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Original Report |
1 Section of Abdominal Imaging and Intervention, Department of Radiology,
Brigham and Women's Hospital, Harvard Medical School, 75 Francis St., Boston,
MA 02115.
2 Department of Radiology, University "La Sapienza," Rome, 0017
Italy.
3 Department of Radiologic Pathology, Armed Forces Institute of Pathology,
Washington, DC 20306.
4 Department of Radiology, Uniformed Services University of Health Sciences,
Bethesda, MD 20814.
5 Department of Pathology, Brigham and Women's Hospital, Harvard Medical School,
Boston, MA 02115.
Received January 13, 2003;
accepted after revision February 19, 2003.
Address correspondence to K. J. Mortele
(kmortele{at}partners.org).
Abstract
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CONCLUSION. Solid pseudopapillary tumor of the pancreas, a tumor typically seen in young women, is a large, well-defined, encapsulated lesion with heterogeneous high or low signal intensity on T1-weighted, heterogeneous high signal intensity on T2-weighted, and early peripheral heterogeneous enhancement with progressive fill-in on gadolinium-enhanced dynamic MR imaging. These features help differentiate this rare tumor from other pancreatic neoplasms.
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On CT, solid pseudopapillary tumor typically presents as a large heterogeneous mass [4–12]. CT can also be used to identify the capsule of a solid pseudopapillary tumor. Reports about its appearance after the administration of iodinated contrast medium, however, are sparse and do not reveal a consistent contrast enhancement pattern. At sonography, the appearance of solid pseudopapillary tumor is variable and lacks correlation with gross pathology [5, 6, 8, 12, 13].
To date, the appearance of solid pseudopapillary tumor on MR imaging is largely unknown; to our knowledge, the MR imaging features of only 19 cases have been reported in the literature, including only three cases with gadolinium administration [8, 9, 12, 14–16]. MR imaging, because of its superior contrast resolution, displays a capsule and intratumoral hemorrhage better than CT. Therefore, MR imaging has the potential to improve our ability to diagnose solid pseudopapillary tumor.
This study was conducted to describe the MR imaging features of solid pseudopapillary tumor of the pancreas.
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All but two patients were female. The mean age was 30 years (range, 14–74 years). No race predilection was observed (Hispanic, n = 6; white, n = 5; black, n = 5; Asian, n = 3). Symptoms were present in 17 (89%) of the 19 patients and were as follows: chronic progressive epigastric pain (n = 13), acute diffuse abdominal pain (n = 2), and abdominal discomfort with sensation of fullness or early satiety (n = 2). The median time interval between onset of clinical symptoms and the pathologic diagnosis was 5 months (range, 0 days–4 years). A palpable mass was found in five patients (26%). In two patients, a mass was found incidentally on imaging: in one, in the setting of trauma; in the other, during sonography performed for pyelonephritis.
Three patients were pregnant at the time of their diagnosis; one of these presented with hirsutism due to the coexistence of a Sertoli-Leydig cell tumor of the ovary. A fourth patient was 18 months postpartum. In the latter patient and in an additional patient, imaging also revealed polycystic ovary disease.
None of the patients had a history of alcoholism, pancreatitis, or biliary tract disease. None of the patients had polycystic pancreatic disease, whether part of hepatorenal polycystic disease or von Hippel-Lindau disease. Results from hepatobiliary and pancreatic laboratory studies were within the normal limits in all but three patients. Two patients presented with an elevated serum bilirubin level, and the third had mildly elevated serum lipase and amylase levels.
Eighteen of the 19 patients underwent complete pancreatic tumor resection by either pancreaticoduodenectomy (i.e., the Whipple procedure) (n = 8) or distal pancreatectomy (n = 10). In the remaining patient, the diagnosis of solid pseudopapillary tumor was established by percutaneous biopsy because the mass encased the mesenteric vessels and small-bowel mesentery and was considered unresectable.
MR Imaging Technique and Analysis
The MR examination consisted of unenhanced T1-weighted spin-echo imaging
(TR range/TE range, 150–632/10–32) and T2-weighted fast spin-echo
imaging (2000–5000/30–112) in all patients. Spoiled
gradient-recalled echo sequences (175–857/18–32) were performed
before and after IV administration of gadopentetate dimeglumine (2 mmol/kg) in
10 patients.
All MR images were evaluated retrospectively by two abdominal radiologists
in a consensus fashion. The following morphologic features were evaluated:
location of the tumor in the pancreas (head, neck, body, or tail); maximal
transverse diameters of the tumor; shape (round, oval, or lobulated); the
presence and thickness of a capsule (thin [< 2 mm] or thick [
2 mm]);
signal intensity features of the capsule (higher or lower than the lesion);
and presence of calcifications. Signal intensity characteristics of the lesion
were compared with those of surrounding pancreas and were described as hypo-,
iso-, or hyperintense and as homogeneous or heterogeneous. Areas of water
signal intensity within the tumor on T1- and T2-weighted images, approximating
the signal intensity of cerebrospinal fluid, were classified as cystic.
Furthermore, areas of high signal intensity on T1-weighted images with
corresponding low signal intensity on T2-weighted images were interpreted as
representing hemorrhagic components. The fraction of tumor composed of cystic
material versus solid was estimated and expressed as a percentage. The amount
of hemorrhage was quantified in relation to the size of the lesion. Finally,
the contrast enhancement pattern of the lesion and capsule, when present, was
evaluated in 10 patients during the arterial (n = 8 patients), portal
venous (n = 7 patients), and equilibrium phases (n = 8
patients) after gadolinium administration. Images were also evaluated for bile
duct obstruction; pancreatic duct obstruction; and spread to regional
vasculature, lymph nodes, adjacent solid organs, and distant abdominal
sites.
Pathologic Examination and Analysis
Two gastrointestinal pathologists at the participating institutions
reviewed the gross tumor specimen and microscopic slides. MR imaging features
were correlated with gross pathologic and histologic findings in each case. In
addition to the morphologic characteristics of the mass, pathologists noted
the presence of lymphadenopathy, visceral metastases, bile or pancreatic duct
dilatation, displacement or encasement of visceral vessels, and involvement of
the surrounding organs.
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Compared with the pancreatic parenchyma, the lesions appeared heterogeneously hyperintense (n = 8), heterogeneously hypointense (n = 6), and homogeneously hypointense (n = 5) on T1-weighted images (Fig. 1A, 1B). Hemorrhage was seen in 14 cases and constituted more than 75% of the mass in six patients, less than 25% in six patients, between 50% and 75% in two patients, and was absent in five patients. Fluid–fluid levels within the mass, representing "hematocrit effect," were observed in two patients. On T2-weighted images, solid pseudopapillary tumor showed heterogeneously hyperintense (n = 12), homogeneously hyperintense (n = 5), and heterogeneously hypointense (n = 2) signal intensity (Fig. 2A, 2B). Solid pseudopapillary tumor appeared greater than 75% cystic in nine patients, less than 25% cystic in five patients, 50–75% cystic in two patients, and completely solid in three patients. All but one lesion had a peripheral hypointense rim (thick, n = 10; thin, n = 8) on T1- and T2-weighted images (Fig. 3A, 3B, 3C, 3D). Tumor calcifications were not observed.
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During the arterial phase of gadolinium administration (n = 8), the lesions showed mild peripheral heterogeneous enhancement (n = 6) or complete homogeneous enhancement (n = 2) (Fig. 4A, 4B, 4C). During the portal venous (n = 7) and equilibrium (n = 8) phases, the majority of the lesions (6/8) showed progressive heterogeneous incomplete fill-in (Fig. 4A, 4B, 4C). In two patients with completely solid lesions, mild complete homogeneous enhancement of the lesions was observed during the portal venous phase, with one lesion appearing isointense in relation to normal pancreas. Similarly, during the equilibrium phase, complete homogeneous enhancement of the lesions was seen in two patients. When compared with normal pancreas, these lesions appeared iso- and hyperintense, respectively. The capsule surrounding the lesion, when compared with the lesion, enhanced early and predominantly in seven of 10 patients, similarly in two patients, and less than the lesion in one patient (Fig. 3A, 3B, 3C, 3D).
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Encasement of mesenteric vessels and invasion of the splenic hilum were observed in one patient each. Displacement of the surrounding vascular structures and organs was observed in four patients, and the mass was contiguous to spleen in two others. Despite the large size of the masses, main pancreatic duct dilatation and bile duct dilatation were absent in all but two cases. No liver metastases were detected on MR imaging.
Correlation with Gross Pathology: Morphologic and Immunohistochemical
Features
Eighteen of the 19 tumors were resected. The tumors appeared oval
(n = 11), round (n = 4), or lobulated (n = 3). The
greatest diameter of the masses was 9.2 cm. All resected tumors were
surrounded by a fibrous pseudocapsule, which measured more than 2 mm in 10 of
the 18 specimens. These gross morphologic features correlated well with the MR
findings.
The proportion of solid and cystic composition of the tumors varied from almost totally solid to markedly cystic, as detected on MR imaging. Most had an intermediate appearance with a mixture of solid and cystic regions. These regions displayed a variegated appearance and were dark red and typically soft and friable with areas of hemorrhage and necrosis. The solid components with little or no hemorrhagic necrosis were gray or yellow macroscopically, were hypo- to isointense with the pancreas on T1-weighted images, and had slightly increased signal intensity on T2-weighted images. The cystic spaces were filled with bloody fluid or tissue debris. Both fluid–fluid levels depicted on MR imaging in two patients corresponded with areas of hemorrhagic degeneration. Microscopically, the solid portions of the tumors revealed sheets of uniform polygonal cells with well-vascularized stroma and variable degenerative changes consistent with the variegated appearance of the gross specimen. The cells adjacent to feeding vessels remained intact, whereas the cells that were most remote from the small vessels had undergone swelling and degenerative changes. Between the solid and cystic portions of the tumor, there was a pseudopapillary pattern characterized by papillary protrusions of epithelium above fibrovascular stalks surrounded by hemorrhage and debris. Although no calcifications were observed on MR imaging, one tumor had microscopic calcifications.
Immunohistochemically, specimens stained positively for
1-antitrypsin (n = 5), neuron-specific enolase
(n = 4), vimentin (n = 2), antichymotrypsin (n =
2), keratin (n = 1), and progesterone receptors (n = 1).
Tumor cells showed negative findings when stained with the following
neuroendocrine markers: insulin, glucagon, somatostatin, serotonin, gastrin,
vasoactive intestinal peptide, bombesin, chromogranin, and lysozyme. On
electron microscopy, no neurosecretory granules were seen.
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Solid pseudopapillary tumor of the pancreas was first reported by Frantz in 1959 [19] who named it "papillary tumor of pancreas, benign or malignant." Solid pseudopapillary tumors are uncommon pancreatic neoplasms accounting for approximately 1–2% of all exocrine tumors of the pancreas [18]. The precise incidence of solid pseudopapillary tumor is not known because it is rare and is frequently misdiagnosed. Todani et al. [20] reported four women with solid pseudopapillary tumor that had been previously misdiagnosed as a non-functioning islet cell tumor or carcinoma. The pathology of solid pseudopapillary tumor of the pancreas was first described by Hamoudi et al. in 1970 [21] and was advocated as a distinct clinical entity by Klöppel et al. in 1981 [22]. In the past, the terminology used to define solid pseudopapillary tumor was not uniform and included "papillary cystic neoplasm of the pancreas" [23]; "solid" and "papillary epithelial pancreatic tumor" [24, 25]; "solid cystic papillary tumor" [26, 27]; "solid" and "cystic acinar cell tumor" [22]; "papillary tumor of the pancreas" or "Frantz's tumor" [19]; and "papillary epithelial neoplasms" [21].
In our series, all but two patients were female (89%) with a mean age of 30 years. Mao et al. [28], in their cumulative review of the literature, found that 90% of patients were female with a mean age of 23.9 years. Although no race predilection was observed in our study, this tumor has been reported to occur predominantly in young black or East Asian women [3, 12, 16].
The pathogenesis of these tumors is still controversial
[27,
29,
30]. Whereas some authors
postulate an endocrine origin
[27], others claim these
tumors arise from ductal cells
[23], acinar cells
[25], or pluripotent stem
cells [30]. The coexistence of
solid pseudopapillary tumor with pregnancy in three of 19 patients, polycystic
ovary disease in two patients, and Sertoli-Leydig cell tumor in one patient
lends support to an endocrine pathogenesis. Moreover, a hormonal influence is
also suggested in view of the tumor's high prevalence in women
[27,
28]. As described by others,
the tumor cells in our patients stained diffusely positive for vimentin and
1-antitrypsin
[31]. Therefore, solid
pseudopapillary tumor shows immunohistochemical and ultrastructural evidence
of both a neuroendocrine and an acinar–ductal differentiation,
suggesting that this tumor arises from a pluripotent stem cell
[30,
31].
Our study patients presented with symptoms in 89% of the cases, which is similar to a prior report [7]. Unlike prior reports, our series showed no predilection for location of these tumors in the pancreatic tail [3, 12, 15]. Although this tumor can grow up to 16 cm [23], it generally displaces surrounding structures rather than invading them. Furthermore, because of its softness, solid pseudopapillary tumor rarely causes bile duct or pancreatic duct obstruction, even when it is located in the head of the pancreas [9]. We observed invasion of mesenteric vessels and the splenic hilum in one patient each, but no evidence of distant metastatic disease was detected radiologically or surgically at the time of diagnosis. Preoperative identification of metastatic disease is, however, of considerable importance because solid pseudopapillary tumor is a low-grade malignant tumor that is associated with an excellent prognosis with complete resection [16]. Disease-free intervals up to 21 years after surgery have been reported [32]. Death due to tumor growth, liver metastases, or peritoneal dissemination is rare [14].
The sonographic and unenhanced CT appearances of solid pseudopapillary tumor have been described in the literature [3–6, 8, 10–13]. Only scattered descriptions of the MR imaging features, consisting mostly of unenhanced MR imaging, in a total of 19 patients have been reported [8, 9, 12, 14–16]. In particular, little is known about the CT and MR imaging enhancement patterns of solid pseudopapillary tumor or about the temporal distribution of the enhancement using multiphasic imaging [3–5, 8, 12, 14–16]. Although in previous reports all solid pseudopapillary tumors contained some high signal intensity on T1-weighted images, in our study, five patients did not have this finding; therefore, the absence of high T1 signal should not exclude the diagnosis.
On T2-weighted images, we observed that 14 lesions (73.5%) appeared heterogeneous, and all lesions but two (89%) appeared predominantly hyperintense. The T2-weighted features were not thoroughly described by Ohtomo et al. [14] and Buetow et al. [12]. Ohtomo et al. reported findings for six patients: the masses appeared hyperintense in four patients and mixed—that is, hyper- and hypointense—in two patients. Buetow et al. reported areas of high signal intensity on T2-weighted images in nine patients and areas of low signal intensity in three patients. However, they did not report the predominant signal intensity of the masses or whether the lesions appeared homogeneous.
Our series shows that the most common enhancement pattern of solid pseudopapillary tumor consists of early, peripheral, and heterogeneous enhancement during the arterial phase with progressive but heterogeneous fill-in of the lesion during the portal venous and equilibrium phases. All but one lesion enhanced less than the adjacent normal pancreas during all phases. This feature helps distinguish solid pseudopapillary tumor from other pancreatic neoplasms, such as islet cell tumors, that typically enhance more than the pancreas.
All but one tumor in our series had a peripheral hypointense tumor capsule of variable thickness on both T1- and T2-weighted images. Compared with the lesion, the tumor capsule showed early and more intense enhancement (70%), identical enhancement (20%), or less enhancement (10%). Ohtomo et al. [14] reported that four patients (67%) presented with a fibrous capsule detected as a band of low signal intensity on T1- and T2-weighted imaging. Conversely, Buetow et al. [12] observed in all their patients a discontinuous low-signal-intensity rim on T2-weighted images. However, the signal intensity characteristics on T1-weighted images and the contrast enhancement behavior were not reported. Our finding of a fluid–fluid level in two patients (10%) was previously reported by Buetow et al. [12] in 10 (18%) of 56 patients examined on CT and MR imaging.
Serous microcystic adenoma, mucinous cystic neoplasm, cystic islet cell tumor, pancreaticoblastoma, and calcified hemorrhagic pseudocyst are differential diagnostic considerations when a pancreatic mass consists of cystic and solid components. However, the former three tumors occur rarely in patients younger than 30 years [16]. On T1-weighted MR images, serous microcystic adenoma may appear as a heterogeneous hypointense mass with hyperintense foci related to prior hemorrhage and could be confused with solid pseudopapillary tumor. However, on T2-weighted images, serous microcystic adenoma shows hyperintense signal intensity with low-signal-intensity central areas due to scar formation and hypointense septa that may radiate toward an enhancing central scar [16, 33]. This radial central scar is not a feature of solid pseudopapillary tumor.
Mucinous cystic neoplasms may be confused with solid pseudopapillary tumor, particularly when the solid pseudopapillary tumor is predominantly cystic. Mucinous cystic neoplasms may also contain fluid–fluid levels, suggesting a hemorrhagic component [16, 33]. Although the mucin-filled cystic spaces are typically hyperintense on T2-weighted images and hypointense on T1-weighted images, mucin occasionally results in high signal intensity on both T1- and T2-weighted images [33]. The signal intensity may vary depending on the proteinaceous content of the mucin. Also, mucinous cystic neoplasms do not exhibit early peripheral and capsular enhancement.
Although islet cell tumors occur in patients who are older and do not have the female predominance observed with solid pseudopapillary tumor, islet cell tumors may appear cystic, contain calcifications, and show areas of internal hemorrhage [34–36]. Islet cell tumors are low in signal intensity on fat-suppressed T1-weighted images, exhibit high signal intensity on T2-weighted images, and show marked ring or diffuse heterogeneous enhancement on immediate gadolinium-enhanced gradient-echo images [34]. Occasionally, a fluid–fluid level may be present [34]. Features that help distinguish solid pseudopapillary tumor from islet cell tumor of the pancreas are the different signal intensity on T1-weighted images and, because islet cell tumors are more vascular than solid pseudopapillary tumor and normal pancreas, the different contrast enhancement pattern after gadolinium injection.
Pancreatoblastoma is more aggressive than solid pseudopapillary tumor and often presents with liver metastases at the time of diagnosis [37]. Cystic components due to liquefaction necrosis may be present, but intratumoral hemorrhage has not been reported in these tumors using either CT or MR imaging [37, 38]. Although pancreatic adenocarcinoma is the most common primary pancreatic malignancy, it is seen in older patients [39]. Pancreatic adenocarcinoma is usually smaller and virtually never contains calcification or cystic degeneration. Pancreatic pseudocysts may be calcified peripherally as a result of internal hemorrhage and may mimic solid pseudopapillary tumor [17]. However, a history of pancreatitis is almost always present, and pancreatic pseudocysts can be distinguished from solid pseudopapillary tumor by the absence of solid components.
In conclusion, in the appropriate clinical setting, MR imaging features can be highly suggestive for the diagnosis of solid pseudopapillary tumor. This tumor should be considered when a well-marginated, large, encapsulated, solid and cystic mass with areas of hemorrhagic degeneration, as revealed by high signal intensity on T1-weighted imaging, and progressive peripheral or slightly heterogeneous contrast enhancement, seen after gadolinium administration on dynamic examination, is detected in the pancreas of a young woman.
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