Main-duct and mixed IPMN

Main-duct IPMN is characterized by segmental or diffuse dilation of the main pancreatic duct without other causes of obstruction. 

According to the International Association of Pancreatology guidelines (2012), the threshold for main pancreatic duct dilation is 5 mm. The main pancreatic duct caliber, as measured on cross-sectional imaging, is useful for stratify the risk of malignancy. In particular: 

  • Main pancreatic duct dilation of 5-9 mm is considered a worrisome feature
  • Main pancreatic duct dilation of >10 mm is one of the high-risk stigmata 

Associated radiologic features (duct wall, mural nodules) contribute to the malignancy estimation. Although IPMN are mostly incidentally diagnosed, main-duct type neoplasms are more likely to cause symptoms, such as:

  • Abdominal pain
  • Acute pancreatitis
  • Jaundice
  • Weight loss
  • Anorexia 
Patients with main-duct IPMN, when surgically fit, should always be candidates for resection because of the high prevalence of high-grade dysplasia and invasive carcinoma found in resected specimens (up to 70%). Of note, there may be malignancy regardless of the presence or absence of symptoms. Accordingly, the radiological aspect of these lesions can determine the indication for surgery. A formal resection (pancreaticoduodenectomy, left pancreatectomy) with regional lymph node dissection is mandatory. Since IPMN extends along the pancreatic duct and may do so without macroscopic tumor, it is important to exclude residual tumor on frozen section. In cases of de-epithelialization, adenoma, or borderline tumor at the surgical margin, the optimal surgical strategy is controversial, but may lead to an extension of the resection or to total pancreatectomy. Patients with malignant main-duct IPMN are older than patients with adenoma or borderline tumor; supporting the theory of an adenoma-carcinoma progression to malignancy. Recurrence in the pancreatic remnant may develop even if the transection margin is negative and even in patients with noninvasive disease. Therefore, an accurate postoperative follow-up is mandatory.
 
In mixed-type IPMN, either the main duct and the branch ducts are dilated. The origin of the mixed form is unknown; that is, whether they originate from main-duct IPMN or arise as a combined form. Regardless of their pathogenesis, their biological behavior is known to be similar to that of main-duct IPMN and their treatment follows the same rules: high risk of malignant change and, for all surgically fit patients, an indication for surgery.
 
The majority of main-duct and mixed IPMN is histologically characterized by an intestinal-type epithelium. These histological subtype typically gives rise to colloid carcinoma, that displays a more indolent biological behavior when compared to pancreatic ductal adenocarcinoma.