Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

PANCREAS

A familial association between autosomaldominant polycystic kidney disease and cystadenocarcinoma of the pancreas and liver was reported in one family (107).

Most patients with cystic pancreatic neoplasms have abdominal pain. Weight loss, jaundice,and an abdominal tumor,at times large,are the presenting clinical findings, although some tumors are discovered in an asymptomatic patient. Complications encountered include obstruction of adjacent gastrointestinal structures, perforation, and hemorrhage.

Cyst fluid analysis aids in differentiating pseudocysts from neoplasms (Table 9.5). Amylase level was >16,000 international units (IU) per milliliter in 13 out of 14 pseudocysts and <83IU/mL in seven of eight cystic neoplasms (108). In general, a pancreatic cyst associated with an elevated serum CA 19-9 level or elevated CA 72-4 cyst fluid level should be considered a neoplasm. Positive cytology is diagnostic.

Table 9.5. Pancreatic cyst fluid analysis

Carcinoembryonic antigen (CEA) levels High in mucinous cystadenomas and

cystadenocarcinomas

Low in pseudocysts and serous cystadenomas

CA 19–9 tumor marker values

High in ductal carcinomas and mucinous cystic neoplasms

Low in pseudocysts

Variable in serous cystadenomas

CA 72–4 tumor marker values

High in mucinous cystic neoplasms

Low in pseudocysts

CA 125 tumor marker values

High in mucinous adenocarcinomas Low in pseudocysts

Variable in serous and mucinous cystadenomas

Fluid amylase and lipase levels

Generally high in pseudocysts

Low in cystic tumors

Cyst fluid relative viscosity High in mucinous tumors

Low in pseudocysts and serous cystadenomas

Cytology

About 50% sensitivity in detecting malignant cysts

Figure 9.20. Cystic pancreatic acinar carcinoma, with most of the tumor located adjacent to pancreas (arrows).

Imaging

A majority of cystic neoplasms are either partially or almost completely cystic, yet even in the latter most but not all have a solid component, a finding not present in a pseudocyst. A minority communicate with the pancreatic ducts. An occasional one develops a pancreaticoenteric fistula.

Computed tomography is often the diagnostic modality of choice in preoperative evaluation of these cystic tumors, yet the published results provide limited guidance. Imaging appears rather limited in distinguishing among a microcystic adenoma, a mucinous cystadenoma, and a mucinous cystadenocarcinoma (Fig. 9.20). On the other hand, the presence of cysts <2cm points toward a serous tumor, and peripheral tumoral calcifications are associated with mucinous tumors (109).

These tumors are readily misdiagnosed as pseudocysts by both CT and conventional US, and less so by endoscopic US. Endoscopic US of 52 pancreatic solitary cystic tumors, consisting of benign and malignant cystic neoplasms and nonneoplastic cysts, found that all neoplasms have a thick wall, tumor protrudes into the cysts, any septa present are thick, or the cysts are small (110); nonneoplastic cysts, on the other hand, contain thin septa or the cysts are simple.

Magnetic resonance signal intensity is of limited use in distinguishing among various cystic neoplasms, which are hypointense on T1and hyperintense on T2-weighted images. They show little or no postcontrast enhancement.

Microcystic (Serous) Neoplasm

Clinical

Microcystic neoplasms invariably are benign, with only a few malignant ones reported. In distinction with their mucinous counterpart, microcystic adenomas are not considered premalignant, with rare exceptions. Most of these tumors, also called serous cystadenomas, are well encapsulated and nodular and contain numerous small cysts and septa. The cysts are filled with serous fluid and contain little mucin.

Some authors believe the term microcystic serous adenoma is too limiting and that a macrocystic variant of the same neoplasm exists; the term serous cystadenoma is thus used for both morphologic variants. Nevertheless, the term macrocystic serous tumor is useful; these tumors are often misdiagnosed as either mucinous cystic neoplasms or pseudocysts. In fact, some tumors are misclassified even with intraoperative frozen section. Serum tumor markers with macrocystic serous tumors tend to be normal, although CA 125 and CA 19-9 are elevated in the cyst fluid in some of these tumors (111). The macrocystic variant of serous adenoma consists of a small unior bilocular cyst containing a thin wall without any mural nodules (112); these are indistinguishable from mucinous cystic tumors.

A rare variant is a histologically solid serous cystadenoma with cytologic, histochemical, and immunohistochemical features indistinguishable from a typical serous cystadenoma. Whether such a pancreatic tumor is a separate pathologic entity is debatable.

Symptomatic patients tend to have a large tumor, with symptoms presumably due to compression of adjacent structures. Smaller tumors often are detected incidentally. An occasional microcystic adenoma having a sponge-like consistency will almost entirely replace the pancreas.

Imaging

A majority of microcystic (serous) neoplasms contain many small cysts, a minority consist of larger cysts (macrolacunar) and a minority are mixed; imaging can suggest the diagnosis in a majority of the small and mixed cystic variety, but the larger cystic tumors are indistinguish-

ADVANCED IMAGING OF THE ABDOMEN

able from other pancreatic cysts by either CT (Fig. 9.21) or MR (Fig. 9.22).

Some of these tumors containing small cysts have a “honeycomb” appearance on imaging. At times the cysts are sufficiently small to suggest a solid tumor (Fig. 9.23). A central scar with radiating septa is evident in some, and occasionally sunburst-like calcifications are detected in the central scar. In some, thin septations enhance on immediate postcontrast images. Any solid component also enhances.

Even when large, these benign tumors tend not to obstruct the pancreatic duct. Also, only a small minority of serous cystadenomas communicate with the pancreatic duct. An occasional microcystic adenoma is sufficiently large to compress and occlude the splenic vein, resulting in hepatopetal collaterals via gastric fundal varices.

Mucinous Cystic Neoplasms

Clinical

A distinction is often made between mucinous cystic neoplasms and intraductal mucinous tumors—a tenuous and somewhat arbitrary differentiation based more on clinical and radiologic findings and tumor location than on pathologic grounds (113). From a simplistic viewpoint, mucinous cystic neoplasms do not communicate with the pancreatic ducts while intraductal mucinous tumors do communicate. Yet even this definition is not satisfactory. Most mucinous cystic neoplasms consist of a cystic septa-containing tumor and no pancreatic duct dilation unless obstructed by the tumor, while intraductal papillary mucinous tumors are small and lobulated—assuming they are visualized at all—and a major finding with them is a dilated main or branch pancreatic duct containing exuberant mucus.

Mucinous cystic neoplasms of the pancreas range from benign (cystadenoma) to malignant (cystadenocarcinoma), with an occasional tumor labeled borderline malignant. A cystadenoma is considered to be premalignant. These tumors range from unilocular to multilocular and their size varies considerably; their cavities are filled with mucin. Pancreatic body and tail are preferred sites. The malignant variety quite often obstructs the pancreatic duct. Tumor aspiration usually simply reveals mucin. A pre-

ones of 83%, and for those with infiltration limited to the pancreatic parenchyma, 74%; but this survival rate is only 28% if infiltration extends to other organs (114).

Imaging

Imaging reveals one or more cysts; typically these cysts are fewer but larger than seen with

Figure 9.24. Pancreatic cystadenoma (arrows) communicating with stomach. This patient was believed to have a pseudocyst, which was anastomosed to the stomach.

serous tumors (Fig. 9.25). Ultrasonography reveals a unilocular or, more often, a multilocular anechoic tumor, with tumor excrescences and debris modifying this appearance. Endoscopic US better evaluates their complex structure. Thus the number of cysts and cyst size are useful criteria in differentiating between the microcystic (serous) and macrocystic (mucinous) tumors. Using criteria that a mucinouscystic neoplasm should contain fewer than six cysts having a diameter of >2cm, out of 17 mucinous cystadenomas and cystadenocarcinomas US correctly diagnosed 88% (115).

Soft tissue nodules project into some of these cysts. Septations tend to be thicker than those seen with microcystic neoplasms. An occasional tumor develops calcifications that are seen with both benign and malignant tumors. Local invasion is evident with some carcinomas. Occasionally these tumors are first detected through liver metastases. Thus a large calcified tumor in the body and tail occasionally manifests its malignant nature with multiple liver tumors.

An MRI identifies multilocular or unilocular cysts having a relatively thick wall (Fig. 9.26). The cysts tend toward hyperintensity on T1weighted sequences, presumably due to their high mucin content, and are, as expected for nonflowing fluid, hyperintense on T2-weighted images. Those tumors containing more mucin have a higher MR T1-weighted signal intensity than those containing more watery fluid; tumors containing gelatinous mucin tend to be

malignant, while those containing more watery fluid tend to be benign. The cysts do not enhance postcontrast.

Benign and malignant tumor varieties have a similar imaging appearance, although thick septa and nodularity point toward malignancy. Invasion of adjoining structures or metastasis signifies malignancy.

An FDG-PET detected only 59% of 22 mucinous carcinomas, with an inverse correlation with the amount of tumor mucin (116).

Intraductal Mucin-Producing Tumors

Clinical

One uncommon type of a mucinous tumor originates from the pancreatic ducts. These tumors have been called mucinous ductal ectasia, duct ectatic mucinous cystic tumor, intraductal hypersecreting neoplasm, mucin-hypersecreting, ductal papillary adenoma or adenocarcinoma, intraductal adenomatosis, intraductal papillary mucinous tumor, diffuse papillomatosis, and the currently generally preferred term of intraductal mucin-producing tumor. As already noted, most authors differentiate them from muci- nous-cystic neoplasms discussed in the previous section. Whether these intraductal papillary mucinous tumors are a variant of pancreatic mucin-producing cystic neoplasms or a separate entity is conjecture (Fig. 9.27). Likewise, a

Figure 9.27. Illustration of an intraductal mucin-producing tumor obstructing the main pancreatic duct (arrow) and a secondary branch (arrowhead). In both instances obstruction results in dilated ducts. The ducts of Santorini and Wirsung are shown to be communicating.

ADVANCED IMAGING OF THE ABDOMEN

relationship of these tumors to papillary cystic (embryonal) neoplasms (discussed in a later section) is not clear.

A slight male predominance was evident among 22 patients at the Endocrine Tumor Registry of the Armed Forces Institute of Pathology, with a mean age at presentation of 64 years (117), findings in distinction to mucinous-cystic neoplasms, which occur mostly in middle-aged women. Pain is a common presentation, often suggesting pancreatitis. Complicating the issue, associated pancreatitis is common. Up to one third of these patients develop diabetes.

Anecdotal reports describe a second pancreatic primary tumor or even a hilar cholangiocarcinoma.

An intraductal papillary mucinous tumor is a rare cause of pseudomyxoma peritonei (118).

In the Armed Forces Institute of Pathology study, only two of 22 patients died of this disease (117); aside from three operative deaths and three deaths from other causes, the remaining patients were alive without evidence of disease an average of 58 months after presentation. Patients with an exclusively intraductal tumor do well after resection, but those with an invasive adenocarcinoma have a poor prognosis. Patients with mucus-filled dilated ducts but without a detectable tumor have had endoscopic sphincterotomy to relieve pain and then were followed by imaging, but in view of the uncertainty for malignancy, even with a biopsy in hand, many of these tumors are resected with a wide margin, at times consisting of a pancreaticoduodenectomy. If they are not corrected, eventual pancreatic atrophy of the obstructed segments ensues.

Pathologic Findings

Not all of these tumors are neoplastic in origin. Histologically, these tumors range from hyperplasia, dysplasia, and adenoma, to mucinhypersecreting carcinoma and various intermediate combinations. A villous structure is common. Papillae lined by columnar epithelium contain varying degrees of epithelial dysplasia, often with an adjacent adenocarcinoma. A benign tumor probably is at risk of malignant transformation. Histologically, these tumors share a number of characteristics with colorectal villous adenomas.