Systematic review and meta-analysis of MRI features for differentiating autoimmune pancreatitis from pancreatic adenocarcinoma
Seung Bae Yoon, Tae Yeon Jeon, Sung-Hoon Moon, Sang Min Lee, Myung-Hwan Kim.
European Radiology (2022) 32:6691–6701
Autoimmune pancreatitis (AIP) is a form of chronic pancreatitis that need to be differentiated from other types of pancreatitis such as alcoholic, hereditary or obstructive [1, 2]. Nowadays it is known that this condition has an autoimmune etiology and is not limited to the pancreas alone, but also involves organs such as bile ducts, retroperitoneum, and lymph nodes [3].
Further histological and clinical profiling of AIP revealed 2 distinct types: type 1, histological descripted as lymphoplasmacytic sclerosing pancreatitis (LPSP); and type 2, called idiopathic duct-centric pancreatitis (IDCP). Both share some histopathological features, such as periductal lymphoplasmacytic infiltrate and storiform fibrosis. A characteristic histopathologic finding of IDCP is granulocyte epithelial lesions. Instead, LPSP can lead to venulitis, with lymphocytes and plasmacells, often leading to obliteration of the affected veins, and is associated with either serum IgG4 elevation and with other organ involvement.
Extrapancreatic involvement of IDCP, reported approximately 30% of cases, is with inflammatory bowel disease.
The most frequent presentation of AIP is obstructive jaundice and/or pancreatic mass. Whereas most patients have pancreatic swelling (diffuse or focal), a few patients may show low-density pancreatic masses. The diagnosis of LPSP is made through a combination of imaging, serologic and clinical findings, making histological confirmation not mandatory. Contrarywise, since IDCP patients are seronegative and lack other organ involvement, definitive histological diagnosis is needed [4]. Both conditions respond to steroid therapy and do not need surgery [5].
The differential diagnosis with pancreatic ductal adenocarcinoma (PDAC) is challenging, especially for focal-type AIP, due to de clinical manifestations and radiological features.
A recent systematic review and meta-analysis conducted by Yoon and colleagues aimed to identify MRI features that will aid the differential diagnosis between AIP and PDAC. After an initial screening or 1214 studies, the authors included 12 retrospective studies published between January 2006 and July 2021. After reviewing 92 overlapping descriptors of MRI findings in the included studies, 16 MRI features were selected:
- Ten favoring AIP: diffuse enlargement, capsule-like rim, multiple main pancreatic duct (MPD) strictures, homogeneous delayed enhancement, low apparent diffusion coefficient value, speckled enhancement, multiples pancreatic masses, tapered narrowing of MPD, penetrating duct sign, and delayed enhancement.
- Six favoring PDAC: target type enhancement, discrete pancreatic mass, upstream MPD dilatation, peripancreatic fat infiltration, upstream parenchymal atrophy, and vascular involvement.
From these features, the pooled diagnostic accuracy, including the diagnostic odds ratios (DORs) with 95% confidence intervals (CIs), was calculated using a bivariate random effects model. MRI features for AIP tended towards a high specificity but low sensitivity. Among them, diffuse enlargement, capsule-like rim, multiple MPD strictures, and homogeneous delayed enhancement had the highest DORs, and showed a high pooled specificities (90–100%). Diffuse enlargement had the highest DOR; however, diffuse-type AIP can be differentiated from PDAC due to the growing awareness of imaging appearance of AIP [6]. Radiologists must be aware that pancreatic lymphoma, diffuse PDAC, or metastasis to the pancreas can also present with diffuse enlargement of the pancreas [7-8]. The capsule-like rim, representing a continuous halo-like soft tissue lesion surrounding the pancreas can be thin or thick depending on disease activity [9]. Multiple pancreatic masses and MPD strictures without intervening duct dilatation are especially helpful in the differentiation of focal-type AIP from PDAC. Delayed enhancement had low specificity compared to the “homogeneous” delayed enhancement (55% vs. 90%). Due to its desmoplastic nature, PDAC can show delayed enhancement. However, radiologists should consider the presence of homogenous enhancement when evaluating the enhancement pattern of a focal pancreatic mass in the differential diagnosis between AIP and PDAC. Penetrating duct sign has a high sensitivity (81%) and is indicative of AIP.
The Apparent Diffusion Coefficient (ADC) values are lower for AIP than PDAC, and had a relatively high sensitivity (84%) and specificity (86%). However, the cutoffs for the ADC value differed among the studies included by the authors.
On the other hand, for the diagnosis of PDAC, MRI features with the highest pooled DOR were target type enhancement and discrete pancreatic mass. The informative MRI features favoring PDAC had moderate specificity. Whereas upstream MPD dilation and upstream parenchymal atrophy had relatively high specificities (88 and 81%, respectively), with low sensitivities (46-59%, respectively).
The results of the current study are important as they provide informative MRI features with their diagnostic performance that may help the radiologists in the differential diagnosis between AIP and PDAC, which can be particularly challenging in clinical practice.
In conclusion, MRI features summarized in this study may aid in the detection and accurate differentiation of AIP from PDAC, in combination with serologic, extra-pancreatic involvement and clinical features.
References:
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Dr. Mario Pace is a fourth-year radiology resident at the University of Palermo in Italy. During his student carrier he was at the Department of Cardiology, Nephrology, Gastroenterology and Endocrinology of the “University Medical Center” in Slovenia and at the Department of Gynecology of the “Hospital San Martin” of La Plata in Argentina. His main interests are abdominal and musculoskeletal radiology, in fact he had a fellowship of 6 month to the IRCCS AOU San Marino in order to improve the knowledge of musculoskeletal and peripheral nerve system.
Comments may be sent to 91mariopace@gmail.com