Morphological abnormalities of the pancreas include pancreatic divisum, annular pancreatic agenesis/hypogenesis of the body and tail of the pancreas, ectopic pancreas, protrusion of the lateral contour of the pancreatic head and pancreatic tail, intrapancreatic parasplenium, and splenic varices [1,2,3]. It is important to differentiate these from pancreatic tumors. Protrusion of the lateral contour of the pancreatic head includes a convex protrusion of the lateral margin of the pancreatic head and a beak-shaped protrusion of the uncinate process. Ross et al. reported that dual-phase helical CT was performed in 119 patients without pancreatic disease; of which, 41 patients (34.5%) had a tissue that protruded more than 1 cm laterally from the gastroduodenal artery or the anterior superior pancreaticoduodenal artery . Contour variants of the pancreatic head and neck are classified into three types according to the direction of protrusion based on the gastroduodenal artery or the anterior superior pancreaticoduodenal artery; the three types are as follows: anterior type (type 1), posterior type (type 2), and horizontal type (type 3). On the other hand, Chandra et al. found that 12 of 21 patients (57%) with intestinal malrotation had an abnormal pancreatic head morphology, which could be classified into three categories: globular-shaped (24%), elongated (24%), and combined globular and elongated type (10%) . Since the pancreatic development is closely related to the development of the intestinal tract, it has been reported that pancreatic morphological abnormality may be more common in patients with abnormal intestinal rotation. The cause of the protrusion of the lateral contour of the pancreatic head is considered to be related to the pattern of fusion of the ventral and dorsal pancreatic primordium. The dorsal and ventral pancreatic primordia arise from the duodenum at 4 weeks of fetal life . At 5 weeks of fetal life, when the duodenal primordium rotates in a clockwise direction, the ventral pancreatic primordium rotates clockwise with the primitive common bile duct, and migrates below the dorsal pancreatic primordium. At 6 weeks of fetal life, the dorsal and ventral pancreatic primordium begin to fuse. At 7 weeks of fetal life, the fusion is completed to form the ventral pancreatic duct, and the proximal part of the dorsal pancreatic duct becomes the peripancreatic duct. If there are doubts regarding the diagnosis, it is essential to confirm the contrast pattern of the normal pancreatic parenchyma by multiphase, thin-slice, contrast-enhanced CT to accurately differentiate morphological abnormalities from neoplastic lesions . Aysel et al. reported that magnetic resonance cholangiopancreatography (MRCP) can capture abnormalities in the pancreatic duct travel and is useful in diagnosing pancreatic morphological abnormalities . If a non-pedunculated broad continuity could be confirmed by imaging studies including EUS, it would have been possible to diagnose pancreatic morphological abnormalities preoperatively. It is possible that only the uniaxial direction was visible on the EUS images. Although the most common type of protrusion of the lateral border of the pancreas is the posterior type (type 2), this case is considered the anterior type (type 1). The ectopic pancreas, which is listed as a differential diagnosis, refers to the pancreatic tissue existing in other organs without continuity with the original pancreas. The ectopic pancreas is found in 0.11–21% of autopsy cases and is preferentially located in the stomach, duodenum, and air conditioning close to the pancreatic base . In the stomach, 88% of cases are reported to be located in the pyloric vestibule and 12% in the body . When they occur in the lumen of the gastrointestinal tract, they may enlarge and cause obstruction or bleeding, resulting in abdominal discomfort and epigastric pain. Although ultrasound endoscopy and ultrasound endoscopic aspiration cytology are used for diagnosing ectopic pancreas, the positive diagnostic rate of ectopic pancreas by EUS-fine needle aspiration is not necessarily high (50–80%) [10,11,12]. Adenocarcinoma in ectopic pancreas is rare (0.7–1.8%) [13, 14] and most of the cases are asymptomatic and can be followed up. Still, surgery is considered for symptomatic patients. A definitive diagnosis cannot be made, and it is necessary to differentiate between gastrointestinal stromal tumor, cancer, insulinoma, and cases in which there is a tendency for the ectopic pancreas to increase in size . In this case, the indication for surgery was appropriate owing to the mild but recurring pancreatitis. However, since there was a discrepancy in the thickness of the continuity between the tumor and the pancreatic head in the preoperative EUS and intraoperative findings, it might have been better to consider biopsy. Moreover, we did not rule out malignancy by biopsy before the surgery; thus, it may have been necessary to perform positron emission tomography–CT.
A search on PubMed using the keywords "pancreas" and "protrusion of lateral contour" revealed no case reports, suggesting that protrusion of the lateral border of the pancreatic head in the anterior type (type 1) is an extremely rare condition, and there is no description of its relationship with pancreatitis. Therefore, the apparent cause of pancreatitis in this case is unknown. Intraoperative findings revealed grossly visible changes that suggested only mild inflammation at the margins of the pancreatic tissue extending into the duodenum. If the pancreatic morphology abnormality is a matter of fusion between the dorsal and ventral pancreatic primordium, the pathology may resemble pancreatitis associated with pancreas divisum . In other words, in pancreas divisum, in which the dorsal and ventral pancreatic ducts perform their excretory functions independently, pancreatitis may develop due to the relationship between the size of the opening and the amount of pancreatic juice to be drained. In general, the opening of the dorsal pancreatic duct into the accessory papillae as a main pancreatic duct is often a problem, and it is considered that such an abnormal running of the pancreatic duct may have been related to the pathogenesis in this case as well. MRCP or Endoscopic retrograde cholangiopancreatography would have been necessary to obtain this definitive diagnosis. As for the choice of surgical procedure, partial resection is considered the first choice when operating on an ectopic pancreas. In this case, we preoperatively judged that partial resection would be difficult because of a broad extension of the tumor from the anterior gastric side of the gastric pylorus to the duodenal bulb and planned to perform a distal gastrectomy including the duodenum where the mass was located. In addition, the possibility of a pancreatic morphological abnormality was also considered because preoperative EUS suspected the continuity of a 5 mm-diameter mass with the pancreatic head. In case of abnormal pancreatic morphology, if the diameter was small and could be resected safely, distal gastrectomy with resection of the pancreatic parenchyma could be performed. Based on the intraoperative findings, it was considered that the present case was not an ectopic pancreas but a thick-diameter protrusion of the lateral contour of the pancreas and that the risk of resection and invasiveness was high compared to the past episodes of mild pancreatitis. We therefore decided to terminate the surgery with observation only. In cases of pancreatic tumors or ectopic pancreas, which are difficult to diagnose in preoperative examinations, it is necessary to determine a treatment plan keeping in mind pancreatic morphological abnormalities such as protrusion of the lateral contour of the pancreatic head.