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From the Department of Pathology, Vanderbilt University Medical Center, Nashville, Tennessee.
Correspondence: Address correspondence to: Kay Washington, MD, PhD, C-3307 Medical Center North, Department of Pathology, Vanderbilt University Medical Center, Nashville, TN 37232; Fax: 615-343-7023; E-mail: kay.washington{at}mcmail.vanderbilt.edu
In recent years, there has been a resurgence of interest in the pathology of the pancreas, with characterization of several rare variants of pancreatic neoplasms, such as solid-pseudopapillary tumor of the pancreas and intraductal mucinous neoplasm. The flurry of papers describing the clinical, radiographical, and pathologic features of these tumors has resulted in increased recognition of these lesions by clinicians. The importance of accurate diagnosis of these special types of pancreatic tumors is obvious: they generally carry a much better prognosis than the typical adenocarcinoma of the pancreas, and surgical excision is usually curative.
The article by Martin et al.,1 although it reports a relatively small number of patients, serves a very useful function in reporting the experience of a single large referral institution with diagnosis and treatment of solid-pseudopapillary tumor of the pancreas. This rare tumor accounts for only 1% to 2% of exocrine pancreatic tumors at most institutions, and 24 cases represents a fairly sizable number when the rarity of this neoplasm is considered. For the most part, this tumor behaves in a low-grade fashion, although in approximately 15% of cases metastases occur. Because these tumors are indolent and metastases are often amenable to resection, patients rarely die as a direct result of this neoplasm. Because of the prolonged natural history and the relative rarity of the tumor, it has been difficult to establish histopathologic criteria predictive of aggressive behavior. The most commonly cited previous study attempting to identify histological prognostic factors is that of Nishihara et al.,2 who compared 3 cases that metastasized with 19 cases without metastasis and found that venous invasion, degree of nuclear atypia, mitotic rate, and prominence of necrobiotic cell nests were helpful in determining malignancy. Of interest in the current study is the presentation of 4 of 24 patients with metastases, a similar percentage of patients with metastatic disease to the series reported by Nishihara (3 of 22). Of note to the pathologist, for the patients who died of tumor in the series reported by Martin et al., the histological appearance of the neoplasm was unusual, with large areas of tumor with a more diffuse growth pattern, increased cytologic atypia, numerous mitoses, and, in one case, a focus of sarcomatoid carcinoma. The feature that malignant cases in both this series and those reported by Nishihara et al.2 have in common is marked nuclear pleomorphism. Although we agree with the authors that definite conclusions cannot be drawn from two cases, it is important to report such findings so that other pathologists can compare their experience with this tumor.
To the pathologist, the solid-pseudopapillary tumor is an enigma. The light-microscopic features are characteristic and generally do not present diagnostic problems, although on gross examination degenerative cystic changes may lead to confusion with cystic neoplasms of the pancreas. Immunophenotyping shows a distinctive pattern that does not correspond to any of the normal pancreatic cell types but that may aid in diagnosis in problematic cases. Solid-pseudopapillary tumors are typically positive for vimentin, neuron-specific enolase (NSE), 
1-antitrypsin, and 1-antichymotrypsin and are negative for chromogranin, epithelial membrane antigen, and cytokeratin. Differentiation along endocrine cell lines has been postulated for this tumor, on the basis of NSE positivity, but the expression of vimentin and 1-antitrypsin does not support this interpretation. Because the pseudopapillary pattern is sometimes mistaken for a trabecular architecture and because the tumors are NSE positive, solid-pseudopapillary tumors may, however, be mistaken for pancreatic endocrine tumors. Some investigators have postulated that instead of being of pancreatic cell origin, the solid-pseudopapillary tumor may be derived from genital ridge/ovarian anlage-related cells attached to the developing pancreas in early embryogenesis,3 an intriguing but unproven hypothesis.
The incidence of this neoplasm may be increasing, but consideration should also be given to whether it is simply recognized and diagnosed on a more consistent basis. Reported associations with oral contraceptive use are tenuous at best and probably reflect the patient population in which this tumor occurs. We agree with the authors that increasing experience with this lesion leads to a greater awareness of the clinical presentation and pathologic features. The presence of a large bulky pancreatic neoplasm in a young woman or a child should obviously prompt consideration of this diagnosis. This series, however, reminds us that this neoplasm may occur in men (4 in 24 patients) and in older patients. For the surgeon, this report emphasizes that even bulky examples of this tumor are usually resectable, and size does not predict surgical resectability. Because of the indolent nature of the solid-pseudopapillary tumor, portal vein or arterial resection with reconstruction has been reported with long-term survival. Aggressive attempts at complete resection seem to be warranted, on the basis of the observation that recurrence has not been reported after complete resection of local disease.
The indolent nature of most examples of solid-pseudopapillary tumor of the pancreas is well established. The challenge at this point is to characterize the pathobiology of the lesion and to identify variants that may behave in a more aggressive fashion. Further radiographical, clinical, and pathologic delineation of this entity will depend on carefully reported series and collaborations between large referral institutions.
Received for publication September 20, 2001. Accepted for publication September 26, 2001.
REFERENCES
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