Eleven cases with mucinous noncystic carcinoma (MC) of the pancreas were studied by histology and mucin immunohistochemistry, to elucidate the mechanism, or route of development, and pre-existing histological type of MC of the pancreas. These MCs were observed in close approximation to, or surrounding, intraductal papillary-mucinous carcinomas (IPMCs), and were centrally situated among ductal adenocarcinomas (DAs). Hence, the 11 cases originated from 8 IPMCs and 3 DAs. The mechanism and routes to MC were divided into four types as follows: IPMC directly invaded the stroma (4 cases), over-production of mucin in IPMC expanded the branches of the pancreatic duct possibly resulting in rupture (3 cases), DA underwent extreme mucinous degeneration (3 cases), and a recurrent form, as MC, at the surgical stump of IPMC (one case). The outcomes of MC cases with IPMC had variable survival rates, while those from DA had short durations. MUC immunoreactivity in MC was divided into three categories; anti-MUC1-positive only (2 IPMCs, 2 DAs), mixed anti-MUC1 and anti-UC2-positive (3 IPMCs, one DA) and anti-MUC-positive only (3 IPMCs). Pre-existing MC histological types included both IPMC and DA. These two pre-existing types of MC involved mucin overproduction and mucinous degeneration. MUC immunoreactivity in MC revealed three patterns, which may be related to variable outcomes.
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