Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Chemotherapy/Radiotherapy

In a setting of unresectable pancreatic cancer, chemotherapy is rarely employed, although an occasional prolonged survival has been reported. Some of these tumors shrink after chemotherapy or radiotherapy, although tumor size is difficult to measure with imaging.

Preliminary results with immunochemotherapy combined with resection show a higher response rate than to surgery alone.

Pancreatic adenocarcinomas are relatively radioresistant, and a dose of >50Gy is necessary,

Figure 9.19. Biliary stent in a patient with pancreatic carcinoma. (Courtesy of David Waldman, M.D., University of Rochester.)

general, jaundice per se is not an indication for therapy; uncontrolled pruritus is. Generally endoscopic biliary drainage is performed if the intrapancreatic portion of the bile duct is obstructed. A percutaneous approach is available with endoscopic failure or a more proximal obstruction.

An unresectable pancreatic head carcinoma tends to invade and eventually obstruct the duodenum. Relative merits of palliation versus resection are still debated. Some of these duodenal obstructions can be palliated with selfexpandable duodenal endoprostheses; the commonly associated bile duct obstruction must also be addressed. Patients undergoing operative palliation for unresected ductal adenocarcinoma achieve a one-year survival of about 20%; to put this in perspective, the operative mortality rate after either resection or palliative bypass can be >10% and the relative merits of palliation versus resection are still debated.

Pancreatic carcinomas tend to invade the adjacent portal vein and lead to portal hypertension, with its associated complications. Stents placed across stenotic or occluded veins during percutaneous transhepatic portography led to a decrease in mean portal venous pressure (95); stent patency was considerably greater in patients without splanchnic venous invasion than in those with splanchnic involvement.

Moderate to severe pain is a common feature of a pancreatic carcinoma. If medical management of pain is insufficient, percutaneous celiac axis plexus blockade should be considered.

Metastasis

Hepatic metastases are common in patients with pancreatic ductal carcinoma; normally pancreatic veins drain into the portal vein, and this would explain why the liver is the most common site for metastasis. In a minority of patients pulmonary metastases and other hematogenous metastases are found without hepatic metastases. The reasons for such occasional odd metastatic distribution are speculative, but possibly include hepatofugal portosystemic shunting. Practically, pancreatic metastases occur throughout the body. An occasional cutaneous metastasis is encountered,

ADVANCED IMAGING OF THE ABDOMEN

most often at the umbilicus, and a metastatic skin lesion has been a first manifestation of pancreatic cancer.

In patients with pancreatic cancer who had no distant metastases at surgery, follow-up CT and autopsy revealed that 60% developed local recurrence, 52% liver metastases, and 32% both (96).

Other Carcinomas

Encountered occasionally are poorly differentiated carcinomas that defy classification. Their imaging findings vary. Some even mimic a pancreatic neuroendocrine tumor.

Adenosquamous Carcinoma

Adenosquamous and squamous carcinomas are rare in the pancreas. Clinically they behave similarly to a ductal adenocarcinoma. An adenosquamous or squamous carcinoma typically presents as a large, cystic tumor. Multiple calcifications have developed. Computed tomography reveals contrast enhancement, and, overall, imaging suggests a mucinous cystadenocarcinoma. Thus a typical appearance is a hypodense, fluid-filled tumor surrounded by calcifications.

Small Cell Carcinoma

Although the rare pancreatic small cell carcinoma has a poor prognosis, its importance lies in its favorable response to chemotherapy. Some small cell carcinomas are cystic and contain a hypervascular rim; late-phase contrast CT reveals tumor staining. US reveals the cystic regions, consisting of necrotic tumor and hemorrhage, to be mostly hypoechoic. Imaging findings suggest a hypervascular pancreatic tumor.

Pleomorphic Carcinoma (Sarcomatoid

Carcinoma)

A pancreatic giant cell carcinoma, also known as sarcomatoid carcinoma or pleomorphic carcinoma, is a rare, highly aggressive variant having characteristic histologic features. In distinction to a more common carcinoma, the giant cell variety often results in massive lym-

PANCREAS

phadenopathy and extensive liver involvement. Its relationship to more common adenocarcinomas is not known. These tumors have a poor prognosis.

These tumors have a sarcoma-like growth pattern and contain monoand multinucleated tumor giant cells. Necrosis and hemorrhage are common. Immunohistochemical study of a mixed pancreatic giant cell tumor revealed osteoclastic and pleomorphic type giant cells together with ductal adenocarcinoma (97); the authors believe this tumor is a carcinosarcomalike neoplasm containing both epithelial and mesenchymal components.

Imaging usually shows a large, wellmarginated, and hypervascular tumor.

Hepatoid Carcinoma

A hepatoid carcinoma, consisting of histologic evidence of hepatocytic differentiation, bile production, and elevated a-fetoprotein levels, is rare in the pancreas. One was part of an islet cell glucagonoma, while another one was part of a pancreatic duct carcinoma (98).

Pancreaticoblastoma

A pancreaticoblastoma is a tumor of childhood, only rarely detected in adults. In children, most pancreatic neoplasms are of acinar origin, rather than ductal as in adults. In distinction to children, prognosis is poor in affected adults, but even in children some of these tumors are aggressive, invade adjacent vessels, infiltrate surrounding structures, and metastasize (99).

A pancreaticoblastoma has a distinct histologic appearance. Both solid and cystic components are common. Hemorrhage, necrosis, and calcification develop in some. These tumors are rather complex and at times both neuroendocrine and mesenchymal cells are present. Some consist of tubular gland-like structures, squamoid components and small round cells, with the latter suggesting neuroendocrine cells having an origin from primitive multipotential stem cells with exocrine and neuroendocrine differentiation.

Most pancreaticoblastomas are located in the head of the pancreas and are large at the initial presentation. Some are associated with elevated serum a-fetoprotein level. They usually metastasize to liver.

Computed tomography in 10 patients (age range, 2 to 20 years) with pancreaticoblastomas identified heterogeneous (nine of 10) enhancing (10 of 10) tumors with well-defined margins (nine of 10) (100); two contained calcifications. An MRI of three of these tumors revealed hypoto isointense signal on T1and hyperintense signal on T2-weighted images. Postcontrast, some of these tumors are multiloculated and contain septations. Ultrasonography reveals a mixed hyperechoic solid tumor.

Lymphoma/Leukemia

Both lymphoma and leukemia involve the pancreas, and either a localized tumor or a diffusely enlarged pancreas is evident to the point of obstructive jaundice. Rarely, lymphoma is confined to the pancreas and a pancreatic malignancy and pancreatitis are then in the differential diagnosis. In particular, pancreatic lymphoma mimics atypical chronic pancreatitis, such as autoimmune pancreatitis. While a positive percutaneous biopsy in these patients establishes the diagnosis, a negative biopsy is nondiagnostic; also, a biopsy is usually not performed in patients with clinically suspected pancreatitis.

Computed tomography in pancreatic nonHodgkin’s lymphoma reveals a homogeneous hypodense pattern with mild contrast enhancement (61); necrosis or calcifications are not evident in either the diffuse or nodular forms. Adjacent lymph nodes may or may not be involved. A characteristic of primary pancreatic lymphoma is that, even when extensive, it tends not to obstruct the pancreatic duct, and thus ductal dilation is not present (101); this is in distinction to the more common adenocarcinomas, which commonly obstruct the pancreatic duct.

Adult T-cell leukemic infiltration led to a diffusely swollen pancreatic body and tail, identified by CT and US (102); a biopsy confirmed leukemic infiltration, and a Southern blot analysis revealed human T-cell lymphotropic virus type I (HTLV-I) proviral DNA both in pancreas and peripheral blood.

Plasmacytoma

Major pancreatic involvement with multiple myeloma is uncommon. Multiple myeloma pre-

senting as a pancreatic tumor and causing jaundice is rare, with pancreatic involvement often being a preterminal event. Computed tomography identifies a homogeneous solid tumor; the appearance mimics lymphoma. In some patients even a biopsy can confuse multiple myeloma with lymphoma.

Sarcoma

Primary sarcomas are rarely encountered in the pancreas. They tend to be rather aggressive. A differential problem is to exclude pancreatic invasion by mesenchymal tumors originating in other extraperitoneal structures.

A rare primary pancreatic malignant fibrous histiocytoma (103) and malignant mesothelioma (104) have been reported. Computed tomography and US simply detect a large tumor. About 70% of malignant fibrous histiocytomas have alterations in the p53 tumor-suppressor gene, a finding generally associated with a poor prognosis.

A pancreatic undifferentiated (anaplastic) carcinoma was associated with a sarcomatous spindle cell component and a cyst composed of epithelial tumor cells (105); metastases were present in the liver and lymph nodes.

Metastases to Pancreas

A number of distant sources, including hepatocellular, lung, breast carcinoma, melanoma, and osteosarcoma metastasize to the pancreas. Probably most common is a renal cell carcinoma, at times being a solitary focus. The time interval between nephrectomy and detection of a pancreatic metastasis can be years.

Even if detected by imaging, most pancreatic metastases cause few symptoms and are overshadowed by symptoms referable to the primary tumor or other metastatic sites. Endoscopic US identifies most metastases as hypoto isoechoic to pancreatic parenchyma, homogeneous, round, and well circumscribed—an appearance different from that seen with most primary pancreatic carcinomas; only an occasional metastasis has a heterogeneous appearance and indistinct margins mimicking a primary pancreatic carcinoma or focal pancreatitis.

In general, the CT and MR appearance of a pancreatic metastasis parallels that seen with

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the primary tumor. Renal metastases range from solitary to multiple tumors or diffuse pancreatic enlargement. Thin-section three-phase postcontrast CT of nine patients with metastatic renal cell carcinoma revealed conspicuous early enhancement (106). They are hypointense compared to the normal pancreas on T1and hyperintense on T2-weighted images. Smaller metastases show immediate postcontrast images enhancement and larger ones have rim enhancement.

A metastasis rather than a primary tumor should be suspected in a patient with a known malignancy and a new solitary pancreatic tumor. Nevertheless, most of these patients require a biopsy for confirmation.

Cystic Neoplasms

Clinical

The most common cystic pancreatic lesions are pseudocysts, followed by cystic neoplasms and true pancreatic cysts. Rarer causes of pancreatic cysts include a cystic teratoma, lymphoepithelial cyst, and a cystic lymphangioma. The importance of cystic pancreatic neoplasms lies in their considerably better prognosis compared to that of adenocarcinomas. Nevertheless, more than one cystic tumor has been either drained or anastomosed to the stomach in a mistaken belief that it represents a pseudocyst, often with dire consequences.

The current classification of cystic pancreatic neoplasms is somewhat arbitrary. Overall, only a minority of cystic neoplasms are malignant. Undoubtedly an occasional cystic endocrine carcinoma is misclassified among these tumors. Pathologists generally differentiate these neoplasms along immunohistochemical lines. Some have questioned whether these tumors are indeed a separate definable entity and believe that most should be classified among more conventional glandular and acinar origin neoplasms. Most of these neoplasms are divided into two basic types: microcystic adenomas (previously called serous cystadenomas) and mucinous cystic (or macrocystic) neoplasms. The distinction is important, because the former are benign while the latter include both mucinous cystadenomas and cystadenocarcinomas.