Most common neoplastic mucinous cystic lesions in the pancreas are MCN and IPMN, both of which have malignant potential.[1
]. Therefore, the clinical management of MCN and main duct IPMN tends to be aggressive and usually involves surgery [1
]. However, studies have found that branch duct IPMN especially small ones (<3cm) tends to have a much lower risk of malignant transformation and may be managed by nonoperative surveillance[15
],. The molecular basis for the different behavior of main and branch duct IPMN is not clear.
A recently described mucinous nonneoplastic cyst, not a well recognized entity, shares many clinical and radiological features with MCN and IPMN; and it also raises a question if all or some of branch duct IPMN is a non-neoplastic process. Recognition of this lesion is clinically important since the management and prognosis are different from other neoplastic mucinous lesions. In this study, we further characterized MNC using morphological, immunohistochemical and biochemical parameters using 15 cases.
Terminology of MNC is another concern. As the nature of this MNC lesion is mucin productive and cystic lesion with non-neoplastic feature, it should be called mucinous non-neoplastic cyst (MNC). There is another entity in the literature called retention cyst that may share some of similarity with MNC; but retention cyst may also cover some nonmucinous cysts.
The incidence of MNC at our institution in 436 resected specimens was 3.4%. This is slightly higher than Kosmahl’s report (2.1%) [3
], probably due to different patient population in different medical centers. Like Kosmahl and others, we also found MNC in both men and women with a slightly preponderance in women (F: M =4:1). In our study, MNC were found in wide age groups (22 to 73 years) but tended to be more common in patients older than 50 years (87%, 13/15). MNC are randomly distributed in the pancreas and may present as a unilocular or multilocular cyst. Most patients (73%) presented with single lesion; 27% presented with multiple cysts. The size of the cysts was usually small (0.5 to 3.5cm). The clinical presentations of MNC patients were nonspecific and include abdominal discomfort, pain, loss of appetite and increased frequency of urination. Radiologically, MNC did not communicate with the main or branch pancreatic duct, but could not be distinguished from other cystic lesion like MCN.
EUS-FNA evaluation of pancreatic cystic fluid has evolved as the initial diagnostic approach in all cystic lesions of the pancreas [1
,18-22]. In our study, 10 of 15 cases had EUS-FNA and all showed extracellular mucin or mucinous epithelium confirming mucinous nature of these cysts. The cytomorphologic features of MNC are usually bland compared with IPMN which commonly show sheets of mucinous epithelium and papillary structures. Rarely, mild atypia or small sheets of mucinous epithelium may present in MNC fluid and make it difficult to distinguish from other neoplastic mucinous cystic lesions especially benign MCN and IPMN.
Cyst fluid analysis for pancreatic enzymes and different tumor markers has been suggested as a diagnostic tool in the differentiation of different cystic lesions of the pancreas [1
,23-25]. Although a tremendous variability has been reported in the levels of these markers in neoplastic and nonneoplastic cysts, certain cutoffs are believed to provide high specificity. For example, amylase < 250 ng/ml and CEA > 800 ng/ml exclude a pseudocyst, and CEA < 5 ng/ml and CA19-9 < 37 excludes a mucinous cyst [1
]. Increased cystic fluid CEA levels have been found in IPMN and MCN [1
,23-25]. Recent studies have suggested extracellular mucin and fluid CEA levels greater than 300ng/mL are considered very sensitive and specific for neoplastic mucinous cyst . However, studies have also found that cyst fluid CEA level can not differentiate between malignant and premalignant cyst, therefore is not predictive for malignancy [1
]. In our study, cyst fluid CEA levels measured in 10 cases were all higher than 500 ng/ml, in which 5 cases were higher than 3000ng/ml. Our data further confirm the association of CEA with mucinous differentiation. However, the occasional high CEA level observed in MNC suggested that CEA level alone is not a reliable marker for neoplasia or malignancy. Using immunohistochemistry, Brunner et al [4
] have demonstrated CEA staining in MNC. But CEA production in MNC is not associated with cytologic atypia or grading of the lesions.
Histological features of MNC were similar in our cases to other reported cases. Unilocular and some multilocular MNC were lined by a single layer of cuboidal to columnar mucinous epithelium; whereas in four multilocular and one unilocular MNC focal papillary features and multilayered epithelium were seen. No cytological atypia, pleomorphism or mitotic activity was seen. Cysts typically were surrounded by sparse fibrous tissue without any specific type of stroma. Interestingly, in the majority of our MNC (80%), the adjacent pancreatic tissue showed acinar ductal mucinous metaplasia, which is a metaplastic lesion commonly seen in chronic pancreatitis and normal pancreatic tissue adjacent to ductal adenocarcinoma. Previously, we have shown that acinar ductal mucionous metaplasia of pancreas arose from centroacinar cells (CAC, pancreatic stem/progenitor cells). The close association of MNC with acinar ductal mucinous metaplasia suggests that MNC may develop from and share the same pathway as acinar ductal mucinous metaplasia.
In previous studies of MNC, expression of MUC1 and MUC5AC was reported in 1/6 and 5/6 cases, respectively. Expression of MUC2 was not observed [4
]. In this study, focal and weak MUC1 positivity in 27% cases; and moderate to strong MUC5AC expression in 67% cases was observed. None of our cases was positive for MUC2. This apomucin profile was also very similar to the adjacent acinar-ductal mucinous metaplasia, further suggesting the possible origin of MNC from acinar ductal mucinous metaplasia.
For the first time we confirmed the nonneoplastic nature of the MNC using HUMARA clonality analysis. Kosmahl et al proposed that these cystic lesions are nonneoplastic based on morphological features and lack of recurrence or malignant transformation in 2 years median follow-up [5
]. Benign IPMN and MCN can show very bland cytology with no mitosis. And the recurrence of the benign IPMN and MCN usually depends on surgical resection margins and if the lesion was completely excised, it may not recur in up to 5 years [1
]. Our clonality assay indicated the polyclonal origin of the cystic epithelial cells, providing a strong experimental evidence of their nonneoplastic nature.
It is important to distinguish this lesion from MNC, MCN and IPMN for proper clinical management. Main duct IPMN are easy to separate from MNC or MCN by radiological studies. The MCN exclusively occurred in female patients, commonly in the pancreatic tail and, importantly, show ovarian type stroma, making it relatively easy to be distinguished from MNC. Distinction between MNC and benign branch duct IPMN may be extremely difficult due to the overlapping clinicopathological features. We compared the mucin profile of MNC with main and branch-duct IPMN, and found the majority of IPMN (100% main duct, 58% branch duct) were MUC2 positive, whereas all MNC were MUC2 negative. Therefore, MUC2 may be used as a biological marker in cases when differentiation of MNC and IPMN is difficult. On the other hand, for branch duct IPMN there were 42% cases showing no MUC2 expression. Thus, it raises the possibility of nonneoplastic nature of these lesions and this remains to be investigated further.
In summary, our results indicate that MNC are rare cystic lesions in the pancreas, commonly associated with adjacent acinar-ductal mucinous metaplasia and share similar mucin profile, suggesting the possible origin from acinar-ductal mucinous metaplasia. The polyclonal nature demonstrated by clonality analysis further supports our hypothesis that these lesions are non-neoplastic. Furthermore, MUC2 may serve as unique biomarker to differentiate MNC from branched duct IPMN.