Objectives: In branch duct intraductal papillary mucinous neoplasm (IPMN) of the pancreas, the importance of the cyst size to predict malignancy is still controversial. Our aim was to elucidate the malignant potential of branch duct IPMN without mural nodules (flat branch duct IPMN). Methods: Seventy-three patients with flat branch duct IPMNs were studied in our institution. Results: There were 6 malignant IPMNs in this series, all of which were 30 mm or more in size, whereas there was no malignancy in IPMNs of less than 30 mm. Statistically significant predictors of malignancy were atypical cytological condition and main pancreatic duct (MPD) diameter of 5 mm or more. The cyst size of 30 mm or more tended to be associated with malignancy. The frequency of malignancy in flat branch duct IPMNs with the size of 30 mm or more and MPD diameter of less than 5 mm was 3.6%, whereas there were 5 malignant cases (26.3%) in flat branch duct IPMNs with the size of 30 mm or more and MPD diameter of 5 mm or more. Conclusions: We conclude that the size criteria (30 mm) to predict malignancy proposed in the international consensus guidelines is appropriate and resection or meticulous follow-up using cytological examination and MPD dilatation is needed in patients with flat branch duct IPMNs.
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