MUCINOUS CYSTIC LESIONS OF THE PANCREAS

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MUCINOUS CYSTIC LESIONS OF THE PANCREAS

INTRODUCTION

Cystic lesions of the pancreas (CLP) include pseudocysts, cystic tumours and true pancreatic cysts. Ninety percent of all CLP are pseudocysts. A majority of the remaining CLP are serous and mucinous cystadenomas (MC). Other cystic lesions are even less frequent (Table I). Certain CLP are pre-malignant or potentially malignant (CK and MC) necessitating surgical resection whereas for benign cysts (pseudocysts and serous cystadenoma), surveillance or drainage are sufficient. Therefore as the therapeutic approach to CLP is different, a pre-operative diagnosis is essential.

There are three types of mucinous cystic lesions of the pancreas: benign (MC), malignant (CK) and intraductal papillary mucinous tumour of the pancreas (IPMT) also referred to as mucinous ectasia of pancreatic ducts. These lesions share many common features: the same embryonal origin (ductal), histological type (mucin-secreting cells) and immunohistological characteristics (sialomucin/neutral mucin ratio) as well as high risk of malignant transformation. In practice, IPMT can be distinguished from CM and CK by their clinical symptoms, imaging methods, proliferation type (endo- vs extraductal), and it is important to note that the treatments of these affections are substantially different.

MUCINOUS CYSTADENOMAS

Histology

Macroscopically, MC present as a large (> 2 cm), unifocal (or paucifocal) tumour. The wall of the lesion may contain microcysts or nodules. It is coated by a cylinder-shaped epithelium secreting acid mucins (sulfomucins, sialomucins and neutral mucins). It may assume a true papillary aspect of several stratified layers. On the outside, the epithelium is fringed with a dense chorion reminiscent of ovarian stroma. These characteristic aspects are commonly seen in female patients but may also be encountered in males.
We shall not discuss the borderline forms of MC with dysplasia.
Clinical symptoms

MC is a benign tumour which occurs predominantly in women (90% of all cases). The patients are usually in their 50-ies. The presenting symptom in 60 to 80% is abdominal pain, which may radiate to the back, suggesting a pancreatic origin. Abdominal pain may however be vague and chronic in nature dating from months to even even years duration. Weight loss is also occasionally seen and in 10% of cases, an abdominal mass is palpable. Episodes of acute pancreatitis are rare. This results from either compression of the main pancreatic duct (MPC) or from a mucus-secreting fistula of the MC. The same mechanisms can lead to jaundice (less than 5% of patients). Fistulae can also occur between the MC and stomach, colon or the peritoneum. These complications appear to be associated with a discontinuity in the epithelium of MC, allowing extravasation of the intra-cyst fluid and the emergence of peri-cystic inflammation (occasionally symptomatic), resulting in the formation of a fistula.

In several patients, we have observed a completely benign form of MC with important local inflammatory changes leading to an erroneous diagnosis of locally advanced CK (unpublished data).

It is also important to state that advances in imaging techniques has resulted in an increase in the incidental diagnosis of MC. A survey by the Association Française de Chirurgie has shown that out of 150 MC, 26% of patients were asymptomatic.
Imaging of mucinous cystadenomas
In cases of suspected CLP abdominal ulmtrasound and CT scan (with and without iodine-based contrast molecule) should be performed as first line investigations.

ULTRASOUND EXAMINATION
CLP are found more commonly in the body or tail of the pancreas (75%), than in the head or cephalic region (25%). Ultrasound examination typically reveals a unique, anechogenic, uni-or paucilocular lesion, with well defined borders. The mean diameter is 6 cm but can be as much as 20 cm.
Small (< 2 cm) MC are sometimes encountered, and can pose major problems in diagnosis. In such cases, fine needle aspiration and analysis of the cystic fluid can often be positively contributive.

TOMODENSITOMETRY
CT scan without contrast shows an isolated hypodense lesion, which is round or oval in shape, with distinct borders. If several lesions are visible, the diagnosis of MC should be reconsidered. We have found that such cases are more likely dilated ducts secondary to IPMT or as a result of congenital polycystic disease. Peripheral rim calcifications are seen in 10 to 15% of cases. With contrast medium, the wall of the cystadenoma is clearly enhanced, and partitions separating the cysts of a multilocular MC can be sometimes visualized. The presence of endocystic vegetations or irregular thickness of the wall (or mural vegetations) of MC is suggestive of malignant transformation. The presence invasion of neighbouring organs (stomach, splenic vessels), peritumoral lymph nodes or distant metastases is indicicative of a diagnosis of CK.
In a study by Vilgrain et al, radiological analysis using CT scan in 134 patients with CPL resulted in a correct diagnosis of MC in 70% of cases. CT scan-guided fine needle aspiration of CLP can also be performed depending on the size of thelesion and its anatomical position (ie. > 2 cm with no intervening vascular or digestive structures).
ENDOSCOPIC ULTRASONOGRAPHY
This examination is usually performed as a second line and is preferable under general anaesthesia. Interpretation of the images depends largely on the experience of the operator in the field of pancreatic pathology. Endoscopic ultrasonography (EUS) has been found to be a sensitive investigation in the differential diagnosis of CLP. Using EUS, MC are shown to be well defined isolated lesions. Small mural cysts are sometimes visible and should not be erronously mistaken for serous cystadenomas. Cystic contents are usually hyperechogenic due to the presence of mucus. A careful examination of the cyst with respect to the pancreatic duct needs to be undertaken, particularly in cases where IPMT are suspected. In a previous study, we found that MC was correctly diagnosed in 60% of cases using EUS, and that this figure significantly increased to 86% following re-examination of the recorded data by an experienced investigator. In MC, EUS is also useful in defining papillary projections and mural nodules or irregularities, features suggestive of malignant transformation.
The recent development of EUS-guided fine needle aspiration in pancreatic disease plays an increasingly important role in the diagnosis and management of CLP. In 9 out of 10 cases, EUS-FNA allows sampling of a sufficient volume of the cystic fluid for analysis of pancreatic enzymes (amylase, lipase) and tumour markers (CEA, Ca 19.9, Ca 72.4 and M1 mucins). Occasionnally, a tissue sample of the cystic wall is obtained and this along with the cystic fluid can be examined using histology and cytology respectively. Complications of EUS-guided fine needle aspiration are rare. In contrast with the experience of Wiersema et al., who reported frequent complications using EUS-FNA in CLP in a small number of patients (n = 22), we have not observed any serious incident (infection, hemorrhage or acute pancreatitis) in more than 100 patients having undergone this intervention.

RETROGRAD CHOLANGIOPANCREATOGRAPHY OF THE MAIN PANCREATIC DUCT (MPC)
As MC rarely communicates with the MPC, ERCP is not an important adjunct in its diagnosis. Currently, only 27 cases of such communicating MC have been reported. Gazelle et al. demonstrated the presence of ductal stenosis in 60% of malignant CLP but in only 4% of benign lesions. As outlined below, the interest of this examination rests essentially in the diagnosis of IPMT.

OTHER IMAGING METHODS

Magnetic resonance imaging is only of modest importance in the diagnosis of MC. Signal intensity on T1-weighted images depends on the mucinous contents of the lesion. T2-weighted images yield a strong signal.

Coeliac axic and mesenteric arteriography may demonstrate peri-cystic hypervascularisation but in practice is not needed for the diagnosis of benign MC.
ANALYSIS OF THE CYST FLUID
Imaging examinations fail to give an exact diagnosis of MC in approximately 30% of cases. For this reason, several groups including ourselves, have developed methods of analysis of the fluid obtained using fine needle aspiration. There is some debate in the literature as to the value of analysing biochemical and tumour markers of cystic fluid in the differential diagnosis of CLP.

Most published data however include studies involving only small numbers of CLP. We however performed a prospective evaluation on a large number of CLP for which a definitive diagnosis has been established using histology or a reliable imaging method.

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