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Intraductal papillary mucinous neoplasm

Intraductal papillary mucinous neoplasms (IPMN's) are rare cystic tumours of the pancreas.

Epidemiology

These tumours are most frequently identified in the elderly (60-80 years of age) 6. Main duct type (see below) appears to present a decade or so earlier on average than branch duct type 5. There may be a male predilection 6.

Clinical presentation

Clinical presentation can be difficult to distinguish from chronic pancreatitis with repeated acute exacerbations. Endocrine dysfunction with new onset of diabetes mellitus has also been reported 5.

Pathology

IPMNs are one of a number of mucinous tumours of the pancreas, and can be further divided both histologically and with respect to their macroscopic appearance 5. They are uncommon ductal epithelial tumours comprising approximately 10-15% of cystic pancreatic neoplasms.

They are histologically divided into

  • adenoma
  • borderline malignant
  • intraductal papillary mucinous adenocarcinoma

Divided macroscopically

  • main duct
    • reminiscent of chronic pancreatitis
    • segmental or diffuse distribution
    • highest malignant potential 6
  • branch duct type
    • mostly seen in the head and uncinate process
    • more localised and mass-like
    • may be macro or microcystic in appearance 5
    • typically indolent behaviour 6
  • mixed type lesions

Solid components are suspicious of malignant transformation.

Radiographic features

The characteristic feature is that these tumours communicate with the pancreatic duct or branches, which helps to distinguish these tumours from mucinous cystadenoma / cystadenocarcinoma which do not.

CT

In main duct type either the entire or segments of the pancreatic duct are demonstrated to be dilated and filled with low density (mucin thus water density) material. The overlying pancreatic parenchyma is thinned.

In some cases, the tumour is very localised and appears cystic. It can therefore be difficult to distinguish from peripheral mucinous cystadenoma / cystadenocarcinoma unless convincing communication with the duct system can be demonstrated.

When such tumours are proximal, the distal pancreatic duct may be dilated without direct involvement. Cystic neoplasms can have a similar appearance.

Solid mural nodules are concerning for malignant transformation, and appear as hyperdense nodules protruding into the mucin filled dilated ducts, and enhance following administration of contrast.

Occasionally mucinous material can be seen to bulge out of a dilated ampulla of Vater. This finding is uncommon but essentially pathognomonic.

In branch duct type the majority of the gland is normal in appearance, except for a single or multiple side branches demonstrating marked dilatation.These have a cystic mass like appearance and often mimic cystic tumours of the pancreas. This has been termed a bunch of grapes due to its appearance. The microcystic variety has appearances similar to serous cystadenomas, but again communication with the main pancreatic duct is the key to correct diagnosis.

MRI

MRI appearances are similar to those seen on CT. Mural nodules appear hypointense c.f. to surrounding fluid / mucin and enhance following administration of contrast. Mucin globules do not enhance and lie dependently within the duct.

Ultrasound

Ultrasound demonstrates dilated ducts which appear hypoechoic. Mural nodules and mucin globules may appear hyperechoic, and difficult to separate from adjacent pancreatic parenchyma 6.

Diffuse main duct type has appearances essentially indistinguishable from chronic pancreatitis, with duct dilatation and parenchymal atrophy 5.

ERCP

Direct imaging of the pancreatic duct demonstrates variable dilatation (segmental or diffuse or branch) depending on the type. Polypoid mural tumour or amorphous mucinous luminal filling defects may be identified 5.

Mucinous material may be seen protruding from the ampulla of Vater 6.

Treatment and prognosis

Although generally indolent, malignant degeneration does occur, with direct invasion into adjacent organs or more frequently dissemination in the peritoneal cavity (pseudomyxoma peritonei).

There seems to be no consensus on treatment. Some advocate that as all types have malignant potential surgical excision is recommended. Others suggest follow up of smaller asymptomatic lesions is reasonable. Patient co-morbidities and wishes clearly have a major impact on the decision to operate.

If the lesion is proximal (either segmental main duct or branch type) then a Whipple procedure may be performed. If distal then a partial pancreatectomy suffices.

Complete resection is curative.

Differential diagnosis

General imaging differential considerations include

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