Microcystic adenoma of the pancreas

Clinical, radiologic, and cytologic features

Authors

  • Aseem Lal M.D.,

    1. Department of Pathology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois
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  • Eleni P. Bourtsos M.D.,

    1. Department of Pathology, Adventist Health System Midwest Region, Hinsdale, Illinois
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  • Denise V. S. DeFrias M.D.,

    1. Department of Pathology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois
    2. Department of Obstetrics and Gynecology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois
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  • Albert A. Nemcek M.D.,

    1. Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois
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  • Ritu Nayar M.D.

    Corresponding author
    1. Department of Pathology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois
    2. Department of Pathology, Northwestern Memorial Hospital, Chicago, Illinois
    • Department of Pathology, Feinberg Pavilion 7-210, Northwestern Memorial Hospital, 251 East Huron Street, Chicago, IL 60611
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    • Fax: (312) 926-6037


Abstract

BACKGROUND

Cytology literature on pancreatic microcystic adenoma is sparse. It is important to separate microcystic adenoma from adenocarcinoma and mucinous cystic neoplasms on aspiration cytology, because patients with microcystic adenoma can be treated conservatively unless they are symptomatic. Potential pitfalls with endoscopic ultrasound (EUS) sampling of these lesions is discussed.

METHODS

From January 1991 through June 2003, 10 patients with microcystic adenoma of the pancreas were diagnosed on fine-needle aspiration cytology. An additional patient, who was diagnosed with a mucinous cystic neoplasm by EUS sampling, was rediagnosed with microcystic adenoma on the excised specimen. Aspirate smears, cell blocks, core biopsies, subsequent excision (if any), and special stains were reviewed. Imaging studies and clinical data were available from the majority of patients.

RESULTS

Clinical results: The patients included 5 females and 6 males who ranged in age from 45 years to 84 years. Radiology studies showed tumors, which were heterogeneous with areas of fluid density and septations, located in the head, body, or tail of the pancreas. The masses ranged in size from 1 cm to 17 cm. The radiographic impression was highly suggestive of microcystic adenoma in six patients; detailed radiologic information was not available from three patients. On follow-up, six patients were alive and well at the last follow-up available, two patients died of unrelated sepsis, and three patients were lost to follow-up. Three of 11 patients underwent a Whipple resection. Cytology results: The cytologic features identified included the following: 1) Bland tumor cells were seen in sheets or small groups with a lack of nuclear abnormalities and moderate-to-scant cytoplasm with occasional clearing or vacuolation. Naked nuclei were present occasionally. Tumor cells were distinguishable from acinar cells based on larger cell size and granular cytoplasm in which prominent nucleoli were seen. 2) Relatively acellular, fibrovascular stroma was seen, usually located between tumor cells. 3) Calcifications were seen in four of eleven tumors. One tumor sampled by EUS revealed fragments of glandular-type epithelium with minimal atypia and was diagnosed erroneously as a mucinous cystic neoplasm. Cell blocks or core biopsies from most tumors showed fragments of dense stroma and cystic spaces lined by flattened epithelial cells. Subsequent Whipple resection in three patients showed histologic features of microcystic adenoma. Special stains performed in select tumors were positive for cytokeratin, carbohydrate antigen 19.9, and periodic acid–Schiff stain. Calretinin staining was negative in the tumor cells.

CONCLUSIONS

A cytologic diagnosis of microcystic adenoma is possible based on the criteria described above. Cell block and/or core biopsy, special stains, and radiologic information are key in making a definitive diagnosis. Patients with microcystic adenoma are spared a major surgical procedure unless they are symptomatic. With the EUS-guided modality of pancreatic sampling, caution should be exercised in misinterpreting benign glandular epithelium derived from the stomach or small bowel as a mucinous cystic neoplasm. Cancer (Cancer Cytopathol) 2004. © 2004 American Cancer Society.

Approximately 2% of pancreatic tumors are cystic. They are divided broadly into mucinous cystic neoplasms of uncertain malignant potential and nonmucinous neoplasms that either are benign or have low malignant potential. The latter category includes microcystic adenoma (MCA), or serous cystadenoma, and solid pseudopapillary neoplasm (SPN).1 MCAs of the pancreas are relatively rare, most often benign neoplasms that usually occur in elderly patients and have no particular gender predilection.2–4 MCAs are distributed evenly throughout the pancreas and most likely originate from the centroacinar cell. The most common clinical presentation is abdominal or epigastric discomfort, nausea, emesis, or weight loss caused by the large size of the mass.5 Approximately one-third of these neoplasms are incidental findings on routine examination or at autopsy.6 Their cystic nature may not always be apparent on imaging studies, because findings from such studies may depend on the technique used.

Cytologic descriptions of cystic neoplasms of the pancreas are few.7–12 Characteristic cytologic features of MCA include clusters of benign appearing, cuboidal epithelial cells with oval-to-round nuclei, inconspicuous nucleoli, and clear cytoplasm. Calcification and/or fibrous stroma can be seen on fine-needle aspiration (FNA). Herein, we report the aspiration cytology findings from 11 patients with MCA of the pancreas, the largest FNA series to date, along with immunohistochemical studies and radiologic correlation.

MATERIALS AND METHODS

This study was approved by the institutional review board of Northwestern University.

The computerized files of the Cytopathology Division of Northwestern Memorial Hospital were searched for patients who were diagnosed with MCA between January 1, 1991 and June 30, 2003. Eleven patients were identified who had cytologic evaluation by FNA with or without a cell block/core biopsy. Three of these patients had surgical follow-up.

Percutaneous FNA of the pancreas was performed in interventional radiology using ultrasonography (US) or computerized tomography (CT) in 10 of 11 patients. Specimens were obtained using 18/22-gauge needles and/or an 18/20-gauge core device. One tumor was sampled using an endoscopic ultrasound (EUS)-guided technique. Direct smears for on-site evaluation were air-dried and stained with a modified Romanowsky staining procedure, and specimen adequacy was determined on site by a cytopathologist. Part of the aspirated material was smeared on glass slides, immediately fixed in 95% alcohol, and subsequently stained with Papanicolaou stain. The needle and syringe were rinsed in RPMI media for the preparation of cell blocks. Core biopsies were fixed in 10% buffered formalin, paraffin embedded, and stained with hematoxylin and eosin. Histochemical and immunohistochemical studies for periodic acid–Schiff (PAS), cytokeratin (AE1/AE3), calretinin, epithelial membrane antigen (EMA), carbohydrate antigen 19-9 (CA 19-9), vimentin, S-100, neuron-specific enolase (NSE), chromogranin, and factor VIII were performed on tumors if tissue was available using the avidin-biotin peroxidase complex technique. Appropriate positive and negative controls were run simultaneously.

RESULTS

Clinical and Radiologic Findings

There were 11 patients, including 5 females and 6 males, who ranged in age from 45 years to 84 years. The most common presenting symptoms (five of nine patients) included abdominal bloating, fullness, or discomfort. One patient presented with weight loss, pruritis, and obstructive jaundice; and another patient presented with pancreatitis. In one patient with von Hippel–Lindau disease, and another with an abdominal hernia the pancreatic mass was an incidental finding, and the clinical presentation was unknown in the remaining two patients. Six of 11 patients were alive and well at the last follow-up available, and 2 patients underwent stent placement after their diagnoses for relief of jaundice and a stricture, respectively. Three patients underwent a Whipple surgical procedure for symptomatic relief in 1 patient (Table 1, Patient 5), for atypical radiologic features and scant biopsy tissue in the second (Table 1, Patient 11) and for a cytologic diagnosis of mucinous cystic neoplasm in the third patient (Table 1, Patient 10). Two patients died of unrelated medical conditions. In three patients, follow-up data were unavailable (Table 1).

Table 1. Clinical and Radiologic Findings
Patient no.Age (yrs)GenderClinical presentationRadiologyLocation within pancreasTreatmentFollow-up
  1. F: female; M: male; VHL: von Hippel–Lindau disease; h/o: history of.

184FWeight loss (20 lbs), pruritis, jaundiceLow-density cystic mass (7.0 cm)HeadStent placedStent replaced 2 yrs after diagnosis
281FAbdominal bloatingLow-density mass (17.0 cm), central calcificationHead + bodyNoneAlive and well at 6 mos
365FAbdominal fullnessLow-density mass (10.0 cm)Head + bodyNoneDied of unrelated sepsis
483MAbdominal painLow-density mass (10.5 cm) with cystic componentHead + bodyDeclined surgeryAlive and well at 5 mos
552MAbdominal bloatingLow-density mass (6.0 cm)Uncinate areaPyloric-sparing WhippleAlive and well at 3 mos
645MNot knownLow-density cystic mass (1.2 cm)Not knownNot knownDied 20 mos later due to unrelated condition
781MPancreatitisLobulated mass (4.0 cm), radiologic characteristics unknownBodyNot knownNot known
851Mh/o VHL, incidentalMass (3.0 cm), higher attenuationHeadStent placedRebiopsy 1 yr later due to increase in size was inadequate
957FAbdominal discomfortMass (3.0 cm), radiologic characteristics unknownHead + neckNot knownNot known
1048FNot knownPancreatic head cyst (2.0 cm), radiologic characteristics unknownHeadWhippleNot known
1170MIncidental to symptomatic abdominal wall herniaPancreatic head mass (2–6 cm) suspicious for malignancyHead − uncinateWhippleAlive and well at last follow-up (2 yrs)

The radiologic impression was highly suggestive of MCA in six patients. On unenhanced CT scans, most tumors were of mixed density, which included fluid and soft tissue densities. Compared with liver or pancreas, the lesions were of lower density in the majority of patients. In two patients, calcified foci were noted in the center of the tumor (Fig. 1), and one tumor was described as cystic. Ultrasound performed on one of the lesions showed a hyperechoic mass with through transmission, indicating a multiseptated but fluid-filled mass. The tumors ranged in size from 1 cm to 17 cm, with the largest tumor demonstrating encasement of the inferior vena cava and portal vein (Table 1, Patient 2)

Figure 1.

Computed tomography scan (Patient 2) shows a 17 cm × 13 cm × 10 cm low-density mass in the head and body of the pancreas. The mass exhibits central calcifications and encases the inferior vena cava and portal vein.

Cytologic Findings

The cytologic features were remarkably similar in all tumors. Romanowsky stain showed small clusters and sheets of round-to-polygonal cells and round-to-oval nuclei with minimal pleomorphism. The cytoplasm was scant to moderate, with a soft texture and ill-defined, wispy cell borders and was light gray-blue to purple on Romanowsky stain (Fig. 2). With the Papanicolaou stain, the nuclei demonstrated bland cytology with fine, evenly distributed chromatin and a single, central, inconspicuous nucleolus (Fig. 3). Five tumors showed slightly coarser chromatin. Rare nuclear grooves were noted in three tumors. Eight tumors exhibited a stromal component on FNA that usually was paucicellular, appearing metachromatic on Romanowsky-stained material and green-blue on Papanicolaou-stained material (Fig. 4). More cellular fibrous tissue, which was less metachromatic and contained plump spindle cells, also was identified. Calcifications were noted in three tumors (Fig. 5). The tumor cells were distinguished from acinar cells of the pancreas because the acinar cells had more abundant, well defined, granular cytoplasm and prominent nucleoli. The epithelial cells in some tumors resembled mesothelial cells due to their morphologic features, such as fine chromatin, nuclear grooves, and scant cytoplasm. Distinguishing features included lack of “streaming pattern” seen in mesothelial cells. One tumor, which was sampled with an EUS-guided technique, revealed scant strips of benign gastric epithelium and was diagnosed erroneously as consistent with a cystic mucinous neoplasm.

Figure 2.

Tumor cells with round-to-oval nuclei, minimal nuclear pleomorphism, and moderate amounts of delicate blue-gray cytoplasm with ill-defined cytoplasmic borders. Diff-Quik stain. Original magnification ×400.

Figure 3.

Sheets of bland tumor cells with fine, evenly distributed nuclear chromatin and small, usually single nucleoli. The cytoplasm is wispy and focally clear, and the cytoplasmic borders are not well defined. Papanicolaou stain. Original magnification ×200.

Figure 4.

Metachromatic paucicellular stroma (left) with attached sheets of tumor cells (right). Diff-Quik stain. Original magnification ×400.

Figure 5.

Calcifications were detected in 3 patients. Papanicolaou stain. Original magnification ×400.

Macroscopic Pathology

The macroscopic specimen from Patient #5 who underwent a Whipple resection revealed a 6 cm × 5 cm × 5 cm, spherical mass with lobulated edges that were well demarcated from the adjacent pancreas. The tumor was spongy and multicystic on the cut surface. The cysts contained clear, serous fluid. A central stellate scar was noted in this neoplasm (Fig 6). The Whipple resection from patient #10 who had a cytologic impression of mucinous cystic neoplasm on EUS-FNA showed a 2.0-cm, multicystic lesion in the pancreatic head. Patient #11 had a 2.6 × 2.5 cm relatively well circumscribed tumor in the uncinate process, abutting the main pancreatic duct. Cut-section was multilocular and sponge-like with central firmness.

Figure 6.

Multicystic mass with a central scar from Patient #5 in an excised (macroscopic) specimen.

Histology and Immunohistochemistry

The histologic sections from two excised specimens were similar. The cytoplasmic clearing seen on hematoxylin and eosin-stained sections correlated with glycogen, as demonstrated by diffuse PAS positivity, which was sensitive to diastase digestion (D-PAS negative)

The cell blocks and/or core biopsies from eight tumors revealed fibrous stroma and variable numbers of cysts lined by flat-to-low, cuboidal epithelium (Fig. 7). Immunohistochemical stains were performed in five tumors. The cuboidal cells were positive for cytokeratin in 3 of 3 tumors, negative for carcinoembryonic antigen (CEA) in 2 of 2 tumors, positive for CA 19.9 in 2 of 2 tumors, and focally positive for EMA in 1 of 3 tumors. Calretinin staining was performed in two tumors and was noncontributory in one tumor (background staining) and negative in the other tumor. Additional immunostains performed in one tumor revealed the following results: chromogranin, positive; NSE, negative; factor VIII, negative; and vimentin, negative.

Figure 7.

Core biopsy sample obtained at the time of fine-needle aspiration. Cysts are lined with low cuboidal epithelium and adjacent paucicellular stroma. Hematoxylin and eosin stain. Original magnification ×400.

DISCUSSION

Cystic lesions of the pancreas include pseudocysts and other nonneoplastic cysts13 as well as cystic epithelial neoplasms. Pseudocysts account for the majority of cystic lesions in the pancreas.13 Other nonneoplastic cysts include squamous lined cysts (i.e., lymphoepithelial cysts, dermoid cysts, and accessory-splenic epidermoid cysts),14 retention cysts, congenital cysts, enterogenous cysts, endometrial cysts, and paraampullary duodenal wall cysts, to name a few.13 Cystic epithelial neoplasms account for approximately 5–15% of cystic lesions in the pancreas.10 These include, but are not limited to, MCAs (glycogen-rich or serous cystadenomas) and mucinous cystic neoplasms.10 Solid-pseudopapillary tumors of the pancreas also may present clinically as cystic pancreatic tumors.1 In addition to these well known cystic neoplasms, cystic forms of typically solid tumors (islet cell tumors, ductal adenocarcinomas, and acinar cell carcinomas) also have been described.15

MCAs are distributed evenly throughout the pancreas and most likely originate from the centroacinar cells.2 These neoplasms generally occur in older patients, have equal gender predilection, and symptoms usually include abdominal pain and bloating.2, 3 The majority of patients in the current series presented with these symptoms. Serous pancreatic neoplasms are comprised of small cysts (0.5–1.5 cm) lined by flattened to cuboidal cells that contain cytoplasmic glycogen. It is believed that mucinous cystic neoplasms arise from the pancreatic ductal epithelium and predominantly occur in women with a mean age of 49 years.16 Symptoms include epigastric pain/discomfort, anorexia, weakness, and weight loss. Mucinous neoplasms are comprised of large, multilocular, or occasionally unilocular cysts lined by tall, columnar, mucin-producing cells that often form papillae.

Imaging studies often reveal the multicystic nature of MCA and show a mixed fluid and soft tissue density on CT scans17 corresponding to the multiple, usually small cysts in these neoplasms. The CT appearance depends on the relative amounts of connective tissue and fluid content. Sonographically, hypoechoic and echogenic areas are seen, often with a central, radiating, echogenic density (representing the central scar). Although ultrasound highlights the cystic component of these tumors, CT scans are for visualizing the central scar. These neoplasms tend to be demarcated poorly due to the absence of a well developed capsule. Two tumors in this series demonstrated central calcifications: Their appearance can be linear, arcuate, or globular; and they represent dystrophic deposition of calcium in the scar, giving rise to the “sunburst” calcification cited frequently in literature.18 In contrast, mucinous cystic neoplasms are larger and hypovascular with peripheral rather than central calcifications; they have fewer cysts and, due to the presence of a thick, fibrous capsule, are well delineated.18 The radiologic impression was highly suggestive of MCA in 6 of 11 patients in this series.

Aspirate smears from MCA tend to be hypocellular. Diagnostic material is characterized by strips of benign-appearing, cuboidal-to-polygonal cells with delicate, clear cytoplasm; central, bland nuclei; and inconspicuous nucleoli. Naked nucleoli are not unusual. With the modified Romanowsky stain, the cytoplasm is gray-blue with wispy borders. The chromatin is fine and is distributed evenly with small, inconspicuous nucleoli. Occasional nuclear grooves can be seen on Papanicolaou stain. Aspiration of watery fluid and intranuclear cytoplasmic inclusions have been described previously; however, we did not observe them in our patients. Furthermore, aspiration of clear fluid is not a reliable feature, because this also has been described in mucinous cystic neoplasms.9

Fibrous stroma and calcifications are usually an integral component of the aspirate and core biopsy, as demonstrated in this series. On FNA smears, the stroma was present in eight of eleven tumors and varied from paucicellular and metachromatic to fibrous tissue comprised of plump, spindled cells. Calcifications can be seen in both serous and mucinous neoplasms but are more common in MCA and are easier to find on FNA. On core biopsy, MCAs usually are comprised of paucicellular, fibrous stroma that separates small cysts lined by a single layer of flat-to-low, cuboidal epithelium. Histologic and cytochemical stains are helpful in supporting a diagnosis of MCA. The tumor cells contain glycogen within the cytoplasm and, thus, stain with PAS, and they are sensitive to diastase digestion. Keratin (AE1/AE3) and CA 19.9 are positive, and EMA is positive focally.

Compared with MCAs, mucinous cystic neoplasms yield cellular specimens comprised of mucus-producing, columnar cells, usually with extracellular mucin in the background. They will stain positively for mucicarmine and CEA and are resistant to diastase digestion (D-PAS positive). Fibrous stroma of the “ovarian type” can be seen in mucinous neoplasms. Their biologic behavior cannot be predicted by cytology (and, often, not on the excised specimen); thus, these neoplasms need to be excised. EUS-guided FNA sampling currently is considered a safe and reliable procedure and is being used increasingly for the diagnosis and staging of pancreatic lesions.19 However, with this technique, glandular epithelium of the gastrointestinal tract maybe sampled, leading to potential misinterpretation as a mucinous neoplasm. Other tumors that yield mucin and epithelial cells include intraductal papillary mucinous tumor (IPMT) and colloid carcinoma. IPMT is characterized by intraductal papillary growth of mucinous cells accompanied by extracellular mucin with pancreatic duct dilatation, thus mimicking mucinous cystic tumors.20 Cytologic findings need to be correlated with endoscopic and radiologic findings for an accurate diagnosis in these tumors.20 Colloid carcinoma (comprised of pools of mucin with scant malignant cells) does not present as cystic cavities. However, it is associated frequently with mucinous cystic neoplasms and IPMT.21

SPNs are rare tumors that are seen in young females and usually are large and encapsulated with extensive cystic degeneration. Cytologically, these are highly cellular aspirates: They have papillary clusters of small, oval cells that have scant cytoplasm, bland nuclear features, small nucleoli, and occasional nuclear grooves; and they may resemble neuroendocrine tumors.9, 22–24 The presence of numerous capillaries and histiocytes, as well as a panel of immunostains with positivity for CD10 and β-catenin, is helpful in making the diagnosis of SPN.25, 26

From a purely pathologic, differential diagnostic view point, MCA should be distinguished from incidental sampling, i.e., mesothelial cells, normal acinar cells, and other pathologic entities, that can occur in the pancreas and its vicinity. These include neuroendocrine tumors lymphangioma, hemagioma, low-grade renal cell carcinoma (clear cell type), and intrabdominal multicystic peritoneal mesothelioma. Tumor cells from MCA are easy to distinguish from percutaneously sampled pancreatic pseudocysts, which contain inflammatory cells and histiocytes and lack an epithelial cell component. Immunohistochemical stains may be needed to differentiate MCA from other diagnostic possibilities. Lymphangioma, hemangioma, and mesothelioma would be negative for PAS, and hemangioma would be positive for factor VIII. Mesotheliomas are positive for cytokeratin and calretinin. The latter stain was negative in one of our MCA samples and may be used to distinguish cells of mesothelial origin from MCA. The clear cell type of renal cell carcinoma is positive for PAS and keratin, similar to MCA; however, renal cell carcinoma cells are larger with more abundant cytoplasm and prominent nucleoli (in higher grade tumors), often are vimentin positive, and lack the fibrous stroma seen in MCAs. A neuroendocrine tumor should also be considered in the differential diagnosis, since cytologic features can overlap with MCA. Immunohistochemistry using neuroendocrine markers can help to distinguish the two entities.

Although MCAs are considered benign neoplasms, they have been known to cause morbidity and, rarely, mortality in some patients, depending on their size and location. In a series of 34 patients with MCA reported by Compagnio and Oertel, 4 patients died of causes related to their pancreatic tumor, 4 patients died intraoperatively, and 6 patients had complications resulting from surgery.4 There are rare reports of malignant counterparts of MCA27, 28 as well as a patient with coexisting MCA and pancreatic ductal carcinoma.29 Cystic neoplasms of the pancreas may be associated with von Hippel–Lindau disease, as was seen in one of the patients in this series.30

It is important to be able to render a precise diagnosis of MCA, so that patients who are asymptomatic or who have potential surgical risks can be spared extensive resection procedures. Adequate FNA smears, ideally with a core biopsy, usually are sufficient to make a definitive diagnosis of MCA, in conjunction with clinical features. More recently, endoscopic, ultrasound-guided FNA has been employed for patients with pancreatic and nonpancreatic tumors,31, 32 and MCA of the pancreas also can be diagnosed using this technique.33 In summary, correlation between clinical, radiographic, and cytologic features, supported by histochemical and/or immunostains, allows for a precise diagnosis of MCA and leads to appropriate patient management.

Note Added in Proof

Recent follow-up (at 28 months) on Patient 8 was obtained when the patient was rebiopsied due to an increase in size of the pancreatic mass. Repeat biopsy was diagnosed as neuroendocrine tumor, supported by immunohistochemical stains. This patient has von Hippel Lindau disease (VHL) and there is an increase in incidence of both MCA and neuroendocrine neoplasms in patients with VHL.

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