CT differentiation of gastric ectopic pancreas from gastric stromal tumor

Ectopic pancreas, which was first described by Klob in 1859, is a rare developmental anomaly that is defined as pancreatic tissue lacking anatomic or vascular continuity with the main body of the gland [1‐3]. It can occur anywhere along distal end of the esophagus to the colon, and of the gastrointestinal lesions, stomach (30%) is the most common area, followed by duodenum (25%) and jejunum (15%) [3]. Ectopic pancreas is usually discovered incidentally during surgery or autopsy and is generally asymptomatic. However, some patients with ectopic pancreas may have clinical symptoms such as abdominal pain, gastrointestinal bleeding, and obstruction when complicated with pancreatitis, pseudocyst, insulinoma, and pancreatic cancer [2, 4‐7].

Although there were many diagnostic imaging methods such as computed tomography (CT), ultrasonography and endoscopic ultrasonography, the diagnosis of ectopic pancreas was difficult due to its non-specific imaging characteristics. Ectopic pancreas may commonly present as a submucosal mass in gastrointestinal tract [8] and were easily misdiagnosed with other submucosal tumor such as gastric stromal tumor (GST) and leiomyoma. As we know, GST is the most common subepithelial lesion and accounts for 90% of gastric submucosal tumors [8, 9]. Additionally, the standard treatment for a GST without metastasis is surgical resection and a GST with metastasis is usually treated by tyrosine inhibitors such as imatinib. However, an ectopic pancreas would not need clinical treatment unless it is symptomatic, in addition, whether to remove suspected ectopic pancreas that is found incidentally is still a controversial issue [10]. So it is of great clinical significance to identify ectopic pancreas and gastrointestinal stromal tumors on image.

As we know, CT is the most commonly used noninvasive modality for preoperative evaluation of gastric tumors due to its well-standardized protocal and easy accessibility. However, CT features of gastric ectopic pancreas (GEPs) as well as differential imaging features of GEPs from GST have not yet been investigated completely. Therefore, our study retrospectively investigates the differential CT features of GEPs from GST.

Gastric ectopic pancreas (GEPs) is a pancreatic tissue which was found outside its normal localization and without any anatomical or vascular connection with pancreas [3, 11]. Although the majority of patients with GEPs were asymptomatic, a few patients may have clinical manifestations due to stimulation of hormones and enzymes secreted by the ectopic pancreas [11]. As the most common subepithelial lesion, GST accounts for 90% of gastric submucosal tumor [8, 9], And it is difficult to differentiate GEPs from GST, so we compare the CT features of GEPs and GST to help us make the correct diagnosis.

Our study indicated that 0 (0%) of 17 cases of GEPs were located in gastric fundus and the majority of GEPs (64.7%) were located in gastric body, moreover, only 5 cases (29.4%) were located in gastric antrum. In contrast, 41 (34.4%) of 119 GST were located in gastric fundus and 56 (47.1%) cases were located in gastric body. However, the location of GEPs is inconsistent with the well-known fact that ectopic pancreases in upper gastrointestinal system are usually located in the gastric antrum [6‐8, 12‐14], and this may be resulted from mall sample size of GEPs.

Moreover, 6 (35.3%) of 17 cases of GEPs in our study had the presence of peritumoral infiltration or fat-line of peritumor, while only 3 (2.5%) of 119 cases of GST performed like that. Our result is similar to previous researches of Cho et al. [14] and Kim et al. [15] and consistent with typical endoscopic ultrasonography (EUS) characteristics, that is indistinct borders, lobulated margins, presence of anechoic duct-like structures, a mural growth pattern, and localization within two or more layers [16, 17]. The feature of peritumoral infiltration is very closely correlated with the histological feature of a lobular structure of the acinous tissue at the margin [16]. Since most GEPs were exophytic growth pattern (64.7% [11/17]) and GEPs was not a true neoplasms but a hamartoma that flat pancreatic acinar formation and duct development histologically [6‐8], so it is of high possibility for GEPs to have fat space between the tumor and serosal layer. As for GST, the main endoscopic finding of it is a nonspecific smooth bulge covered with normal mucosa, which is common to all subepithelial lesions [9], so the possibility for GST to have fat space between the tumor and serosal layer is extremely low. Furthermore, Mietinenn [18] reported the metastatic risk of GST increases according to the tumor size irrespectively of the mitotic count and the probability of malignancy was significantly increased when the tumor was larger than 5 cm in diameter. In our series, 86 cases of GST (72.3% [86/119]) were less than 5 cm in diameter and it is of low probability for GST in our study to invaded the serosal layer, which lead to the seldom presence of peritumoral infiltration.

In our series, 0 (0%) of the 17 cases of GEPs had the presence of calcification, surface ulceration and lymph node, just 1 (5.9%) case of GEPs had necrosis, our results were supported by the fact that GEPs was not a true neoplasm but a hamartoma of flat pancreatic acinar formation and duct development [6‐8]. Our study also showed that the LD and SD of GEPs were shorter than GST, but the previous study [14] regarding the CT features of GEPs didn’t regard it as a characteristic CT finding.

Our study demonstrated that the CT attenuation values of CTu, CTa and CTp of GEPs were significantly higher than that of GST. Besides, the degree of enhancement was much heavier for the GEPs than that of GST, both in the DEAP and DEPP. Furthermore, ROC analysis showed that the test efficiency of CTp in differentiating them was perfect (AUC = 0.9). The majority of GEPs appeared as homogeneously extramucosal masses with similar or higher attenuation to pancreas and this result may be attributable to the histologic similarity of GEPs to normal pancreatic tissue, especially acini. Microscopically, GEPs consist of pancreatic acini and ducts and rarely contain islet cells. If the GEPs was mainly composed of pancreatic acini, the lesions would show greater enhancement and have a higher CT attenuation value in portal venous phase than the pancreas. On the contrary, if the GEPs consisted of predominantly ducts, the lesions would have lower CT attenuation values than pancreas and even the back muscles [14, 19, 20]. Our result showed that 16 of 17 GEPs mainly consisted of pancreatic acini and 1 of 17 GEPs contained many ducts and a few acini, and it is similar to the report of Yamagiwa et al. [20]. But for GST, as we know, GST had malignant potential and fast growth rate so as to tumor cell prone to degeneration, which undoubtedly decreased the degree of enhancement of GST [9, 21].

Our study has several limitations. Firstly, this was a retrospective study, the selection bias and the use of various CT scanners were inevitable. Secondly, only two types of gastric mesenchymal tumors were compared and other subepithelial lesions were ignored. Besides, in order to avoid the influence of size bias, we excluded larger GST because most of larger-sized gastric mesenchymal tumors can be diagnosed as GST eventually [22]. Thirdly, our results were based on clinical data obtained from an eastern country at a single institution, which could not reflect fully the image differences between the different races and regions of two groups.

In conclusion, gastric ectopic pancreas had characteristic CT findings that differ from gastric stromal tumor. When characteristic CT imaging findings are used in combination, ectopic pancreas can be differentiated from with a high degree of diagnostic accuracy.