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Original Research |
1 Department of Radiology and Institute of Radiation Medicine, Seoul National
University Hospital and College of Medicine, 28 Yongon-dong, Chongno-gu, Seoul
110-744, Korea.
2 Present address: Department of Radiology, Chungnam National University
Hospital, Seoul, Korea.
Received February 27, 2005;
accepted after revision June 17, 2005.
Address correspondence to J. K. Han.
Abstract
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MATERIALS AND METHODS. Forty-one patients with histologically confirmed macrocystic neoplasms of the pancreas were enrolled: serous oligocystic adenoma in 10 patients, mucinous cystadenoma in 13, and intraductal papillary mucinous tumor in 18. Location, greatest dimension, shape, presence of mural nodules, presence of wall calcification, and the extent and degree of main pancreatic duct (MPD) dilatation were analyzed with CT. The lesions were categorized into seven groups according to their shapes: multicystic, lobulated contour with and without internal septation, smooth contour with and without internal septation, pleomorphic cystic, and clubbed fingerlike cystic. Comparative studies were performed using Fisher's exact test and the Mann-Whitney U test.
RESULTS. Significant differences in lesion shape were found between serous oligocystic adenoma and the other macrocystic neoplasms (mucinous cystadenoma [p < 0.05], intraductal papillary mucinous tumor [p < 0.05]). Serous oligocystic adenoma had a multicystic or lobulated contour with or without septation, whereas mucinous cystadenoma had a smooth contour with or without septation and intraductal papillary mucinous tumor had either a pleomorphic or a clubbed fingerlike cystic shape. Serous oligocystic adenoma showed proximal MPD dilatation from the lesion, whereas intraductal papillary mucinous tumor showed distal or whole MPD dilatation (p < 0.05). No significant difference was apparent among the three diseases in terms of location, greatest dimension, or presence of calcification or mural nodules.
CONCLUSION. Serous oligocystic adenoma of the pancreas has characteristic CT findings that differentiate it from other cystic tumors. It appears as a multicystic or lobulated cystic lesion with septation.
Keywords: abdominal imaging • CT • intraductal papillary mucinous tumor • macrocystic serous adenoma • mucinous cystadenoma • pancreas • pancreaticobiliary imaging • serous oligocystic adenoma
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Pathologists say that the macroscopic approach to diagnosis makes it easy to differentiate serous oligocystic adenoma from serous microcystic adenoma because the latter macroscopically tends to be a spongelike cystic lesion lined by uniform glycogen-rich cuboidal epithelial cells. The question is whether serous oligocystic adenomas have macroscopic findings similar to other macrocystic tumors such as mucinous cystadenoma or intraductal papillary mucinous tumor. Since the World Health Organization subclassified serous macrocystic adenoma (a synonym for serous oligocystic adenoma) as a subgroup of pancreatic serous cystic tumors, for example, many authors have reported that several cases of serous oligocystic adenoma have been misdiagnosed as mucinous cystadenoma and inappropriately managed [6-16].
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Sixty patients were identified as having one of these three pancreatic macrocystic neoplasms. Seventeen were excluded because of the nonavailability of appropriate CT images. Thus, 43 patients were selected for pathologic review. An experienced pathologist reconfirmed the diagnoses by microscopic review of pathology slides. However, as a result of the pathologic review, two patients were excluded because their tumors were microcystic serous cystadenomas, even if they were included initially because their initial pathologic diagnosis was serous cystadenoma. Therefore, 41 patients were finally enrolled. These comprised 13 men and 28 women (mean age, 54 years; age range, 32-77 years). The final diagnoses were serous oligocystic adenoma in 10 patients (age range, 34-70 years; mean age, 48 years; 2 men, 8 women), mucinous cystadenoma in 13 (age range, 32-77 years; mean age, 49 years; all women), and intraductal papillary mucinous tumor in 18 (age range, 49-74 years; mean age, 60 years; 11 men, 7 women).
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Helical CT was performed using a single-detector scanner and the following parameters: 5-mm collimation, 1:1 table pitch, and 5-mm reconstruction intervals. If an MDCT scanner was used, the parameters were 2.5-mm detector collimation, 20 mm/s table speed, 3.2-mm slice thickness, and 1.6-mm reconstruction interval. Unenhanced CT was performed using the same parameters. Multiphasic helical CT scans were obtained at 30 seconds (arterial phase), at 40 seconds (pancreatic phase), and at 70 seconds (portal venous phase) after the initiation of the contrast injection.
CT Imaging Analysis
Two experienced abdominal radiologists who were blinded to the specific
diagnoses and clinical information reviewed the CT images by consensus in
terms of the following morphologic features of the lesions: location (head,
neck, body, or tail), greatest dimension (measured in centimeters), presence
of mural nodules (present or absent), presence of wall calcification (present
or absent), extent of main pancreatic duct (MPD) dilatation (none, diffuse,
distal to, or proximal from the tumor), degree of MPD dilatation (normal,
mild, moderate, or severe), and tumor shape. All CT scans were reviewed on a
PACS workstation.
The presence of wall calcification was determined using unenhanced images. The greatest diameter of the tumor and the extent of MPD dilatation were measured with images obtained during the portal venous phase by one of the reviewers. The degree of MPD dilatation was considered mild if the diameter was more than 2 mm, moderate if more than 4 mm, and severe if more than 6 mm. CT images of axially reconstructed sectional planes were reviewed.
Categorization of Shape
To identify morphologic features that can be used to differentiate these
tumors, tumor shape was categorized as multicystic, lobulated contour with or
without internal septation, smooth contour with or without internal septation,
pleomorphic cystic, and clubbed fingerlike cystic. A "multicystic
shape" was defined as a conglomeration of two or more round evenly sized
cysts (Fig. 1A). A
"lobulated shape" was defined as the shape of a simple closed
curve that could not be described as the borders of the same circle (Figs.
1B and
1C). A "smooth
shape" was a simple closed curve with the borders of the same circle
(Figs. 1D and
1E).
In order to differentiate the multicystic and multilobulated shapes with internal septation, a "pleomorphic cystic shape" was defined as one containing three or more cysts that should include more than one oval or tubular cyst [1] (Fig. 1F). A tubular or oval-shaped cyst was defined as having a conic section with a plane that was not parallel to the axis, base, or generatrix of the intersected cone differentiating it from a round cyst. Finally, a "clubbed fingerlike cystic shape" was defined as one having one or two tubular or oval cysts (Fig. 1G).
Because we needed to differentiate the lobulated type with septation from the multicystic, pleomorphic cystic, and clubbed fingerlike cystic shapes, we defined an internal locule, seen in the lobulated and smooth shapes with septation, as an angular closed curve without protrusion from the border of a main lesion, whereas we defined a cyst as a simple closed curve without concavity.
Statistical Analysis
The mean longest diameters of each type of lesion were compared using the
Mann-Whitney U test. Fisher's exact test was used to compare other
morphologic features. A p value of less than 0.05 was considered to
indicate a statistically significant difference. Because we wanted to identify
features capable of differentiating serous oligocystic adenoma of the pancreas
from other cystic tumors of the pancreas, statistical analysis was not
performed for comparisons between mucinous cystadenoma and intraductal
papillary mucinous tumor.
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Mucinous cystadenomas ranged from 2.0 to 12.5 cm (average, 6.07 ± 3.27 cm). Eleven (84.6%) of the 13 cysts were located in the tail of the pancreas. Twelve patients (92.3%) had no mural nodules, and 4 (30.8%) had wall calcification. Three patients (23.1%) showed MPD dilatation (2, mild proximal dilatation; 1, diffuse moderate dilatation). In terms of shape, 12 cysts (92.3%) had a smooth shape (septate, 7; nonseptate, 5) (Fig. 4; Tables 1 and 2).
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When serous oligocystic adenoma was compared with mucinous cystadenoma, serous oligocystic adenoma showed a statistically significant difference in lesion shape (p = 0.001). Serous oligocystic adenoma tended to have the multicystic or the lobulated shape (Figs. 2, 3A, and 3B). On the other hand, mucinous cystadenoma tended to have a smooth shape (Table 1). Tumor location (p = 0.108), greatest dimension (p =0.111), presence of mural nodules (p = 1.0), presence of wall calcification (p = 0.104), and extent (p = 1.0) and degree (p = 0.476) of MPD dilatation were statistically inadequate for differentiating serous oligocystic adenoma from mucinous cystadenoma (Table 2).
Intraductal papillary mucinous tumors ranged from 1.0 to 5.1 cm (average, 3.09 ± 1.25 cm). In 12 (66.7%) of 18 patients, cystic lesions were located in the body or the tail of the pancreas, and in 5 patients (27.8%) lesions were located in the head of the pancreas. Only one intraductal papillary mucinous tumor (5.6%) had mural nodules, and only 3 (16.7%) had wall calcification. Six patients (33.3%) had diffuse MPD dilatation. In terms of shape, 10 lesions (55.6%) showed the pleomorphic cystic shape. Four lesions (22.2%) had the clubbed fingerlike cystic shape (Figs. 5A, 5B, 5C, 6A, and 6B; Tables 1 and 2).
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Several authors have attempted to identify radiologic findings capable of differentiating serous oligocystic adenoma from other macrocystic neoplasms of the pancreas and have reported the limited value of CT for this purpose [1, 2, 6-8, 10-13, 15, 16]. However, those previous reports were limited because detailed CT findings such as tumor shape or accompanying MPD dilatation were not considered during image analysis. More recently, Cohen-Scali et al. [14] found that CT was helpful for differentiating serous oligocystic adenoma and mucinous cystadenoma, and they described specific CT findings indicating serous oligocystic adenoma—namely, location in the pancreatic head, a lobulated contour, and the absence of wall enhancement. However, those authors did not include intraductal papillary mucinous tumors in their study population. On the basis of our clinical experiences, serous oligocystic adenoma is sometimes misdiagnosed as intraductal papillary mucinous tumor, especially as the branch duct type. Thus, we included intraductal papillary mucinous tumor as a subgroup.
In this study, we found that lesion shape was the most useful feature among the seven imaging features for the differential diagnosis of the three types of cystic neoplasms of the pancreas. The multicystic and lobulated shapes were prevalent in serous oligocystic adenomas, the smooth shape in mucinous cystadenomas, and the pleomorphic cystic and the clubbed fingerlike cystic shapes in branch duct type intraductal papillary mucinous tumors. This result is in accord with the findings of Santos et al. [20] regarding serous oligocystic adenoma, in which serous oligocystic adenomas were most commonly found to be composed of a few cysts and to contain tiny cysts that were irregularly arranged and separated by mostly broad septa. The key morphologic feature of intraductal papillary mucinous tumor can be considered to be a presentation of branch pancreatic duct dilatation. In our study, dilated branch pancreatic ducts appeared as clusters of small cysts with a grapelike appearance, or as a multilocular cyst with papillary projections, or as a single cystic lesion with lobulated or irregular margins communicating with a dilated or normal MPD. This result is in accordance with the findings of previous studies [17, 18].
Of the seven imaging features, the extent of MPD dilatation was also an important morphologic feature that allowed differential diagnosis between serous oligocystic adenoma and branch duct type intraductal papillary mucinous tumor. If combined with dilated MPD, diffuse or distal MPD dilatation was exclusively observed in intraductal papillary mucinous tumor, whereas proximal MPD dilatation tended to be observed in serous oligocystic adenoma. Diffuse MPD dilatation in intraductal papillary mucinous tumor was mostly associated with mucin secreted from the tumor, whereas proximal MPD dilatation in serous oligocystic adenoma is probably a mass effect due to extrinsic compression [18, 21].
In previous reports [7, 13], wall calcification was considered to be more frequent in serous cystic tumors than in mucinous cystadenomas. However, in our study, no case of serous oligocystic adenoma showed wall calcification. On the other hand, mucinous cystadenoma and intraductal papillary mucinous tumor had some calcified walls. Thus, wall calcification was not found to be a useful feature for the differentiation of the three diseases. In addition, several previous studies [14, 16, 19] reported that lesion location and patient sex and age may be distinguishing features for these three disease entities at diagnosis. In our study, these features did not show statistical significance. However, because our study population was small, these findings must be confirmed by a large series.
Our study has several limitations. First, this was a retrospective study and was subject to all biases affecting such studies, including that radiologic-pathologic correlation was not possible. Second, different CT scanners were used, and therefore CT parameters are not the same in each patient (5-mm detector collimation in single-detector CT vs 2.5 mm in MDCT). Last, the number of patients who were enrolled in this study was small. Because MDCT provides improved spatial resolution that allows good-quality multiplanar reformation compared with single-detector CT, further study in large population using MDCT will be warranted.
In conclusion, serous oligocystic adenoma of the pancreas has typical CT findings that differ from other macrocystic neoplasms such as mucinous cystadenoma and intraductal papillary mucinous tumor. The typical imaging features of serous oligocystic adenoma are either a multicystic or a lobulated cystic lesion with or without internal septation (specificity, 90%). The typical imaging features of mucinous cystadenoma are a smooth cystic lesion with or without internal septation, and those of intraductal papillary mucinous tumor are either a pleomorphic cystic or clubbed, fingerlike cystic lesion. In terms of MPD dilatation, if MPD dilatation is detected, serous oligocystic adenoma has proximal MPD dilatation on CT.
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