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Extra‐gastrointestinal stromal tumor of the pancreas: Clinical characteristics, diagnosis, treatment, and outcome

Pisters and Patel [1] presented an excellent review on the management of gastrointestinal stromal tumors (GISTs), which are the most common mesenchymal tumors of the gastrointestinal (GI) tract. Extra-GISTs, which arise from outside the GI tract but histologically resemble their GI counterparts, have been only scatteredly reported in literatures. Pancreatic GIST is a rare neoplasm with prognosis and biological behavior unknown. Here, we presented the management and outcome of a case with pancreatic malignant GIST. Furthermore, other cases reported in English literature were reviewed, with clinical characteristics, prognosis, and predictors for adverse outcomes analyzed. A 55-year-old man was found to have a large epigastric cystic lesion during check-up for abdominal discomfort. Laboratory and radiological examinations revealed a suspected pancreatic pseudocyst or cystic tumor. Distal pancreatectomy plus splenectomy was performed for this large lesion that arised from pancreatic body and tail. Pancreatic malignant GIST was diagnosed by histopathology and immunohistochemistry with positive staining for CD117 and CD34 [2]. He was treated with imatinib (400 mg orally once daily) after surgery for nearly 2 years and followed-up with 3–6 monthly CT scans. He did well until 6 months after drug withdrawal, when CT scan showed two low-density masses measuring 6 cm 6 cm and 2.5 cm 3 cm located in the left epigastric region. He received relaparotomy, and curative resection was performed for the tumors, which were confirmed recurrence and intraperitoneal metastasis of GIST by histopathology and immunohistochemistry. The patient continued taking imatinib and remained disease-free for 17 months after surgery. To date, only 12 cases have been reported (including our case) in the English literature [2–7]. The clinicopathologic features of these patients are listed in Table I. The majority have been diagnosed in patients older than 40 years (10/12), and the mean age at diagnosis is 54.1, with a male/female ratio of 1:2. Major clinical presentations included abdominal pain or discomfort, weight loss, and fatigue, however, one-third of them were incidental found during a routine workup. One patient presented with a 4-year history of increasingly hemorrhagic pancreatic cyst [3]. None of the patients had obstructive jaundice or history of trauma. The tumor could occur in any part of the pancreas with slightly more frequent in the body and tail. CT scan usually showed large cystic mass (in eight cases larger than 10 cm) with a few solid components. Tumor markers including CA19-9 and CEA were not elevated. The rate of pre-operative misdiagnosis was rather high. Follow-up ranging from 1 to 60 months were mentioned for 10 patients, all underwent surgical resection. Among them, four received adjuvant post-surgical therapy with imatinib. During the follow-up period, all 10 patients were still alive, however, recurrence and metastasis (liver, retroperitoneal nodal, or intra-abdominal) developed in three cases 1 month, 9 months and 28 months, respectively, after resection. Only our patient received re-resection, and was alive and free of disease to date. On univariate analysis, none of the features including sex, age, tumor size, and cell type except mitoses were significant predictors for recurrence and metastasis of this disease (Table II). Pancreatic GIST should be ruled out for patients with large periampullar tumors but no jaundice. Although it is rare, it should be considered in the differential diagnosis of both cystic and solid pancreatic masses. Pre-operative diagnosis is difficult and relies on endoscopic ultrasound guided fine needle aspiration cytology (EUSFNA). EUS is unequivocally the best modality to determine whether the lesion arises from the duodenal wall, given its ability to distinguish the layers of the digestive tract with high accuracy [4]. Despite the relatively small sample size, the indolent natural history, subtle symptoms, and favorable survival suggest that pancreatic GIST has low malignant potential. There is limited data with regards to predicting aggressive behavior of this entity. Most pancreatic GISTs (9/12) were considered lesions with high risk for their large sizes and high mitotic counts. Our analysis showed the only significant predictor for adverse outcome was mitoses 10/50HPF, while no association was found between tumor size and outcome. This may be attributed by the fact that the majority of pancreatic GISTs are large ( 10 cm) when detected. Surgical resection is the definitive treatment for resectable primary GISTs. However, aggressive and reiterative surgery may be potentially curative for local recurrence or metastasis. Targeted therapy with imatinib should be recommended in the treatment of those with high mitotic counts.