Malignant Potential of Intraductal Papillary Mucinous Neoplasms of the Pancreas, Especially Minimally Invasive Carcinoma

Purpose: To elucidate the malignant potential of intraductal papillary mucinous neoplasms (IPMNs) and the biological behavior of minimally invasive carcinoma. Methods: Seventy patients with 71 IPMNs were operated on in our department. There were 46 benign IPMNs (intraductal papillary mucinous adenoma; IPMA, low and intermediate grade dysplasia) and 25 malignant IPMNs (intraductal papillary mucinous carcinoma; IPMC, high grade dysplasia and invasive carcinoma). We classified IPMNs by the diameter of the main pancreatic duct (MPD) and the depth of stromal invasion, evaluating them in terms of malignant potential and prognosis. Results: There were 16 IPMNs with an MPD diameter of ≥10 mm, 24 with an MPD diameter of 5–9 mm, and 31 had an MPD diameter of ≤5 mm. There were 7 IPMCs with an MPD diameter of ≥10 mm (malignant rate: 43.7%), those with an MPD diameter of 5–9 mm were 9 (37.5%), and those with an MPD diameter of ≤5 mm were 9 (29.0%). Among the malignant predictive factors examined, only the size of mural nodules was significant (P0.5 cm to ≤1.0 cm), 5 T1c IPMCs (>1 cm to ≤2 cm), and 3 T2 IPMCs (>2 cm). A total of 132 patients with pancreatic invasive ductal carcinoma (IDC) were operated on in our department from 1996. Compared with patients with IDC, patients with T1a or earlier IPMCs (5 year survival rate: 100%) showed significantly better prognosis than those with T1b or more advanced IPMCs (38.5%, P=0.013) and T1 IDC (41.6%, P=0.03). Conclusions: These results suggested that IPMNs with an MPD of ≥10 mm and/or the existence of the mural nodules were recommended for surgical resection. The patients with T1a or less IPMCs showed favorable prognosis, therefore they were suggested to be early cancers or early IPMCs.