Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

phoma are unusual with tuberculosis. An enlarged isolated lymph node chain is nonspecific; for instance, periportal adenopathy, detected by CT, can be an isolated manifestation of tuberculous adenitis, but the differential diagnosis also includes not only malignancies but also such benign diseases as sarcoidosis. Enlarged tuberculous lymph nodes tend toward a lower CT density than lymphomatous nodes. In tuberculous peritonitis a low-density center was seen in some enlarged nodes, and calcification eventually develop in a minority; the low-density portion probably represents caseation necrosis. Even more striking is peripheral rim contrast-enhancement with tuberculosis; lymphomatous nodes are more homogeneous in their enhancement (25).

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Fine peritoneal septations are common in tuberculosis, and ascites is often enclosed in a lattice-like mesh. Tuberculous ascites tends to have a higher density than most ascitic fluid. Diffuse peritoneal thickening and infiltration of the greater omentum is common.

In general, CT and US reveal similar findings, although bowel wall thickening is probably better appreciated with CT. At times CT and US complement each other in patients with suspected tuberculous peritonitis. With pelvic involvement the CT appearance often mimics a malignancy. An abdominal cocoon-like appearance can be associated with tuberculous pelvic involvement (26) (Fig. 14.8). Ultrasonography reveals a hyperechoic, thickened mesentery, mesenteric adenopathy, and less often dilated

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

small bowel loops, minor ascites, and occasional omental thickening.

A cone-shaped, contracted cecum and a dilated, edematous terminal ileum are diagnostic findings in the dry type of ileocecal tuberculosis.

Filariasis

Infection with Wuchereria bancrofti, Brugia malayi, B. timori, and Loa loa affects not only the lymphatics but also such structures as the kidneys; renal damage can occur even in asymptomatic carriers.

Ultrasonography detects live adult filarial worms by identifying twirling motions in dilated lymph channels; successful chemotherapy ends the worm activity.

Lymphoscintigraphy is of value in assessing lymphatic damage. Extensive lymphoscintigraphic abnormalities are found in W. bancrofti-endemic regions even in clinically asymptomatic persons. Chyluria is common. Among patients with filarial chyluria undergoing unilateral pedal lymphography, lymphatic crossover was identified in all and lymphaticorenal fistulas were detected in 98% (27); the authors advocate unilateral lymphography over

a bilateral study because lymphatic crossover identifies lymphaticorenal fistulas regardless of the side of involvement.

Echinococcosis

A primary echinococcal cyst in the peritoneum is rare. A study from Tunis found that retrovesical and extraperitoneal sites represent about 10% of their operated hydatid cysts (28); a retrovesical site presumably represents secondary implantation in the pouch of Douglas by intraperitoneal rupture of a more conventionally located cyst (Fig. 14.9).

Parasitic Infestation

Parasitic infestations involve myriad end organs, with the peritoneal cavity often serving as an intermediary pathway. Peripheral eosinophilia often suggests underlying parasitic infestation, although it is present only in about half of infected patients, and generally during the acute phase. Acute or vague abdominal pain is common. Serologic tests (enzyme-linked immunoabsorbent assay) are available for some infestations.

Computed tomography detects focal omental, peritoneal, mesenteric, or bowel wall infiltration, often heterogeneous and partly cystic in nature. The imaging differential diagnosis often includes other inflammatory conditions and neoplasms.

Diffuse Infiltration

Fibrosis/Desmoid

Diffuse extraperitoneal fibrosis, also known as retroperitoneal fibromatosis, desmoid tumor, and Ormond’s disease, is a locally invasive but benign chronic condition. Extraperitoneal fibrosis is idiopathic in about two thirds of patients. Some evidence suggests an underlying autoimmune response manifesting as a vasculitis and fibrosis. Both mesenteric and extraperitoneal fibromatosis (fibrosis) present most often as a discrete desmoid tumor; a diffuse infiltrate is less common. Fibrous tissue is noncapsulated, poorly marginated, and infiltrates surrounding structures. Its relationship to sclerosing peritonitis and mesenteritis is conjecture. Indeed, some of the patients discussed previously (see Sclerosing Peritonitis/Mesenteritis) probably should be included here (and vice versa).

Etiology

Secondary forms of fibromatosis/desmoid are associated with both malignant and benign disorders. It develops in response to some infiltrating malignancies, and at times the term malignant retroperitoneal fibrosis is used to describe a reactive fibrosis secondary to such neoplastic infiltration; malignant cells tend to be scattered throughout the fibrosis. A distinction between malignant and nonmalignant retroperitoneal fibrosis is of obvious clinical importance and thus an extensive biopsy is often indicated.

An association with methysergide and ergotamine use appears to be greater than by chance. Extraperitoneal fibrosis has been reported following treatment of Parkinson’s disease with L- dopa analogues; a number of these patients have underlying atherosclerosis and it is difficult to put the relationship among atherosclerosis, drug therapy, and the subsequent development of fibrosis into proper perspective.

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An inflammatory abdominal aortic aneurysm is commonly associated with extraperitoneal fibrosis (discussed in Chapter 17). Pathogenesis of fibrosis in this clinical setting is unclear, although severe atherosclerosis appears to induce an immune response in some individuals.

A rare patient develops extraperitoneal fibrosis secondary to actinomycosis and other chronic infection. Extensive extraperitoneal fibrosis develops as a reaction to some foreign bodies. Thus fibrosis is induced by extravasated barium during bowel perforation (29); similar to some of the other reactive fibroses mentioned above, such fibrosis is a local reaction surrounding the barium crystals and is not associated with fibrosis at other sites.

Desmoid tumors are common in familial adenomatous polyposis. In fact, this polyposis syndrome should be suspected in a patient with a newly discovered rectus abdominis muscle desmoid tumor (Fig. 14.10).

Prior abdominal surgery increases the risk for desmoids; in an occasional patient desmoids begin to develop in trocar sites within months after laparoscopy. Less often a desmoid develops in a patient without familial polyposis and at a site unrelated to prior surgery. These sporadic desmoids are less likely to recur after resection than familial ones.

A mesenteric desmoid tumor can develop after radiation therapy. These patients often also develop leg edema and protein-losing enteropathy due to intestinal lymphangiectasia.

Figure 14.10. Rectus sheath desmoid tumor (arrow). (Courtesy of Patrick Fultz, M.D., University of Rochester.)

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Classification of desmoplastic fibroblastomas (collagenous fibroma) is not clear. One slowly growing heterogeneous tumor was hypointense on T1-weighted images, mixed hypoand hyperintense on T2-weighted images and showed heterogeneous contrast enhancement (30); the hypointense regions consisted mostly of collagen.

Clinical

Extraperitoneal fibrosis usually involves structures from the renal hila inferiorly. A somewhat atypical variant is to find fibrosis limited to the pelvis. Although benign, desmoids invade locally and have a high potential for recurrence. Complications of desmoid tumors account for part of the mortality associated with familial adenomatous polyposis.

Symptoms result from the gradual compression of adjacent structures, including ureters, retroperitoneal vessels and nerves, and portions of the gastrointestinal tract. Extraperitoneal fibrosis encasing the ureters results in hydronephrosis, at times unilateral. The ureters are displaced medially. Once a diagnosis of extraperitoneal fibrosis is established, in a setting of hydronephrosis, ureteral decompression is necessary to save the kidney. Laparoscopic ureterolysis is feasible, although often in vain. Placement of double-J ureteric stents is helpful on a temporary basis. With time, soft polyurethane stents tend to become obstructed due to continued extrinsic ureteral compression, and they need to be replaced with more rigid ones.

Occasionally extraperitoneal fibrosis involves the bile ducts and leads to obstructive jaundice; imaging findings can suggest extrahepatic sclerosing cholangitis or a pancreatic malignancy. At times fibrosis leads to both hydronephrosis and biliary obstruction.

Abscesses can develop within a desmoid tumor, at times communicating to the bowel. Some of these abscesses can be managed initially with percutaneous abscess drainage.

Extraperitoneal fibrosis tends to respond to steroid therapy. Immunosuppressive therapy has also achieved some success. Surgical therapy is difficult due to the often-diffuse infiltration. Also, surgery often tends to exasperate recurrence. Once the diagnosis is established, follow-up is by CT or US. Some

unresectable desmoid tumors regress after being treated with radiation therapy. The role of hyperthermia is not clear. Spontaneous regression of extraperitoneal fibrosis has also been reported, albeit rarely.

Both growth and response to therapy of a desmoid are readily evaluated with imaging.

Iliac vein and inferior vena caval stenosis secondary to extraperitoneal fibrosis is often approached surgically but can be treated by venous thrombectomy and bilateral iliac stenting, but keep in mind the often temporary nature of such therapy.

Imaging

Most desmoids develop in the mesentery and abdominal wall, with only an occasional one being extraperitoneal in location. A rare one involves both extraand intraperitoneal structures. In some patients a specific site of origin cannot be determined.

Although at times imaging suggests the diagnosis, no specific finding is pathognomonic. Imaging identifies a desmoid either as a discrete tumor or a poorly marginated infiltration. With diffuse extraperitoneal infiltration, the first task is to establish that the diagnosis is indeed fibrosis rather than an underlying malignancy. Rather than being displaced, adjacent vessels and bowel become encased. Imaging cannot differentiate primary benign fibromatosis from a secondary one due to malignant infiltration, although malignancies increase in size faster than a benign infiltration. Extraperitoneal fibrosis is usually diffuse, but occasionally is well marginated or even nodular. Contour irregularities are not a reliable differential. Either extensive percutaneous biopsies or surgical exploration is often necessary. Negative percutaneous biopsies should be interpreted with caution.

Precontrast CT of most extraperitoneal fibrosis shows a density similar to that of adjacent muscle or is even slightly hyperdense. The tissue tends to enhance postcontrast and may appear somewhat hyperdense. At times the appearance mimics that of a diffuse tumor.

Magnetic resonance imaging of diffuse fibrosis and desmoids usually reveals hypointense signals on both T1and T2-weighted images. An occasional complex infiltrating tumor exhibits isoor even hyperintense signal

intensities on T2-weighted images. Postcontrast enhancement is variable.

Desmoids and retroperitoneal fibrosis show Ga-67-citrate uptake. Some correlation exists among clinical symptoms, disease activity, and Ga 67 scintigraphic activity.

Amyloidosis

Amyloid lymphadenopathy is common in uremic patients, in one study being identified in 22% of hemodialysis patients’ lymph nodes (31).

Occasionally a patient with amyloidosis develops extensive calcified extraperitoneal amyloid deposits which are visible with conventional radiography and CT.

Sarcoidosis

The CT appearance of abdominal sarcoidosis varies considerably. Often systemic sarcoidosis manifests with hepatosplenomegaly and extraperitoneal adenopathy, at times massive and mimicking lymphoma. Magnetic resonance imaging also readily detects the adenopathy.

Sarcoid-involved lymph nodes show FDGPET uptake.

Xanthogranulomatosis/Erdheim-

Chester Disease

Xanthogranulomatosis is more common in the kidneys than in other extraperitoneal soft tissues. Systemic involvement is called ErdheimChester disease, which is a nongenetic systemic condition most often presenting with characteristic and almost pathognomonic bilateral symmetric foci of sclerosis in the long bones, at times containing focal osteolysis within these regions of sclerosis. Bone involvement, however, is only an early manifestation of this systemic condition affecting multiple organs. The etiology of Erdheim-Chester disease is unknown, and its specific histologic classification is unclear. Many pathologists characterize it as a non–Langerhans cell histiocytosis, with histology typically revealing a xanthogranulomatous infiltrate by foamy histiocytes in a bed of fibrosis. Macrophages and giant cells are evident in some biopsies. It is distinct from histiocyto-

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sis X. Although it shares similar clinical and radiologic findings with Langerhans cell histiocytosis, most authors believe these are separate entities. Considerable evidence suggests that it is neither a lipid storage disorder nor a primary macrophage cell disorder. It occurs most often in the middle aged and elderly and tends to progress slowly. For instance, in one patient it developed over a 20-year period and led to renal artery stenoses, bilateral ureteric stenosis, and adhesive renal capsule involvement (32). At times it evolves into upper urinary tract obstruction. A diffuse periaortic infiltrate is evident in some,with the CT appearance termed the coated aorta (33).

Although case reports mention the rarity of this condition, it is probably not uncommon, and it is underdiagnosed and underreported. In an extensive review of Erdheim-Chester disease, bone pain was the most frequent clinical finding (47%), followed by exophthalmos (27%) and diabetes insipidus (29%) (34); long bone diaphysial osteosclerosis was detected in 76% of patients and extraperitoneal involvement in 39%.

Imaging findings of retroperitoneal ErdheimChester disease (and xanthogranulomatosis in general) are those of a diffuse infiltrating solid tumor. Technetium-99m accumulates in regions of bone involvement and presumably also at other sites of involvement. The differential diagnosis includes retroperitoneal fibrosis, diffuse lymphoma, inflammatory fibrosarcoma, malignant fibrous histiocytoma, and other solid tumors, conditions distinguishable only pathologically.

Extramedullary Hematopoiesis

Extramedullary hematopoiesis implies deposits of erythroid precursors outside of bone marrow. The rare patient with retroperitoneal extramedullary hematopoiesis presents with an extensive soft tissue infiltrate.

Systemic Mast Cell Disease (Mastocytosis)

Systemic mast cell disease typically involves the skin, and imaging plays no role in diagnosis, although many patients with systemic mastocytosis have ascites, hepatosplenomegaly, thick-

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ened omentum and mesentery and abdominal adenopathy. Some patients develop gastrointestinal lymphonodular hyperplasia. Ascites developed in a patient with systemic mastocytosis and hypertension (35). The usual skin findings point to this condition, but an occasional patient presents with hepatomegaly and lymphadenopathy, and the diagnosis is established from lymph node biopsies.

Fluid Collections

Ascites

Clinical

Ascites means fluid in the peritoneal cavity. Classic ascites is either a transudate or exudate based on total protein concentration, but blood, pus, various organ secretions, or a mixture of fluids can be found in the peritoneal cavity.

The most common cause of ascites in North America and Europe is portal hypertension due to liver disease. Peritoneal carcinomatosis leads to an exudate, often with an obstruction of draining lymphatics. Tumor infiltration of the porta hepatis or liver parenchyma also results in portal hypertension and ascites. Other, less common causes include some infections and conditions leading to hypoalbuminemia.

Congestive heart failure results in increased right heart pressure with concomitant hepatic congestion. Leakage from bile ducts results in bile ascites; often the bile is also infected. Chylous ascites forms from lymphatic obstruction, usually due to a neoplasm such as a lymphoma, or lymphatic transection, usually secondary to surgery.

Ultrasonography in children with mild dengue hemorrhagic fever revealed ascites in 34% (36); in those with severe disease US detected ascites in 95% and pararenal and perirenal fluid in 77%. Ultrasonography thus appears useful for predicting the severity of this condition in children.

Peritonitis, regardless of etiology, is usually associated with varying amounts of peritoneal fluid. Some infections result in blood-tinged ascites. Severe salmonella enteritis or massive Fasciola hepatica infection are uncommon causes of moderate ascites.

Common causes of ascites in neonates include urinary tract obstruction with subse-

quent rupture, and chylous ascites due to birth trauma. Ascites in the young is easier to detect with CT or US than with conventional radiographs.

Ascitic fluid white blood cell count should be routinely obtained on all fluid samples. An elevated count is present with both an inflammation and malignancies. Polymorphocytes predominate in acute bacterial peritonitis, while in more chronic conditions such as peritoneal tuberculosis or carcinomatosis a preponderance of lymphocytes is usually found.

Imaging

As discussed above (see Trauma), the volume of peritoneal fluid first detected by US is about 600mL. Detection limits for CT and MR are unknown. Larger amounts of fluid are visible even with conventional radiography. Uncomplicated ascites generally has an attenuation of 0 to 40HU, but in clinical practice the attenuation values alone are not a reliable indicator for differentiating among a transudate, malignant ascites, or a hemorrhage. Most ascites is readily differentiated from other intraabdominal fluid, but care is necessary in the pelvis. An adnexal cyst, such as a fallopian tube cyst, may mimic pelvic ascites.

Although both ascites and intrathoracic fluid abut the diaphragm, CT can nearly always distinguish between them. Fluid in the subphrenic space tends to extend to the paracolic gutters and the posteriorly located Morison’s pouch, and forms a sharp outline with abdominal viscera, but there is sparing of the bare area of the liver. With massive ascites CT detects the medial umbilical folds, representing peritoneal reflections, in about two thirds of patients. With the patient supine, intrathoracic fluid extends more medially than ascitic fluid and collects in the posterior sulci.

Occasionally ascitic fluid enhances on delayed contrast-enhanced CT. In most patients this phenomenon is of little significance and is even a potential pitfall suggesting perforation.

Especially in a setting of adhesions or tumor, US is useful in guiding both fluid aspiration and a biopsy or cytology needle.

Lesser sac fluid is readily identified with imaging. Some peripancreatic fluid that appears to be in the lesser sac on CT, however, may actually be located within adjacent tissue planes.