Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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Ectopic Pancreas

Ectopic (heterotopic) pancreatic tissue is most often found in the gastric antrum and proximal duodenum, although it can be located anywhere in the gastrointestinal tract,including a Meckel’s diverticulum.

Inflammation does develop in ectopic pancreatic tissue (called cystic dystrophy of an ectopic pancreas by some authors). If chronic, fibrosis and bowel stenosis ensue. Acute pancreatitis and even a (pseudo)cyst have developed in an ectopic pancreas. Similarly, this tissue is not immune to tumor formation; a car-

Figure 9.3. Duodenal stricture (arrow), believed to be secondary to pancreatitis in an annular pancreas.

Figure 9.4. Annular pancreas. After oral and IV contrast CT shows the pancreas surrounding descending duodenum (arrow).

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cinoma originated in ectopic pancreatic tissue in the esophagus, and a papillary cystic neoplasm developed in ectopic pancreatic tissue in the omentum.

Aberrant Duct Insertion

During embryonic development a network of duct branches eventually fuses and forms a primary channel. Aberrant fusion leads to a number of duplication anomalies of the pancreatic ducts and results in one or more main duct. This congenital anomaly is often associated with an apparent tumor on CT imaging.

The pancreatic duct can communicate with a duodenal duplication or even involve aberrant bile ducts and the patient presents with a periampullary cystic tumor; CT and ERCP are helpful in defining the underlying anatomy and planning surgery.

Cysts

Congenital cysts of the pancreas are either true cysts or pseudocysts. Both are rare. They range from solitary to multiple. True cysts are lined by epithelium. These cysts in neonates tend to be large and their clinical differential diagnosis often includes renal, choledochal, mesenteric, ovarian, and gastrointestinal origin cysts.

An increased prevalence of simple cysts is found in patients with polycystic kidney disease, tuberous sclerosis, and von HippelLindau disease. Incidentally detected multiple pancreatic cysts, especially in a young patient without pancreatic disease, should suggest one of these entities. Patients with von HippelLindau disease are also prone to developing cystic pancreatic neoplasms. In fact, in an occasional patient pancreatic lesions are the only abdominal manifestation of von Hippel-Lindau disease.

Computed tomography or MR confirms a cyst’s location and its relationship to pancreatic parenchyma; imaging tends not to differentiate among congenital cysts, a cystic neoplasm, and other cystic lesion.

Precontrast CT of a lymphoepithelial cyst revealed a heterogeneous water-density tumor, with septa identified postcontrast (9); MRI showed a hypointense mass on T1and a hyper-

intense mass on T2-weighted imaging, with septa identified on postgadolinium images. Endoscopic US confirmed the presence of septa and also detected fine hyperechoic structures within the cyst. A cystic neoplasm was suspected preoperatively.

Histiocytosis

Langerhans cell histiocytosis is the current term for disorders previously known as histiocytosis X. This is a rare disorder, with pancreatic involvement being even rarer.

Hereditary Pancreatic Dysfunction

Pancreatic dysfunction due to an isolated embryogenetic defect is rare. More often dysfunction is part of an inherited abnormality manifesting at multiple sites. Cystic fibrosis is an example of the latter. It is the most common cause of hereditary pancreatic dysfunction.

Cystic Fibrosis

In this autosomal-recessive disorder obstruction of small and large pancreatic ducts by thick, viscid secretions and surrounding inflammation induces fibrosis and an eventual atrophic pancreas. As a result, most patients develop exocrine pancreatic insufficiency. Acute pancreatitis is not common in cystic fibrosis patients, with some authors postulating that their pancreatic atrophy prevents a major inflammatory response.

Pathologically, parenchymal atrophy, fibrosis, and fatty infiltration are evident. In some patients fatty infiltration predominates and the pancreas appears enlarged; in others, atrophy predominates with varying degrees of fatty infiltration.

Calcifications are not common in chronic pancreatitis secondary to cystic fibrosis. When present, calcifications are scattered and not as prominent as in hereditary pancreatitis. Cysts, fatty replacement, and atrophy are detected with imaging. Once small, the pancreas may have a CT density approaching that of fat, similar to severe lipomatosis. Cysts are interspersed with segments of dilated pancreatic ducts or

they may essentially replace the pancreatic parenchyma.

Computed tomography in adolescents and adults with cystic fibrosis revealed less pancreatic tissue and more fat than in controls (10); no relationship was found between the degree of pancreatic fat and pancreatic endocrine dysfunction, but a relationship existed between the degree of fatty replacement and exocrine dysfunction.

The varied composition is reflected in the US appearance, which can range from normal to a hyperechoic pattern caused by fibrosis and fatty infiltration. In some, the pancreas blends into surrounding extraperitoneal fat and is difficult to define. Likewise, smaller cysts are not identified.

Magnetic resonance signal intensity varies depending on the degree of fatty infiltration and fibrosis present. Pancreatic T1-weighted MR in patients with cystic fibrosis reveals hyperintense signals ranging from homogeneous to a lobular outline,focal sparing,and even a normal appearance. Magnetic resonance cholangiopancreatography is useful in evaluating pancreatic duct dilation and cyst formation.

Hereditary Pancreatitis

The rare hereditary (familial) pancreatitis eventually leads to pancreatic exocrine dysfunction. It has an autosomal-dominant mode of inheritance with variable penetrance, but tends to affect only a few members of each family. This genetic defect is believed to be due to mutations in the trypsinogen gene leading to failure in inactivating prematurely activated cationic trypsin within acinar cells (11); two types are described: type I involves mutations in trypsinogen R117H and type II in trypsinogen N211. Accumulation of active trypsin mutants are believed to activate a digestive enzyme cascade process in pancreatic acinar cells, leading to autodigestion, inflammation, and acute pancreatitis. An enzymatic mutation detection method is accurate in screening individuals for known trypsinogen gene mutations.

Two types of pancreatic stones develop in this condition. Calculi in members of 10 families were composed of degraded residues of lithostathine, a pancreatic secretory protein inhibiting calcium salt crystallization (12); in

ADVANCED IMAGING OF THE ABDOMEN

one family with five affected individuals calculi were composed of >95% calcium salts.

This condition typically develops in childhood or early adolescence as acute episodes of pancreatitis, evolves into chronic pancreatitis, and gradually progresses with episodes of pancreatitis, including pseudocyst formation, to pancreatic insufficiency. An early diagnosis prior to onset of chronic pancreatitis is often not considered. Compared to patients with chronic alcoholic pancreatitis, patients with chronic hereditary pancreatitis have an earlier onset of symptoms, a delay in diagnosis, and a higher prevalence of pseudocysts, but otherwise the natural history is similar for both conditions. These patients are at increased risk for developing a pancreatic duct adenocarcinoma, the risk being approximately 50 to 60 times greater than expected (13); smoking increases this risk and lowers the age of onset by approximately 20 years.

Prominent calcifications in the pancreatic ducts are a common feature and appear similar to those seen with chronic alcoholic pancreatitis, except that this finding is almost pathognomonic of chronic hereditary pancreatitis in a young patient. A dilated pancreatic duct is also often detected.

Hereditary pancreatitis has a different clinical presentation and imaging findings, and usually is not in the differential diagnosis with cystic fibrosis.

Less Common Syndromes

The syndromes listed below are rare and have little radiologic relevance, but are occasionally raised in a differential diagnosis.

Shwachman-Diamond syndrome is a rare congenital disorder consisting of pancreatic insufficiency, growth retardation, and other abnormalities. These patients have steatorrhea, a normal sweat test, and normal intestinal mucosa. Some eventually improve their enzyme secretions and have pancreatic sufficiency. Imaging shows extensive replacement of pancreatic tissue by fat.

Pearson marrow-pancreas syndrome manifests during early infancy with failure to thrive and involves the hematopoietic system, exocrine pancreas dysfunction, and others. A primary defect involves deletions in mitochondrial DNA.

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Johanson-Blizzard syndrome is an autoso- mal-recessive syndrome consisting of ectodermal dysplasia. Among other abnormalities, these infants develop both exocrine and endocrine pancreatic insufficiency. The primary defect is in acinar cells; at autopsy some of these infants have total absence of acini and the pancreas is replace by fat.

Histopathologically, patients with the Johanson-Blizzard syndrome and SchwachmanDiamond syndrome have preserved ductular output of fluid and electrolytes; this distinguishes them from patients with cystic fibrosis, who have a primary ductular defect.

Multiple Endocrine Neoplasia

Syndromes

Multiple endocrine neoplasia (MEN) syndromes are familial autosomal-dominant disorders exhibiting incomplete penetrance and variable expression and manifesting by an increased incidence of various endocrine hyperplasias or neoplasms. They consist of synchronous or metachronous neoplastic and hyperplastic neuroendocrine tumors in several glands. These syndromes are usually divided into three types:

Type I (Wermer’s syndrome) consists of lesions in the pancreas, parathyroid, adrenal cortex, pituitary, and thyroid. Some gastric carcinoids also share a common development with enteropancreatic and parathyroid MEN-I tumors.

Type II (Sipple’s syndrome) consists of lesions in the adrenal medulla, parathyroid, and thyroid.

Type III (multiple mucosal neuroma syndrome) consists of lesions in the adrenal medulla, thyroid, mucosal tissues, and bones.

Whether von Hippel-Lindau disease and neurofibromatosis are part of multiple endocrine neoplasia syndromes is a matter of classification.

The presence of pancreatic tumors in a patient with MEN syndrome signifies type I. The genetic defect for MEN-I appears to involve a tumor suppressor gene located in the long arm of chromosome 11. A majority of MEN-I individuals occur in familial clusters, with only an

occasional sporadic one reported. Affected patients tend to have multiple organs involved, multicentric tumors within one organ are common, and complex hormone secretions are encountered. MEN-I tumors have a low but not insignificant malignant potential. Pancreatic tumors tend to be multicentric and most are hormone secreting, some being multihormonal; in fact, in the same patient different tumors often have different predominant hormonal secretions. Most common tumors are a-cell (glucagon) and b-cell (insulin) secretors. Less common are PP-cell, gastrin, and vasoactive intestinal peptide tumors. Concomitant nesidioblastosis occurs in a minority of patients. These tumors range from hyperplasia, adenoma to a carcinoma.

The majority of MEN-I patients with Zollinger-Ellison syndrome also have primary hyperparathyroidism, 30% have pituitary adenomas (about half are prolactinomas), 30% have adrenal tumors, and a smaller minority have bronchial and thymic carcinoids (14); gastrinomas tend to be multiple and often are duodenal in location. Hypercalcemia and nephrocalcinosis are common initial clinical presentations.

Because of multiplicity of tumors and their malignant potential, management of MEN-I patients with symptomatic pancreaticoduodenal tumors remains controversial. One aggressive surgical approach in a setting of hypergastrinemia and Zollinger-Ellison syndrome includes a duodenotomy, peripancreatic lymph node dissection, enucleation of pancreatic head or uncinate tumors, and a distal pancreatectomy (15); such an approach in MEN-I patients with Zollinger-Ellison syndrome resulted in 68% of patients remaining eugastrinemic during follow-up. The effect on survival of a Whipple resection is not clear.

Gastrinomas in MEN-I syndrome patients tend to be duodenal in location and difficult to locate with imaging. Some are detected only during surgical exploration by palpation, intraoperative endoscopy with transillumination, or duodenotomy.

Multiple endocrine neoplasia type II is an autosomal-dominant condition with the gene abnormality located on chromosome 10, with the responsible gene, called RET, being an oncogene. These patients are at sufficiently high risk for medullary thyroid carcinoma at a young age