According to the reported histological procurement yield of the end-cutting needles, the investigators supposed that the use of EUS-FNB in probable AIP patients, generally aimed only to rule-out malignancy, could provide histological tissue samples useful in enhancing the diagnostic level reached without histology, or defining the type of AIP.

The primary aim is to evaluate the capability of EUS-FNB to enhance diagnostic levels of AIP, that means to obtain definitive diagnosis of type 1 and type 2 AIP. Secondary aims are to evaluate: Safety of EUS-FNB in this setting of patients; the possibility to obtain a definitive diagnosis in focal/segmental form of AIP (that are the most clinically relevant because of the mimicking of cancer); diagnostic yield of EUS-FNB in this patient population; the rate of pancreatic neoplasms mimicking AIP.

Consecutive patients ≥ 18 years old, with suspected focal or diffuse AIP, according to ICDC (probable type 1 AIP; probable type 2 AIP; NOS-AIP) with written consent to participate in the study will be enrolled.

Exclusion criteria include: previous diagnosis of AIP; definitive type 1 AIP; steroid administration within 3 months before the EUS-FNB; coagulation disorders; pregnancy and lactation; unability to give informed consent .

The histopathology of the preparations will be evaluated, according to the ICDC, including the LPSP findings and the IDCP findings and recording the absence or presence of specific criteria: Periductal lymphoplasmacytic infiltrate without GELs, Storiform fibrosis, Obliterative phlebitis >10 IgG4-positive cells per HPF; GEL of duct wall; Granulocytic and lympho- plasmacytic acinar infiltrate.

According to the current clinical practice, when the diagnosis of AIP is confirmed (and malignancy is ruled-out) patient will be treated with the standard therapy for AIP. Patients will be followed for 12 months, in order to exclude misdiagnosed malignancy, and to evaluate the clinical course of the disease (response to steroid therapy, relapses, changes in imaging findings or surgical pathology in resected patients).

The final diagnosis of AIP will be confirmed on surgical pathology (in resected patients) or when a compatible clinical course is observed during a follow-up of at least 1 year (significant improvement on imaging after steroid therapy, no appearance of metastasis or sign of infiltration).

The number of cases where EUS-FNB histology improves the diagnostic level will be summarized using absolute and relative frequencies. The 95% confidence interval of this proportion will also be computed. The number of adverse events, the number of cases where histological finding diagnostic of AIP are observed in focal/segmental form and the number of pancreatic neoplasm will be summarized using absolute and relative frequencies. Sensitivity, specificity, positive likelihood ratio (LR+), negative likelihood Ratio (LR-) and the ROC curve will be used to analyze the capability of EUS-FNB to obtain a definitive diagnosis of type 1 and 2 AIP.

Inclusion Criteria:

• Consecutive patients ≥ 18 years old;
• Suspected focal or diffuse AIP, according to ICDC:

Probable type 1 AIP Probable type 2 AIP NOS-AIP

• provision of written consent to participate in the study.

Exclusion Criteria:

• Previous diagnosis of AIP
• Definitive type 1 AIP
• Steroid administration within 3 months before the EUS-FNB
• Coagulation disorders
• Pregnant and lactating women
• Unable to give informed consent

Patients with suspected AIP who need histological sampling for diagnosis.