This case of pancreatic cancer with a moderate-sized cyst developing into an extrapancreatic growth is extremely rare and has not previously been reported. In pancreatic ductal carcinoma, retention cysts may occur secondary to intraluminal obstruction of the pancreatic ducts. Nitta et al. [1] described a series of cystic pancreatic ductal adenocarcinomas that had distinctive morphologic and immunohistochemical features. In that report, the maximal cyst diameter was 2.0 cm or more in all patients (range, 2.0–5.0 cm; mean, 3.7 cm). The average tumor size including the multiple large cystic structures was 6.0 cm (range, 4.0–8.0 cm) [1]. Cystic tumors of the pancreas occur with less frequency than solid lesions, and are often detected incidentally, as many of these lesions are small and asymptomatic [2]. Cystic lesions of the pancreas can be classified into three groups: pseudocysts, pancreatic cystic tumors, and true cysts. Pseudocysts occur following acute pancreatitis or acute exacerbation of chronic pancreatitis; pancreatic carcinoma; and abdominal trauma. Pancreatic cystic tumors consist of three types: serous tumors, mucinous tumors, and solid pseudopapillary tumors. For pancreatic cysts, the aim of imaging is to differentiate cystic tumors from tumor-like lesions, to further characterize the cystic tumors, and to distinguish benign tumors (which do not usually require surgical excision) from borderline or malignant tumors that must be resected whenever possible [3-7].

Kosmahl et al. [8] reported that a considerable number of pancreatic ductal adenocarcinomas and their variants display cystic features and must therefore be considered in the differential diagnosis of cystic neoplasms of the pancreas. Moreover, not all of the cystic structures they observed were neoplastic in nature. They may also represent nonneoplastic changes, such as retention cysts and inflammatory pseudocysts. Other report showed that most of the cystic pancreatic ductal adenocarcinomas reported in the literature were poorly differentiated tumors with pseudocystic changes [9-11]. In our case, the content of the cyst was nonmalignant and the cyst was a simple pseudocyst. We could not determine the histological cancer type because we could not obtain an adequate sample from the pancreatic cancer.

In our case, the patient had no history of pancreatitis. Of course, we suspected pancreatic ductal carcinoma with presence of a pancreatic cyst, but there was neither a mass lesion nor a low-density area suggestive of pancreatic cancer. We did not immediately suspect pancreatic cancer at first, as the development of a moderate-sized cyst with extrapancreatic growth is extremely rare and the levels of known tumor markers were not elevated. Therefore, we initially suspected that a massive benign cyst (MCN, SCN, or IPMN) resulted in stenosis of the main pancreatic duct. We were unable to reach a definitive diagnosis prior to the operation. We planned a pancreaticoduodenectomy to reach a definitive diagnosis; however, we could not remove the tumor because of significant invasion of the surrounding tissue. The fluid content of the cyst was serous and aspiration cytology from the pancreatic cyst was deemed to be Class III (no malignancy), but the surrounding white connective tissue samples were found to be positive for pancreatic adenocarcinoma on pathological examination during surgery. In this case, the content of the cyst was not malignant and the cyst was a simple pseudocyst. We repeated imaging (CT, MRI, endoscopic ultrasound, etc.) once more after the operation, and there were no mass lesions or low-density area suggestive of pancreatic cancer. In retrospect, we think that the slight pancreatic duct dilation was the only finding suggestive of pancreatic cancer.

If a pancreatic cyst is found in patients who have normal tumor marker levels and no history of pancreatitis, we should always consider the possibility of pancreatic cancer. Slight pancreatic duct dilation may be a diagnostic clue in these cases. In the end, we could not resect the tumor because it was too advanced. However, regardless of whether the tumor is resectable, we need to identify these cases because they strongly influence disease prognosis.

In the present case, we could not conduct an in-depth pathological examination for pancreatic cancer because we could not obtain an adequate sample from the pancreatic cancer. However, we had the impression that this tumor was more invasive than the usual invasive ductal carcinoma.

Despite the development and widespread use of imaging techniques, it remains difficult to preoperatively establish an accurate diagnosis of pancreatic cancer that is accompanied by moderate-sized pancreatic cystic lesions. An accurate diagnosis requires careful consideration of the complete medical history of the patient in addition to the appropriate use of imaging and endoscopic approaches. It is important to carefully observe changes of the pancreatic parenchyma around massive cysts, regardless of their size. If a pancreatic cyst with slight pancreatic duct dilation is found in patients who have normal tumor marker levels or no history of pancreatitis and changes of the pancreatic parenchyma that are suggestive of pancreatic cancer are not observed, we should always consider the possibility of pancreatic cancer. Slight pancreatic duct dilation may be a diagnostic clue in these cases.