patients gave written informed consent. From a prospective clinico-radiological database since 2012, consecutive patients with type 1 AIP who were treated and followed up (≥18 months) were identified. The presence/absence of pancreaticobiliary lesion(s) development during follow-up were assessed. The etiology was determined and the imaging pattern was compared to the initial attack. Risk factors were identified by univariate and multivariate analysis.

Results: Among 103 patients with treated type 1 AIP, 44 (42.7%) developed pancreaticobiliary lesions during follow up (median time interval to initial diagnosis: 17 months, range 3-62 months), mostly after steroid discontinuation (63.6%) or during maintenance therapy (29.5%). All lesions were disease relapse, which responded to steroid treatment. At relapse, pancreatic involvement was less frequent (81.8% vs 100%, p = 0.003), and the pancreas size was smaller (p < 0.01), whereas extra-pancreatic bile duct (ExPanBD) involvement was more severe and extensive (both p < 0.01). Multivariate analysis revealed ExPanBD involvement at initial diagnosis (hazard ratio 2.437, 95% CI 1.343-7.402, p = 0.002) and serum IgG4 response ratio at the induction phase (hazard ratio 0.357, 95% CI 0.055-0.804, p = 0.011) as significant independent predictors of relapse.

Conclusions: In treated type 1 AIP, although imaging pattern may differ, pancreaticobiliary lesions are usually manifestations of disease relapse. ExPanBD involvement and poor serum response suggests high risk of relapse.