What is an IPMN?
An intraductal papillary mucinous neoplasm (IPMN) is a mucin-producing pre-invasive tumor that grows within the pancreatic ducts, characterized by papillary epithelial proliferation that causes cystic dilation of the main pancreatic duct and/or its branches. 1, 2
Pathological Definition
IPMNs are mass-forming pre-invasive neoplasms (tumoral intraepithelial neoplasms) that must be greater than 1 cm in diameter to distinguish them from incidental microcysts and large pancreatic intraepithelial neoplasms (PanINs). 1
The defining histological features include:
- Papillary epithelial proliferation within dilated pancreatic ducts filled with mucin 2
- Friable papillary projections within dilated mucin-filled ducts 3
- Thick mucin secretion that causes segmental or diffuse ductal dilatation 4
Spectrum of Disease
IPMNs represent an adenoma-carcinoma sequence, encompassing a spectrum from benign to malignant lesions:
- Low-grade dysplasia → intermediate-grade dysplasia → high-grade dysplasia → invasive carcinoma 1
- The lesions range from innocuous-appearing (formerly called "hyperplasia") to full-blown intramucosal carcinomatous lesions (formerly designated as "papillary adenocarcinoma"). 1
Anatomical Classification
IPMNs are classified into three types based on ductal involvement, which has critical implications for malignancy risk:
- Branch duct type (BD-IPMN): Involves only the branch ducts; most are benign 4, 5
- Main duct type (MD-IPMN): Involves the main pancreatic duct; frequently malignant with higher risk of progression 4, 5
- Mixed type (MT-IPMN): Combines both branch and main duct characteristics 4, 5
The main duct type presents a significantly higher risk of malignant progression than the branch duct type. 4
Histological Subtypes
The American College of Surgeons classifies IPMNs into four main cell types, each with distinct morphological characteristics and genetic drivers: 3
- Gastric type: Columnar cells with mucinous cytoplasm, simple and short papillae, and frequent pyloric gland-like elements 3
- Intestinal type: Pseudostratified columnar cells with basophilic appearance and apical mucin, villous growth pattern similar to colonic adenomas 3
- Pancreatobiliary type: Cuboidal cells with enlarged nuclei and scant mucin production, complex papillary configurations 3
- Oncocytic type: Cuboidal cells with oncocytic cytoplasm arranged in 2-5 layers, complex arborescent papillae with intraepithelial lumens 3
Approximately 15% of cases present with overlapping characteristics and should be classified as "mixed." 3
Invasive Carcinoma
When IPMNs progress to invasive carcinoma, two main types are recognized:
- Ductal/tubular invasive carcinoma: Atypical cells forming irregular and complex tubular or glandular structures 3
- Colloid carcinoma: Muconodular pattern with distinct pools of mucin containing scant groups of carcinomatous cells 3
A critical diagnostic pitfall is distinguishing true invasive colloid carcinoma from duct rupture with mucin extrusion into the stroma, as this distinction dramatically affects patient prognosis and management. 3
Key Diagnostic Features
The main imaging characteristic distinguishing IPMN from other pancreatic lesions is dilatation of the pancreatic duct without the presence of an obstructing lesion. 6
IPMNs must be differentiated from: