Solid pseudopapillary tumor of the pancreas is a rare neoplasm with a low malignant potential, usually affecting young women in the second or third decade of life. The pathogenesis of the tumor is unknown, although its tendency to affect young women has suggested that sex hormones may be involved in the origin of SPT. However, no differences in immunohistochemical stains for sex hormone-receptor proteins or in clinicopathologic characteristics had been found attributable to sex alone. Sun et al. reported that 62.5% of SPT patients had been infected by Hepatitis B virus (HBV), which may be involved in the pathogenesis of SPTs. However, this association has not been confirmed by other researchers.
The clinical presentation of SPTs is usually unspecific and two or more symptoms usually coexist. Most of the patients presented with unclear clinical features, including abdominal pain, abdominal discomfort, poor appetite, and nausea, which are related to tumor compression of the adjacent organs. Because patients lack distinctive symptoms, the majority of these tumors are diagnosed during complementary imaging investigations, such as CT or US of the abdomen. On US or CT, the lesion is usually seen to be large, and its internal structure ranges from cystic thick-walled or with an inner irregular margin to a predominantly solid mass with some cystic component. On dynamic contrast-enhanced CT, the tumor is enhanced less than the adjacent normal pancreas. Magnetic resonance imaging is better than CT in differentiating the cystic or solid component inside the tumor and providing information about resectability. The use of FNAC, either percutaneously or endoscopic ultrasound guided, can help distinguish SPTs from other pancreatic tumors. However, seeding of the needle tract by neoplastic cells and such complications as bleeding, pancreatic fistula, and biliary fistula during the procedure have also been reported. Despite widespread availability of high-quality imaging systems, preoperative diagnosis was difficult. Only six patients were diagnosed as or suspected of SPTs in our series, and the misdiagnosis rate in other groups was reported as ranging from 38.5% to more than 70%[10, 11]. According to our experience, data from CT or MRI scans combined with age and sex should be sufficient for the decision to operate, and FNAC should be performed where the radiological diagnosis is not clear enough.
Currently, complete aggressive surgical resection is the treatment of choice for SPTs, even in the case of local invasion or metastasis. The surgical approach depends on the location, size, and nature of the neoplasms, as well as the time of surgery. Intra-operative frozen section may be helpful to ascertain the adequate of the resection margins. Extensive lymphatic dissection is not warranted, as SPTs rarely have lymph node metastases. For the case of local invasion or metastases, there is also a consensus that surgical therapy should be performed. Because of the excellent outcomes after complete resection, surgeons should always aim for complete en-bloc resection including adjacent structures preferably with microscopically clear margins. In our study, the infiltrated splenic vein and adjacent tissues were resected en bloc and a long-term survival was observed in these patients. Resection of distant metastases should be performed at the time of primary resection or even for recurrences. This aggressive approach is supported in some studies, which showed that most patients were alive at long-term follow-up after extended resection[12, 15].
In our study, four patients with splenic vein infiltration or pancreatic parenchyma invasion were diagnosed as malignant SPTs. Some studies have shown a correlation between tumor size above 5 cm, tumor necrosis, the male sex, and SPTs with malignant potential[16, 17]. However, several univariate analyses indicated that clinical factors, including sex, age, tumor size, tumor location, increased tumor markers, and tumor characteristics were not intensively related to the malignant potential of SPTs[4, 10, 18]. These results were consistent with that in our study. Moreover, we found that positive immunoreactivity for Ki-67 was detected in three patients with pancreatic parenchyma invasion. Our findings are similar to the report from Yang and indicate that the detection of Ki-67 may correlate with the malignancy and poor outcome of SPTs. However, these results are only limited to a small sample of SPTs, and more cases should be detected for Ki-67 and other new biomarkers in further studies.
Solid pseudopapillary tumors are readily diagnosable, based on their pathological and immunohistochemical features. The tumors contain a mixture of solid, cystic, and pseudopapillary patterns in various proportions. The solid portions of the tumor are composed of uniform and polygonal epithelioid cells with well-vascularized stroma and a discohesive arrangement. Immunohistochemically, SPTs are typically positive for vimentin, α1-antitrypsin, α1-antichymotrypsin, and neuron-specific enolase, but the unique immunohistochemical features with expression of CD56 and CD10 were not consistent in recent studies. Cells from SPTs may also reveal focal immunoreactivity for cytokeratin and synaptophysin, demonstrate abnormal nuclear localization of β-catenin and the presence of progesterone receptors and may express galectin-3, all of which are useful in differentiating SPTs from endocrine pancreatic tumors.
The prognosis of SPTs is good, even with local recurrence, as well as metastases or invasions. More than 95% of patients with SPTs limited to the pancreas are cured by complete surgical excision. Local recurrence is reported to be less than 10%, and usually within 4 years of surgery. Recurrence, local invasion, and limited metastases are not contraindications for resection, and long-term survival has also been observed in patients with malignant SPTs. The overall 5-year survival was estimated to be 95% in a review of 718 patients reported in the English literature. Owing to the favorable prognosis and excellent long-term survival, even in the presence of local recurrence or stable metastases, predictive factors of survival are difficult to identify.