Annular pancreas is a rare congenital anomaly, which consists of a ring of pancreatic tissue, confluent with the head of the pancreas, and partially or completely encircling the second segment of the duodenum. It was first described by Tiedemann in 1818[ ¹] and first named “pancreas anulare “ by Ecker in 1862[ ²].

Studies based on cases where the diagnosis was made on autopsy or during surgery estimate the incidence of annular pancreas of 3 in 20 000[ ^{3 , 4}]. When the diagnosis is based on modern imaging methods, such as multislice computed tomography (MSCT), magnetic resonance imaging (MRI), magnetic resonance cholangiopancreatography (MRCP) or endoscopic retrograde cholangiopancreatography (ERCP), the incidence seems to be higher; up to 1 in 250[ ^{5 , 6}]. About 700 cases of annular pancreas have been reported in the literature[ ⁷]. During normal organogenesis, the pancreas is formed by merging the ventral with the dorsal pancreatic bud during the first four to nine weeks of gestation. Two main theories, sustained by Lecco[ ⁸] and Badwin[ ⁹] respectively, describe the pathogenesis of annular pancreas. Lecco[ ⁸] explained the development of the anomaly as an adherence of the ventral pancreatic bud to the duodenum, which leads to an improper rotation of the ventral bud, resulting in the encirclement of the duodenum. Badwin[ ⁹] contends that the persistence and enlargement of the left ventral bud causes the pancreatic obstructing ring. Although the pancreatic band usually adheres to the muscularis of the duodenum, it may also be loosely applied to the serosa of the duodenum[ ¹⁰]. The annular pancreas has been associated with other congenital anomalies, such as trisomy 21, tracheoesophageal fistula, cardiac anomalies, and intestinal malrotation[ ⁷]. Association of annular pancreas with malignancy is rare. It has been described only 14 times in the English literature (five pancreatic carcinomas, six ampullary carcinomas, two cholangiocarcinomas, and one Insulinoma)[ ^{11 - 22}]. A review of 151 cases of annular pancreas in Japan by Ogawa et al[ ¹¹] revealed an association with malignant tumors of the duodenopancreatobiliary system in 15 patients (five cholangiocarcinomas, four gallbladder carcinomas, four duodenal, and two pancreatic carcinomas). Primary carcinoma of the duodenum is rare, representing about 0.3% of all malignant neoplasms of the gastrointestinal tract and 25%-45% of malignant neoplasms of the small intestine[ ²³].

A 55-year-old Caucasian woman was admitted to our hospital with a six-week history of persistent postprandial nausea and vomiting, associated with a weight loss of 7 kg compared to a total weight of 70 kg. There was no associated pain, hematemesis, fever, diarrhoea, or jaundice. Domperidone therapy was ineffectual. Four years prior to the diagnosis, the patient had intermittent occasional mild epigastric pain. She also reported occasional vomiting during the previous 15 years, which was not medically investigated.

On clinical examination, the patient appeared weak and dehydrated. Her abdomen was soft, without tenderness or palpable masses. Laboratory results revealed a hypokalemia of 1.9 mmol/L, which was substituted, and a metabolic alkalosis with an initial pH of 7.65. Serum amylase, bilirubin, and C-reactive protein (CRP) levels were not elevated.

Gastroscopy showed a reflux esophagitis, a massive distension of the stomach and the first part of the duodenum, respectively, and a stenosis of the second part of the duodenum, which could not be passed with the thin gastroscope.

Abdominal MSCT depicted a fluid filled stomach and second segment of the duodenum (Figure 1). The pancreatic head was enlarged, surrounding the second segment of the duodenum; no dilation of the main pancreatic duct and common bile duct was noticed. For further evaluation, an MRCP was done. This showed an aberrant pancreatic duct encircling the duodenum, linked to the main pancreatic duct (Figure 2). No dilation of the common hepatic bile duct or the main pancreatic duct was evident. Due to the annular pancreas, there was an obstruction at the level of the second segment of the duodenum. During surgery, we found a massively distended and elongated first segment with a conic stenosis of the second segment of the duodenum due to the annular pancreas (Figure 3). A duodenojejunostomy was performed. There was no visible prestenotic tumor in the duodenum and the duodenal mucosa appeared to be normal. No tumor mass was palpated. The patient had an uneventful recovery and was discharged home with no complaints nine days later. After eight weeks, the patient was readmitted presenting with painless jaundice. She had no other complaints. No further weight loss was reported. Laboratory investigations showed elevated total bilirubin (112 μmol/L; reference range: 5-18) and alkaline phosphatases (178 U/L; reference range: < 104). An MRCP demonstrated dilatation of the common bile duct (Figure 4). On explorative laparotomy, a hard mass was palpated in the pancreatic head region. The frozen section of one of several suspicious superior pancreatic lymph nodes revealed adenocarcinoma cells. A duodenopancreatectomy was performed. The tumor was clearly located in the duodenum, with infiltration of the surrounding structures (Figure 5). No tumor was observed in the head of the pancreas. Pathological examination showed a poorly differentiated, infiltrating adenocarcinoma of the duodenum, surrounded by the incomplete annular pancreas (Figure 5). The tumor stage was pT4, pN1 (8/12), G3, L0, V0, R0. The tumor seemed to have arisen from the duodenal epithelium.