Heterotopic pancreas (HP) is a congenital anomaly defined as pancreatic tissue that has no contact with the orthotopic pancreas and its own duct system and vascular supply. The most common locations of HP are the upper gastrointestinal tract, specifically, the stomach, duodenum, and proximal jejunum. Involvement of the mesentery is rare. Here, we describe a rare case of mesenteric heterotopic pancreas (MHP) in a 12-year-old girl who presented with acute abdomen. The patient underwent emergency laparotomy, and the mass and adjacent small bowel were resected. Results of the postoperative histopathologic examination confirmed the diagnosis of MHP. Observation of the patient for 12 mo postoperatively showed no evidence of recurrence. Preoperative diagnosis of HP is difficult, even in a symptomatic patient. Increased awareness and understanding of the image characteristics of MHP will aid in correct preoperative diagnosis and appropriate patient management.

Heterotopic pancreas (HP) is most commonly found in the proximal gastrointestinal tract. Although the reported incidence of HP varies, the true incidence is difficult to determine because the patients are usually asymptomatic and the condition is usually found incidentally at autopsy or during laparotomy[1,2]. Male preponderance is seen among adults, with the incidence of disease peaking during the fourth to sixth decades of life[3,4]. The most frequent location of heterotopic pancreatic tissue is the stomach (47%), followed by the jejunum (35%), duodenum (11.7%), and ileum (5.8%)[5]. Involvement of the mesentery is rare. Until now, no more than ten cases of mesenteric heterotopic pancreas (MHP) have been described in the medical literature[6-13]. Here, we describe a case of MHP in a 12-year-old female patient.

A 12-year-old girl with no significant medical history was admitted to our department with intermittent vomiting and abdominal pain for 3 d. Her growth and development were normal. On admission, she was in a good general condition. Her temperature was 36.6 °C, heart rate was 106 beats/min, and the blood pressure was 100/69 mmHg. Her physical examination showed abdominal tenderness with peritoneal irritation. The laboratory results were unremarkable. Abdominal ultrasonography revealed a well-defined, heterogeneous, medially echoic mass located at the margin of the intestinal mesentery in the abdominal cavity that measured approximately 4.9 cm × 2.6 cm. Contrast-enhanced computed tomography (CECT) of the abdomen showed an enhanced oval, soft-tissue mass (42 mm × 25 mm) in the mesentery at the level of the umbilicus (Figure 1). The clinical diagnosis of a mesenteric mass was made. The differential diagnosis included intestinal duplication cyst, inflammated Meckel’s diverticulum, and mesenteric lymphangioma.

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Figure 1 Contrast-enhanced computed tomography images of the abdomen. A: Axial contrast-enhanced computed tomography (CECT) image of the abdomen showing an enhanced oval, soft-tissue mass in the jejunal mesentery at the level of the umbilicus (black arrow); B: Coronal CECT image showing that the mass had its own blood supply (white arrow).

Because of peritoneal irritation, the patient underwent emergency laparotomy 12 h after hospitalization. Laparotomy revealed a yellowish, soft-tissue mass measuring 4 cm in diameter that was located in the mesentery of the proximal jejunum and adhered to the serosal surface of the jejunum (Figure 2). The mass and the adjacent jejunum were resected, and an end-to-end anastomosis was performed.

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Figure 2 Photograph of the resected mass and the adjacent small bowel. Photograph of the gross specimen demonstrates a 4 cm × 3 cm, yellowish, soft-tissue mass (black arrows) located in the jejunal mesentery and adhered to the serosal surface of the jejunum (white asterisks).

Gross pathology demonstrated a 4 cm × 3 cm mass in the jejunal mesentery that was adhered to the serosa of the jejunum. Postoperative histopathologic examination of the resected specimen revealed ectopic pancreatic tissue consisting of acini, islet cells, and pancreatic ducts, adjacent to the jejunal serosa (Figure 3). There was no evidence of malignant change in the ectopic pancreatic tissue. The pathologic diagnosis was MHP.

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Figure 3 Histopathologic examination of the resected specimen. Microscopic appearance showing that the lesion consisted of heterotopic pancreatic tissue (yellow arrows), including acini, islet cells, and pancreatic ducts, extending to the jejunal serosa (H and E staining; A: Magnification, × 100, scale bar = 200 μm; B: Magnification, × 200, scale bar = 100 μm).

The patient’s postoperative course was uneventful, and she was discharged on postoperative day 8. Follow-up of the patient by abdominal ultrasonography for 12 mo postoperatively showed no evidence of relapse.

HP was first described by Jean Schultz in 1727, which is a congenital anomaly defined as pancreatic tissue that is anatomically separate from the main gland, without vascular or ductal continuity. No more than ten cases of MHP have been described in the medical literature[6-12]. Including our case, only four cases of pediatric MHP have been reported in the medical literature[6,7,9], with patients ranging in age from 12 to 15 years.

The embryologic basis of HP is controversial, but the most widely held theory is the misplacement theory (Figure 4), according to which, deposits of pancreatic tissue are “dropped” into the developing gastrointestinal system, in anatomic isolation from the main body of the pancreas[11,13]. This theory accounts for the fact that heterotopic pancreatic tissue was mostly located in the upper gastrointestinal tract near the pancreas (derivatives of the primitive foregut).

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Figure 4 Schematic diagram of the “misplacement theory”. A, B: The pancreas develops from the ventral and dorsal pancreatic buds, which develop at the junction of the foregut and midgut during the 4^th week of gestation; C: As the foregut elongates, the developing ventral pancreas, gallbladder, and bile duct rotate clockwise posterior to the duodenum and join the dorsal pancreas in the retroperitoneum. The ventral pancreatic bud rotates clockwise and fuses with the dorsal bud at the 7^th week of gestation; D: According to the misplacement theory, deposits of pancreatic tissue are “dropped” into the developing gastrointestinal system during rotation of the foregut when fragments of pancreas become separated and develop into mature elements. Yellow points in D indicate possible locations of heterotopic pancreas. F: Foregut; M: Midgut; VPB: Ventral pancreatic bud; DPB: Dorsal pancreatic bud; LB: Liver bud; S: Stomach; D: Duodenum; P: Pancreas; L: Liver; J: Jejunum.

HP is well differentiated and cannot be distinguished histologically from orthotopic pancreas[1]. Gross specimens always show a firm intramural mass that has a lobular shape and a well-defined interface with surrounding tissues[14]. Most lesions (80%) are solitary and smaller than 3 cm, but they can range in size from 0.2 cm to 5.0 cm[14,15]. The first and most common histological type of heterotopic pancreatic tissue is composed of all the elements of the normal pancreas, including the acini, ducts, and islet cells[16]. However, not all three of these components were identified in a single case. The morphologic feature of MHP closely resembles that of the orthotopic pancreas and manifests as a homogeneous, well-enhanced, and elongated mass with pancreas-like clefts or lobulations[17].

Uncomplicated HP is typically asymptomatic, which is always discovered incidentally during surgery, imaging examination, or autopsy. Any complication that can occur in the orthotopic pancreas can also occur in HP. Associated complications include pancreatitis, pseudocyst formation, abnormal hormone secretion, bowel obstruction, common bile duct obstruction, gastrointestinal bleeding, intussusception, and malignant degeneration[3,4,18-20]. Depending on its location and size, and involvement of the overlying mucosa, HP can cause symptoms. The most common findings were abdominal pain, abdominal distension, nausea and vomiting, malaise, anorexia, anemia, body weight loss, jaundice, and upper gastrointestinal bleeding according to the study by Zhang et al[1].

Because of its rarity and nonspecific clinical manifestation, accurate preoperative diagnosis of HP remains difficult[5,21]. The definitive diagnosis has been always achieved after postoperative pathology. CECT and magnetic resonance imaging (MRI) may demonstrate the lesion that enhances similarly to the orthotopic pancreas[22-26]. The most common CT manifestation of HP is a small intramural and endoluminal mass with microlobulated margins[22]. On an MRI scan, HP is isointense to the orthotopic pancreas, with characteristic T1-weighted high signal intensity and early avid enhancement after the administration of intravenous contrast material[17,22], which is particularly helpful for differentiating HP from other lesions. Heterotopic pancreatic tissue has a rudimentary ductal system. The existence of a central duct along the long axis of the mass is another key finding for the diagnosis of HP[8,17,22]. Sometimes the duct can be confirmed as draining into the fourth portion of the duodenum by magnetic resonance cholangiopancreatography[8]. Knowledge of the characteristic imaging appearance of MHP is key to confirming the diagnosis preoperatively. First, as with HP in other locations, MHP has morphologic and enhancement characteristics similar to the orthotopic pancreas, named “another pancreas in the mesentery”[17,22]. Second, the mean long axis diameter/short axis diameter ratio of MHP was much greater than that of gastric HP (3.0 vs 1.4-1.5)[17]. Third, a duct-like structure paralleling the long axis of MHP is seen more frequently than HP in other locations[17].

MHP should be considered in the differential diagnosis. A gastrointestinal stromal tumor, carcinoid tumor, lymphoma, and metastasis can manifest as a homogeneous and well-enhanced soft tissue mass located in the mesentery, and the enhancement pattern is unreliable to differentiate from MHP on an enhanced CT scan[17]. Because of the potential serious complications and malignant change, local excision of MHP is the optimal treatment[2,26].

We list the cases of MHP in medical literature (Table 1) and found some characteristics of MHP. First, MHP was not seen in children younger than 10 years old. Of the eight cases of MHP, 50% (4/8) were teenagers aged from 12 to 15 years old, 50% (4/8) were adults aged from 38 to 75 years old. Second, there is a female preponderance (75%, 6/8) of MHP, which is opposite to the male preponderance of HP. Third, the majority of MHP (75%, 6/8) were located in jejunal mesentery. Fourth, CECT and MRCP were the most useful diagnostic method for MHP. These characteristics can be applied in clinical diagnosis of MHP.

In conclusion, MHP is very rare and an unusual cause of acute abdomen in patients older than 12 years. Preoperative diagnosis of MHP is difficult, even in a symptomatic patient. When a mesenteric mass has morphology and enhancement similar to the orthotopic pancreas, MHP should be considered in the differential diagnosis. Increased awareness and understanding of the imaging characteristics of MHP will aid in correct preoperative diagnosis and appropriate patient management.