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Division of Oncology, Department of Medical Oncology, Mayo Clinic College of Medicine, Rochester, Minnesota, USA¹ Department of Surgery, Rochester, Minnesota, USA² Department of Health Sciences Research, Rochester, Minnesota, USA

(Correspondence should be addressed to T R Halfdanarson who is now at Division of Hematology, Oncology and Blood and Marrow Transplantation, University of Iowa Hospitals and Clinics, 200 Hawkins Drive, Iowa City, Iowa 52242, USA; Email: thorvardur-halfdanarson{at}uiowa.edu)

	Abstract

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

 
Pancreatic endocrine tumors (PETs) are uncommon tumors with an annual incidence <1 per 100 000 person-years in the general population. The PETs that produce hormones resulting in symptoms are designated as functional. The majority of PETs are non-functional. Of the functional tumors, insulinomas are the most common, followed by gastrinomas. The clinical course of patients with PETs is variable and depends on the extent of the disease and the treatment rendered. Patients with completely resected tumors generally have a good prognosis, and aggressive surgical therapy in patients with advanced disease may also prolong survival. The epidemiology, prognosis, and established and novel prognostic markers of PETs are reviewed.

	Introduction

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

 
Pancreatic endocrine tumors (PETs) are uncommon neoplasms with an incidence of <1 per 100 000 person-years in population studies (Moldow & Connelly 1968, Buchanan et al. 1986, Eriksson et al. 1989, Watson et al. 1989, Carriaga & Henson 1995, Lam & Lo 1997, Halfdanarson et al. 2007). The incidence is higher in autopsy studies, ranging from 0.8 to 10% suggesting that these tumors frequently go unnoticed (Grimelius et al. 1975, Kimura et al. 1991). PETs comprise <3% of all pancreatic neoplasms (Cubilla & Hajdu 1975, Carriaga & Henson 1995, Fesinmeyer et al. 2005, Öberg & Eriksson 2005). Pancreatic endocrine tumors are generally more indolent than adenocarcinoma of the pancreas and have a better prognosis (Carriaga & Henson 1995, Fesinmeyer et al. 2005). The origin of these tumors is not fully known, but they may arise from pluripotent cells within the exocrine pancreas (Vortmeyer et al. 2004). PETs are frequently divided into two groups based on their functional status, but unfortunately there is no uniformly accepted definition of a functional PET. Patients with ‘functional’ PETs commonly manifest symptoms resulting from hormone production of the tumor, although these tumors may also produce hormones without the patient having any symptoms secondary to the overproduced hormones. For the purpose of this review, we consider patients without symptoms of hormone production to have non-functional tumors even though elevated levels of hormones are detected in the blood, but we acknowledge the limitations of that definition. Multiple studies have addressed the epidemiology and prognosis of PETs but there are few large population studies available. A substantial proportion of the literature regarding these tumors stems from case reports and case series, often involving highly selected groups of patients, limiting the generalizability of the results. The purpose of this article is to review the epidemiology and prognosis of PETs and the commonly used prognostic predictors. We briefly discuss the role of novel prognostic markers.

	Pathology and classification of PETs

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

 
Pancreatic endocrine tumors are frequently graded and classified according to the WHO classification of endocrine tumors (Table 1; Solcia et al. 2000, Heitz et al. 2004, Klöppel et al. 2004). The diagnosis of PETs rests upon confirming the neuroendocrine nature of the malignant cells. These tumors can have heterogeneous microscopic findings, and immunohistochemical staining with markers, such as chromogranin A, synaptophysin, and neuron-specific enolase, can usually confirm the neuroendocrine origin. It can be difficult to accurately assess the degree of malignancy of pancreatic endocrine tumors but the current WHO classification provides guidance in that respect. Other features of the tumors, including local invasion and metastases to lymph nodes and distant organs, have also been helpful in defining their malignant nature. The European Neuroendocrine Tumor Society has recently published guidelines on the management of PETs (Falconi et al. 2006, de Herder et al. 2006, O'Toole et al. 2006).

	Epidemiology of PETs

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

The diverse nature of pancreatic tumors has been known for more than a century. It is of historical interest to review earlier reports on pancreatic tumors other than adenocarcinoma (Table 2). These studies have to be interpreted with caution as PETs were not well-defined entities at the time they were conducted, and there likely are substantial inaccuracies regarding the diagnoses. An autopsy study from the early twentieth century by Nicholls reports a case of pancreatic adenoma arising in the islet of Langerhans among 1514 patients (Nicholls 1902). Korpássy (1939) found four cases (0.8%) of macroscopic islet cell adenomas in 500 autopsies in 1938. Twenty-four cases (0.3%) of ‘benign islet cell neoplasms’ were observed in a series of 9158 consecutive autopsies reported by Frantz (1959). Warren et al. reported 24 islet cell tumors among 2708 autopsies of patients without diabetes and 18 tumors in 1858 diabetic patients, corresponding to a prevalence of 0.9% (Warren et al. 1966). Similar prevalence of 1.4% was reported by Becker where 62 ‘islet cell adenomas’ were found in 4280 autopsies (Becker 1971).

Several studies on the incidence of PETs in defined populations have been performed (Table 3; Moldow & Connelly 1968, Buchanan et al. 1986, Eriksson et al. 1989, Watson et al. 1989, Carriaga & Henson 1995, Lam & Lo 1997, Lepage et al. 2004, Halfdanarson et al. 2007). Moldow & Connelly reported on all patients diagnosed with pancreatic tumors in Connecticut during 1957–1963 (Moldow & Connelly 1968). Out of the 856 pancreatic tumors, islet cell tumors accounted for <5%. In this study, no effort was made to distinguish between islet cell tumors and other rare pancreatic tumors. These tumor types in addition to PETs comprised 5% of all pancreatic tumors and the incidence was <1/100 000 (Moldow & Connelly 1968). A Swedish study reported an annual incidence of 0.4/100 000 (Eriksson et al. 1989) and a study from Northern Ireland found an annual incidence of 0.18/100 000 (Buchanan et al. 1986). The latter study was later updated reporting the incidence to be 0.23/100 000 (Watson et al. 1989).

The frequency of the various subtypes of functional PETs has been described in several studies (Tables 4 and 5; Jacobsen et al. 1986, Cullen & Ong 1987, Eriksson et al. 1989, 1990, Watson et al. 1989, Service et al. 1991, Stamm et al. 1991). Insulinoma is the most frequently encountered functional PET and is usually a benign tumor and almost always located in the pancreas (Soga & Yakuwa 1994, Öberg & Eriksson 2005). The incidence of insulinoma in a well-defined population in Olmsted County in southeastern Minnesota was found to be 0.4 per 100 000 person-years (Service et al. 1991). Other investigators have reported annual incidence rates ranging from 0.07 to 0.12/100 000 in populations less well defined than in Olmsted County (Kavlie & White 1972, Cullen & Ong 1987, Eriksson et al. 1989, Watson et al. 1989). The annual incidence of malignant insulinoma in the SEER registry is 0.1/million (Halfdanarson et al. 2008). Gastrinoma is the second most commonly encountered functional PET but gastrinomas are also frequently found outside the pancreas (Soga & Yakuwa 1998a, Norton et al. 1999, Roy et al. 2000, Öberg & Eriksson 2005). Pancreatic gastrinomas may be more aggressive and frequently associated with liver metastases (Weber et al. 1995). Up to 30% of gastrinomas are associated with multiple endocrine neoplasia type 1 (MEN-1; Soga & Yakuwa 1998a, Roy et al. 2000). Gastrinoma is the most common functional PET seen in patients with MEN-1 and the prognosis may be worse than that in sporadic gastrinoma (Norton et al. 1999, Gibril et al. 2001, Norton 2005). Investigators in Denmark estimated the incidence of gastrinoma to be 0.5 per million per year (Jacobsen et al. 1986). A higher incidence of 2–4 per million has been found in Switzerland (Stamm et al. 1991). Other studies have reported an annual incidence of 0.5–1.2 cases per million (Eriksson et al. 1989, Watson et al. 1989). Our recent study using the SEER registry suggested an annual incidence of 0.1/million, but this may be a substantial underestimate given the way that SEER registers these tumors (Halfdanarson et al. 2008).

Non-functioning tumors comprise a substantial proportion of all PETs and have been reported to comprise 25–100% of all PETs. The annual incidence of symptomatic non-functional PETs has been estimated to be 0.07–0.1/100 000 (Eriksson et al. 1989, Watson et al. 1989). Autopsy studies have shown much higher incidence than that reported in clinical series (Kimura et al. 1991).

	PETs associated with hereditary syndromes

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

 
Pancreatic endocrine tumors are commonly observed in MEN-1 and less frequently in von Hippel–Lindau disease (VHL). Cases of PET in association with the tuberous sclerosis complex and type 1 neurofibromatosis have also been reported but are rare (Tan et al. 1996, Verhoef et al. 1999, Fujisawa et al. 2002, Francalanci et al. 2003).

The MEN-1 syndrome is an autosomal dominant inherited disorder characterized by multiple endocrine and non-endocrine tumors (Brandi et al. 2001, Doherty 2005, Lakhani et al. 2007). The endocrine tumors most frequently described in patients with MEN-1 include parathyroid adenomas, pituitary adenomas, and PETs. Multiple other tumors of varying penetrance have been reported in association with MEN and include tumors of the adrenal glands, carcinoid tumors, angiofibroma, collagenoma, and lipoma. The penetrance of PETs in MEN-1 patients ranges from 30 to 75% and these tumors are frequently multifocal and metastatic at the time of diagnosis (Vasen et al. 1989, Skogseid et al. 1991, Le Bodic et al. 1996, Burgess et al. 1998a,b). Gastrinomas are the most commonly encountered PETs, followed by non-functioning tumors and insulinomas (Brandi et al. 2001, Gibril & Jensen 2004, Triponez et al. 2006). A recent study suggested that non-functioning tumors were more common than gastrinomas in MEN-1 patients (Triponez et al. 2006). PETs are a major cause of morbidity and mortality in patients with MEN-1, but discussion of screening and treatment of these patients is outside the scope of this review (Wilkinson et al. 1993, Doherty et al. 1998, Dean et al. 2000). VHL disease is an autosomal dominant tumor predisposition syndrome caused by a germ line mutation in the VHL gene (Lonser et al. 2003). The typical features of VHL disease include retinal and brain hemangioblastoma, renal cell carcinoma, renal cysts, pheochromocytoma, and pancreatic tumors and cysts (Lonser et al. 2003). Pancreatic endocrine tumors are found in 9.5–17% of patients with VHL disease (Binkovitz et al. 1990, Libutti et al. 1998, Hammel et al. 2000, Blansfield et al. 2007). The PETs associated with VHL disease are virtually always non-functional (Blansfield et al. 2007).

	Prognosis following resection

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

 
Functional and non-functional tumors

Patients with PETs generally have a much better prognosis than those with pancreatic adenocarcinoma. Recent studies using the SEER database have reported improved survival after resection or a median overall survival of 58–97 months compared with 15–21 months in patients not undergoing surgery, although the number of patients with information regarding the surgery was small (Fesinmeyer et al. 2005, Halfdanarson et al. 2007). Numerous retrospective reports on PETs have been published, which provide valuable information about the mode of presentation and the prognosis of patients with these tumors, but there is marked heterogeneity among the patient populations studied as well as a large potential for referral bias, decreasing the generalizability of the results (Tables 6–8; Cubilla & Hajdu 1975, Kent et al. 1981, Broughan et al. 1986, Eckhauser et al. 1986, Legaspi & Brennan 1988, Thompson et al. 1988, Venkatesh et al. 1990, Service et al. 1991, Grama et al. 1992, Cheslyn-Curtis et al. 1993, Evans et al. 1993, White et al. 1994, Closset et al. 1996, Lo et al. 1996, La Rosa et al. 1996, Madura et al. 1997, Phan et al. 1997, 1998, Furukawa et al. 1998, Madeira et al. 1998, Bartsch et al. 2000, Hellman et al. 2000, Matthews et al. 2000, Yang et al. 2000, Corleto et al. 2001, Solorzano et al. 2001, Chu et al. 2002, Hochwald et al. 2002, Sarmiento et al. 2002, Gullo et al. 2003, Norton et al. 2003, Dralle et al. 2004, Guo et al. 2004, Lepage et al. 2004, Liang et al. 2004, Pape et al. 2004, Jarufe et al. 2005, Kang et al. 2005, Kouvaraki et al. 2005, Panzuto et al. 2005, Tomassetti et al. 2005, House et al. 2006, Kazanjian et al. 2006, Schurr et al. 2007).

Taken together, these studies of heterogeneous cohorts of patients with both functional and non-functional tumors have not consistently shown functional status to be a prognostic factor in terms of survival when the benign insulinomas have been excluded. The heterogeneity of the studies makes all comparisons difficult. Ninety percent of insulinomas are benign and have excellent prognosis after resection (Service et al. 1991). Patients with gastrinoma seem to have a better survival than those with other malignant and functional PETs, especially after surgery with curative intent (Norton et al. 1999). As expected, patients with more advanced and metastatic diseases as well as those with residual disease following resection had shorter survival.

Non-functional tumors

Several studies have been limited to non-functional PETs (Table 7; Kent et al. 1981, Eckhauser et al. 1986, Cheslyn-Curtis et al. 1993, Evans et al. 1993, Closset et al. 1996, La Rosa et al. 1996, Madura et al. 1997, Furukawa et al. 1998, Bartsch et al. 2000, Matthews et al. 2000, Yang et al. 2000, Solorzano et al. 2001, Gullo et al. 2003, 2004, Liang et al. 2004, Kang et al. 2005). Similar to the studies combining functional and non-functional tumors, the presence of distant metastases and incomplete resection predict worse survival. The 5-year overall survival ranges from 26 to 58% and appears lower than those in the series combining both functional and non-functional tumors. However, the heterogeneity among the studies and the potential for selection bias make any comparison problematic (Dralle et al. 2004, Kouvaraki et al. 2005). Non-functional PETs seem to have inferior prognosis when compared with functional PETs, even after adjusting known prognostic factors such as age, stage, and grade (Halfdanarson et al. 2007).

La Rosa et al. (1996) studied 61 patients with non-functional PETs. The tumors were considered malignant if there was a direct invasion into adjacent tissues or organs or if distant metastases were present. Multiple tumor characteristics predicted malignant behavior in a univariate analysis, including tumor diameter, vascular and perineural invasion, the presence of mitoses, nuclear atypia, and high proliferative index (>2%) as evaluated by Ki-67 immunohistochemical staining. The tumors were classified according to the histological features and Ki-67 proliferative index (Ki-67 PI) into four groups. Malignant tumors were also classified as poorly differentiated based on the appearance of the tumor cells and the presence of mitoses and areas of necrosis. All other PETs were classified into limited risk tumors (LRT) and increased risk tumors (IRT) based on the presence of either high Ki-67 PI (>2%) or vascular and/or perineural invasion. These subtypes were found to predict survival in a univariate analysis. LRT had better prognosis than IRT, which in turn had better prognosis than well-differentiated carcinomas. The poorly differentiated carcinomas had the worst prognosis. Even though capsular penetration, the presence of distant metastases, vascular microinvasion, and high Ki-67 PI all were found to adversely affect prognosis in a univariate analysis, the predictive value disappeared on a multivariate analysis (La Rosa et al. 1996).

Functional tumors

Several studies have focused solely on therapy and outcome of functional PETs (Table 8; Harrison et al. 1973, Lundstam et al. 1979, Danforth et al. 1984, Zeng et al. 1988, Service et al. 1991, Grama et al. 1992, Weber et al. 1995, Boukhman et al. 1998, Norton et al. 1999, Chen et al. 2002, Feng et al. 2002, Matthews et al. 2002, Grant 2005, Hirshberg et al. 2005, Starke et al. 2005, Kang et al. 2006). The largest study of insulinomas is a retrospective review by Service et al. from the Mayo Clinic in Rochester, spanning a 60-year period from 1927 to 1986 (Service et al. 1991). The study included 244 patients with insulinoma, including eight patients who were residents of Olmsted County in southeastern Minnesota. Thirteen patients (5.8%) had malignant insulinoma and 17 patients (7.6%) had MEN-1 in addition. As expected, the survival of patients with benign insulinoma was long following therapy and did not differ from expected survival of this population. The 10-year survival of patients with benign insulinoma was 78%. The factors adversely affecting the prognosis were malignant phenotype, advanced age, and patients diagnosed early in the study period. Patients with MEN-1 had shorter survival but the difference was not statistically significant. A more recent report from the same institution reported 225 patients with benign insulinoma, who underwent resection from 1982 to 2004 (Grant 2005). The outcome for this cohort of patients was excellent, with 98% of patients being cured with resection. The general good outcome of patients with insulinoma may thus skew the outcome results in a series where patients with insulinomas are grouped with patients having other functional or non-functional tumors.

Norton et al. 1999 reported their experience with 151 patients with gastrinoma undergoing surgery. Their cohort of patients included 36 (24%) patients with pancreatic gastrinoma, of which 19 had MEN-1. Gastrinoma was localized to the pancreas in 17 out of 123 (14%) patients with sporadic tumors. The 5- and 10-year disease-specific survival of all patients with sporadic gastrinoma was 100 and 95% respectively and 40% of the patients were free of disease at 5 years postoperatively. A previous study by the same investigators showed that survival was primarily determined by the presence of liver metastases (Weber et al. 1995). Gastrinomas associated with the Cushing syndrome seem to have a particularly poor prognosis (Maton et al. 1986, Ilias et al. 2005).

	Other prognostic factors

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

 
Several investigators have attempted to evaluate previously established and novel prognostic factors in PETs. The WHO classification of PETs has been shown to be useful in predicting the clinical behavior of these tumors Heymann et al. (2000). Histological findings such as grade and the number of mitotic figures have been found to predict survival in a few studies. Hochwald et al. (2002) retrospectively evaluated 136 patients with low-grade or intermediate-grade PETs who had undergone tumor resection. After adjusting for other prognostic factors including the presence of distal metastases, tumor necrosis was found to be associated with shorter disease-free survival (DFS). Higher tumor mitotic rate (>2 per 50 high-power fields (HPFs)) was associated with shorter disease-specific survival (DSS). There was no difference in DFS or DSS between functional and non-functional tumors. The authors proposed a simple system for grading these tumors based on the presence of necrosis and the number of mitoses where patients with a low-grade tumor (<2 mitoses per 50 HPFs and no necrosis) had a significantly longer DFS and DSS (Hochwald et al. 2002). Other authors have suggested a prognostic model for well-differentiated gastroenteropathic neuroendocrine tumors (GEP-NET) using abnormal liver chemistries and urinary excretion of 5-HIAA, but the model has not been validated in patients with tumors limited to the pancreas (Formica et al. 2006). Proliferation markers such as Ki-67 immunohistochemistry have been found to predict prognosis in patients with PETs (Lloyd 1998). However, the results have not uniformly supported the prognostic value of increased Ki-67 expression, especially after adjusting for other known prognostic variables such as stage (La Rosa et al. 1996, Pelosi et al. 1996, 1997, Clarke et al. 1997, Gentil Perret et al. 1998, Lloyd 1998, Jorda et al. 2003, Goto et al. 2004, Böhmig et al. 2005, Panzuto et al. 2005, Couvelard et al. 2006). Recent studies have identified additional markers that may be of prognostic value. Positive staining for CK19 was shown to be a powerful prognostic factor predicting shorter survival, even after controlling for variables such as the number of mitoses, tumor necrosis, and Ki-67 expression (Deshpande et al. 2004). CK19 may not be predictive of survival in patients with non-functional PETs (La Rosa et al. 2007). Expression of CD10 has also been shown to predict worse survival, and a correlation was found between CD10 expression and the WHO classification where the more malignant tumors were more likely to express CD10 (Deschamps et al. 2006). There was also a correlation between positive CD10 staining and higher proliferative index, larger tumor size, and the presence of hepatic metastases. Loss of expression of CD99 has been suggested to predict worse outcome by some authors but not others (Goto et al. 2004, Ali et al. 2006) and expression of CD44 isoforms v6 and v9 may be indicative of more benign behavior and better prognosis in patients with PET (Imam et al. 2000).

With advances in genetic and molecular biology, multiple potential prognostic markers have been identified. These markers have not yet been validated in large cohorts of patients and have not found their way into routine clinical use. The molecular genetics of GEP tumors have been reviewed in detail elsewhere (Zikusoka et al. 2005). Certain chromosomal aberrations have been found more frequently in patients with metastatic PETs when compared with non-metastatic tumors. These aberrations involve multiple chromosomes, including 1,3,5,6,7,14,22 and the X chromosome (Speel et al. 1999, Barghorn et al. 2001a,b, Zhao et al. 2001, Guo et al. 2002a,b, Wild et al. 2002, Chen et al. 2003, 2004). Chromosomal instability as manifested by the number of aberrations per tumor has been shown to be an indicator for the development of metastases in patients with sporadic insulinoma, and loss of sex chromosomes may predict shorter survival in patients with functional and non-functional PETs (Missiaglia et al. 2002, Jonkers et al. 2005).

Methylation of tumor suppressor genes has been implicated as an important factor in the etiology of various tumors. House et al. have shown that silencing of multiple tumor suppressor genes by promoter hypermethylation is frequent in PETs and may be associated with more advanced tumor stage and shorter survival (House et al. 2003b). The most frequently silenced genes were RASSF1A, p16/INK4A, O⁶-MGMT, RAR-β, and hMLH1 (House et al. 2003b). The association between RASSF1A and p16/INK4A methylation and more advanced stage was confirmed by other authors (Liu et al. 2005). Methylation of hMLH1 has also been shown to result in microsatellite instability in patients with PETs and may be associated with a favorable prognosis (House et al. 2003a). Telomerase activity has also been suggested as being useful in the diagnosis of PETs, and it has been suggested that the presence of telomerase activity may predict an unfavorable outcome (Lam et al. 2000, Tang et al. 2002, Vezzosi et al. 2006).

Studies using gene expression analysis can be powerful tools for prognostication of various tumors. Several investigators have used gene expression analysis in tumor tissue from patients with PET using microarray methods (Maitra et al. 2003, Bloomston et al. 2004, Durkin et al. 2004, Hansel et al. 2004, Capurso et al. 2006, Couvelard et al. 2006). Numerous genes have been found to be either over- or underexpressed, and these findings have been validated with immunohistochemical studies and PCR studies for several of the overexpressed genes. Genes found to be overexpressed in metastatic PETs when compared with non-metastatic PETs include Met proto-oncogene, IGF-binding protein 3 gene (IGFBP-3) as well as various genes involved in angiogenesis, signal transduction, cell cycle control, and ion transport (Hansel et al. 2004, Couvelard et al. 2006). Other investigators using a different set of overexpressed genes did not show a significant difference in gene expression between primary and metastatic lesions (Capurso et al. 2006).

Angiogenesis is important for tumor growth and formation of metastases, and several studies have evaluated the prognostic role of angiogenesis markers and mediators. Expression of Vascular endothelial growth factor (VEGF) has been associated with more aggressive tumor growth, the presence of metastases, and shorter progression-free survival in patients with low-grade neuroendocrine tumors when compared with tumors not expressing VEGF (Hansel et al. 2003, Phan et al. 2006). Microvessel density (MVD) in PETs has also received attention recently and decreased MVD may be an adverse prognostic factor according to some studies but not others (Marion-Audibert et al. 2003, La Rosa et al. 2003, Tan et al. 2004, Couvelard et al. 2005, 2006, Takahashi et al. 2007).

	Conclusions

Top Abstract Introduction Pathology and classification of... Epidemiology of PETs PETs associated with hereditary... Prognosis following resection Other prognostic factors Conclusions References

 
Pancreatic endocrine tumors (PETs) are uncommon tumors thought to originate from pluripotent cells in the exocrine pancreas. PETs account for only 1–3% of all neoplasms of the pancreas and their clinical behavior is much more indolent than that of adenocarcinoma of the pancreas. PETs are uncommon tumors with an annual incidence of <0.4 cases per 100 000. Asymptomatic PETs appear to be much more common according to large autopsy studies and frequently are undiagnosed in life. Functioning PETs are rare except for insulinomas and gastrinomas. The prognosis of PET is much better than that of pancreatic adenocarcinoma, even though patients are frequently diagnosed with metastatic disease. Multiple studies have shown that metastatic tumor and incomplete resection portend worse prognosis. Aggressive resection of the primary tumor as well as metastatic lesions may improve survival. Functional tumors may have a better prognosis in some studies, which may partly be explained by the much more benign nature and the favorable prognosis of the hormonally active insulinomas. Patients with functional tumors may also be diagnosed at an earlier stage, especially if they present with classical symptoms of hormone overproduction. Other prognostic factors include higher proliferative rate as manifested by increased number of mitoses as well as tumor necroses but those histological features are not universally reported by pathologists. Novel prognostic factors include increased expression of Ki-67, overexpression of angiogenesis markers, chromosomal aberrations, and overexpression of various genes as identified on microarray studies. Given the heterogeneous nature of PETs, it is unlikely that there will be a prognostic model applicable to all subtypes of these uncommon tumors.

	Acknowledgements

 
We thank Ricardo V Lloyd MD, PhD for reviewing the manuscript. Research support: Mayo Clinic SPORE in Pancreatic Cancer (P50 CA 102701). The authors declare that there is no conflict of interest that would prejudice the impartiality of this scientific work.

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