Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

combination increases extent of coagulation compared with use of radiofrequency alone [124].

Tumor necrosis is identified as a hypointense region on T2-weighted images and as a lack of enhancement on dynamic MR images. Whether the reverse is true, namely, whether residual hyperintensity on T2-weighted images and postcontrast enhancement signify residual tumor, is unclear. Appearance of new nod-

ules signifies treatment failure, with new lesions often being more common than local recurrence.

A multicenter survey of 1139 patients by the Korean Study Group of Radiofrequency Ablation found major complications in 2.4%, with intrahepatic abscess being most common (125); one procedure-related death occurred due to peritoneal hemorrhage. Needle tract tumor seeding was reported by several institu-

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tions. Whether heating surrounding tissue during needle withdrawal decreases the risk of tumor seeding remains to be established. Gastrointestinal perforation due to thermal damage is a potentially lethal complication.

Contrast-enhanced arterialphase CT reveals recurrence as a nodular, halo, or simply gross nodule enlargement pattern (126). FDGPET/CT is also useful in evaluating therapy and following these patients after therapy.

Radiofrequency ablation has a high success rate in eliminating smaller tumors. It is effective for recurrent cancers after a partial hepatectomy. The major long term limitation to patient survival is recurrence of new cancers in these usually cirrhotic livers. Ablation can, however, serve as a bridge to liver transplantation.

Photocoagulation: Interstitial laser photocoagulation is performed by passing modified needles, under US, CT, or MR guidance, into a tumor, inserting a fiberoptic probe, and then heating the surrounding tissues using a highpower output laser generator. A wide region of necrosis is achieved with this technique. On-line MR thermometry provides control during laser ablation. Follow-up contrast-CT evaluates the degree of tumor necrosis. In general, best results are achieved with tumors <3cm in diameter.

Using an open low field strength MR unit, lesion conspicuity and ease of puncture planning were significantly improved when the liverspecific contrast agent mangafodipir (Mn-DPDP) was used (7).

A neodymium:yttrium-aluminum-garnet (Nd:YAG) laser probe introduced with a percutaneously positioned laser application system achieved tumor necrosis with a 5-mm safety margin in 98% of 61 tumors (127); tumors up to 2cm in diameter were treated with a single laser application. The number of needles used during therapy varies with tumor size; a single optical fiber and a single needle insertion were used in nodules < or = 2cm, while two sessions with two laser illuminations per session were used for nodules >4cm.

Percutaneous laser photocoagulation can be combined with other therapy, such as arterial chemoembolization; the practical advantages of such therapy are yet to be established.

Microwave Coagulation: Microwave coagulation therapy requires insertion of probes that act as antennae for externally applied microwave energy. One or more electrode is

inserted into a tumor and the resultant heating leads to tumor coagulation necrosis. This technique is performed both via laparotomy and percutaneously. Superficially located hepatocellular carcinoma nodules appear most amenable to microwave therapy. Preliminary results, mostly from the Far East, appear encouraging (128).

Computer tomography shortly after microwave coagulation therapy identifies peripheral enhancement, which gradually disappears.

After microwave coagulation, MRI reveals tumors becoming more hypointense and heterogeneous on T1-weighted images and surrounded by a hyperintense rim on T2-weighted images, which enhances postgadolinium. This enhancing rim represents granulation tissue and hyperperfusing surrounding liver parenchyma. Lack of CT and MR contrast enhancement within the tumor and lack of flow signals on Doppler US imply tumor necrosis.

Complications of this technique include intratumoral hemorrhage, pleural effusion, ascites, abscess, subcapsular hematoma, portal vein thrombosis, and tumor dissemination.

Cryoablation: Cryoablation therapy, also called cryosurgery and cryotherapy, destroys tumor tissue by freezing and is used to treat both primary and metastatic tumors. Cryoablation appears to have a role in patients with multiple or advanced tumors deemed unresectable, as an adjunct to surgical resection and in patients with recurrent tumors. The size of induced freezing can be controlled, and thus precise control is feasible. Previously the relatively large size of available cryoablation probes necessitated at least a laparoscopic approach. Smaller probes are becoming available, and a percutaneous approach using imaging guidance is feasible. Liquid nitrogen is the freezing medium, although argon gas using less bulky equipment is also available.

Freezing causes ice crystals to form in tissue close to the cryoprobe. Cell death results from subsequent thawing and cell membrane rupture. Ice forms in small blood vessels slightly further from the probe and results in cell hypoxia.

Cryoablation is feasible on lesions adjacent to major blood vessels because flowing blood protects these vessels from freezing. On the other hand, a tumor edge may be incompletely frozen if the tumor abuts a blood vessel and temporary

interruption of blood flow through the vessel involved may be necessary. Cryoablation of tumors close to major bile ducts is avoided; duct wall necrosis and bile leakage are potential complications. Likewise, tumors close to the liver surface are preferentially resected rather than frozen.

Coagulation abnormalities are common after cryoablation. Over half of patients develop a reduction in platelets >50%. Possible hypothermia is a complication with prolonged therapy. Bleeding, at times massive, is believed to be due to parenchymal fragmentation secondary to freezing. A syndrome of multiorgan failure, severe coagulopathy, and disseminated intravascular coagulation has been described, called cryoshock phenomenon. A survey of centers performing hepatic cryoablation identified cryoshock in 1% of over 2000 patients, and cryoshock was believed to be responsible for some perioperative deaths (129). The procedure is not innocuous.

Intraoperative US reveals the ice ball to be hyperechoic with posterior shadowing, thus permitting control of the size of the frozen tissue. Ultrasonography shortly after freezing reveals a hypoechoic center surrounded by a hyperechoic rim, representing the interface between frozen and unfrozen tissues.

Computer tomography, US, or MRI monitors results of cryoablation. Probes and equipment designed to work with MRI should aid follow-up.

Follow-Up

Patient survival after initial tumor detection varies considerably; serum creatinine, alkaline phosphatase,and Okuda’s stage are independent predictors of survival. Serum a-fetoprotein is of limited value in detecting early recurrence posthepatectomy.

Liver scintigraphy appears to have a role in detecting recurrence. Shortly after nonsurgical therapy, however, scintigraphy reveals wider tracer uptake than the actual tumor.

Recurrent tumors have been treated by carbon dioxide-enhanced US guided percutaneous ethanol injection, achieving cumulative patient survival rates of 81%, 71% and 44% for 1, 2 and 3 years, respectively (130).

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Regression

An occasional hepatocellular carcinoma regresses spontaneously, a phenomenon generally associated with tumor necrosis.

Fibrolamellar Hepatocellular Carcinoma

A distinct variant is a fibrolamellar hepatocellular carcinoma. Developing in adolescents and young adults, usually it is not associated with underlying liver disease. These often large, solitary cancers tend to be well differentiated and have a good prognosis if completely resected, yet a number have already spread when first detected, with metastatic lymphadenopathy being not uncommon. In 31 patients with fibrolamellar hepatocellular carcinoma, 81% had a solitary tumor, 42% developed intrahepatic biliary obstruction, 87% had portal or hepatic vein invasion, extrahepatic grown was present in 42%, distant metastases were evident in 29%, and lymphadenopathy identified in 65% (131).

Serum a-fetoprotein levels tend to be normal in these patients. Transforming growth factor-b (TGF-b) is a pluripotent regulatory molecule present in liver but not in hepatocytes. It induces fibrosis in a number of diseases. An inverse correlation exists between TGF-b and serum a-fetoprotein levels. Although TGF-b is detected in a minority of hepatocellular carcinoma cells, it is present in a majority of fibrolamellar hepatocellular carcinomas. Nevertheless, the place of TGF-b in evaluating hepatocellular carcinomas is still unclear. TGF-b levels decrease after successful therapy.

Computed tomography shows a wellmarginated, solid tumor with more uniform contrast enhancement than seen with conventional hepatocellular carcinomas. Some fibrolamellar carcinomas contain small, punctate calcifications. Central fibrosis (scar) is common, but hemorrhage and necrosis are uncommon. Arterial-phase images reveal heterogeneous enhancement with regions of hypervascularity.

Magnetic resonance imaging reveals a heterogeneous focal tumor that is hypointense on T1and hyperintense on T2-weighted images. Intense, heterogeneous immediate postcontrast enhancement is common with these tumors

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Figure 7.37. T2–weighted MRI identifies a fibrolamellar carcinoma as a slightly hyperintense right lobe tumor (arrow). It was almost isointense on T1-weighted images (not shown). (Source: Burgener FA, Meyers SP, Tan RK, Zaunbauer W. Differential Diagnosis in Magnetic Resonance Imaging. Stuttgart: Thieme, 2002, with permission.)

(Fig. 7.37). A central fibrous radiating scar is hypointense on both T1and T2-weighted images, in distinction to the central scar of focal nodular hyperplasia, which is characteristically hyperintense on T2-weighted images. Also, fibrolamellar carcinoma scars are more prominent and often extend to the tumor periphery, findings not seen with FNH. The central scar enhances on delayed images in some tumors. Nevertheless, some fibrolamellar carcinomas mimic focal nodular hyperplasia.

Exceptions do occur. Some of these tumors are homogeneously hypointense on T1and hyperintense on T2-weighted images with the central scar being markedly hyperintense on T2-weighted images, findings at variance from most fibrolamellar carcinomas.

Angiography shows these tumors to be hypervascular and contain numerous septa.

A more aggressive surgical approach than with a similar more conventional hepatocellular carcinoma appears reasonable with these tumors; even vascular reconstruction is justified.

Hepatocellular Carcinoma with

Sarcomatous Changes

An occasional hepatocellular carcinoma contains a sarcomatous component, suggesting differentiation along both epithelial and mesenchymal lineages. Chondrosarcomatous differentiation and even a combination of chondrosarcomatous and osteosarcomatous components have developed, although in some of these tumors osteoclast-like giant cells probably are reactive and not neoplastic.

The diagnosis is usually made after resection. These tumors have a poor prognosis.

Some are multiple, suggesting intrahepatic metastases.

Biopsy results vary, depending on the site chosen for biopsy.

Combined Hepatocellular

and Cholangiocarcinomas

Rare single tumors contain components of both hepatocellular carcinoma and cholangiocellular carcinoma. The original cell type is often conjecture. Some of these tumors presumably have one of two origins—either a double cancer or a stem cell origin, with the latter representing an intermediate type between a hepatocellular and a cholangiocarcinoma. Complicating the picture is that both a hepatocellular and a cholangiocellular carcinoma have occurred synchronously or metachronously independent of each other within the same liver; the lack of direct tumor contact, no histologic transition, and different immunohistochemical characteristics suggest separate histogenesis.

These combined tumors have one of two CT appearances: Those grossly resembling a hepatocellular carcinoma show early contrast enhancement and become hypodense during the portal venous phase, while those grossly resembling a cholangiocarcinoma reveal early peripheral contrast enhancement and contain heterogeneous hypodense regions or only central enhancement during the portal venous phase.

Adenosquamous/Squamous Carcinoma

A primary squamous liver carcinoma is rare and the is pathogenesis unknown. One hypothesis is

that the squamous cell component originates from a metaplastic transformation of adenocarcinoma cells. A number of these tumors develop in a setting of chronic infection or bile stasis. An occasional one presents as a liver abscess.

These tumors exhibit an aggressive growth pattern with early local and distant spread.

Lymphoma

Primary

Primary hepatic lymphomas are rare and their diagnosis difficult. The occasional association with hepatitis C virus has already been mentioned. Most of these tumors are non-Hodgkin’s T-cell lymphomas. In distinction to secondary liver lymphomas, these usually present as a discrete tumor. Less common are primary low-grade B-cell mucosa-associated lymphoid tissue (MALT) lymphomas, which often consist of a dense portal tract lymphoid infiltrate mimicking hepatitis or a bile duct inflammatory condition.

These homogeneous lesions appear either anechoic or hypoechoic with US and mimic a

ADVANCED IMAGING OF THE ABDOMEN

cyst, although unlike a cyst they lack throughtransmission. Some are surrounded by a halo. They tend to increase in size rapidly, with tumor volume doubling time being measured in days.

Most primary liver lymphomas are hypointense on T1and hyperintense on T2-weighted images. They show heterogeneous postcontrast enhancement.

Secondary

Non-Hodgkin’s lymphoma is more often focal than Hodgkin’s lymphoma. Some secondary liver lymphomas infiltrate diffusely and lead to acute liver failure. Imaging tends to underdetect liver involvement. Calcifications develop in some tumors. Portal and periportal intrahepatic infiltrates compress adjacent bile ducts and are a cause of jaundice (Fig. 7.38).

Secondary lymphomas typically are hypointense on T1and hypoto hyperintense on T2-weighted images. Postcontrast enhancement varies considerably; tumors that are moderately hyperintense on T2-weighted images tend toward intense enhancement during early postgadolinium images, while those mildly

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hypoto mildly hyperintense on T2-weighted images show only minimal early postgadolinium enhancement. Not uncommonly, immediate postcontrast images reveal a perilesional or peripheral enhancement.

Use of phosphorus MR spectra from P-31 MR spectroscopy appears potentially useful in evaluating liver involvement with lymphoma.

Most patent portal vein branches within a tumor detected by imaging imply a benign lesion, the exceptions being intrahepatic cholangiocarcinoma and lymphoma; even with periportal lymphomatous infiltration, not uncommonly patent portal vein branches are evident. Nevertheless, some primary liver lymphomas do invade the portal vein and result in tumor thrombi.

Leukemia

Leukemic infiltration of the liver ranges from focal to diffuse. At times a chronic lymphocytic leukemia transforms into a high-grade lymphoma. Some infiltrates surround portal vessels.

Hepatoblastoma

Clinical

A hepatoblastoma is the most common primary liver neoplasm in children under 3 years. These tumors occur throughout the pediatric age range, with only a rare one encountered in an adult. A male predominance is evident. It occurs in siblings. Prevalence is increased in BeckwithWiedemann syndrome and in hemihypertrophy. In some patients prior maternal exposure to certain drugs and chemicals appears to play a role.

A common presentation in young children is an asymptomatic abdominal mass. A large liver tumor is typical in adults. These tumors produce a number of hormones and some patients develop hypoglycemia, hypercalcemia, polycythemia, and precocious puberty in males. Most of these tumors are associated with an elevated serum a-fetoprotein level, which aids in distinguishing them from other, similarappearing benign tumors.

This tumor probably originates from embryonal hepatic tissue and usually consists of epithelial cells or, less often, a mixture of epithelial and mesenchymal cells. The latter occasion-

ally contains differentiated cells, such as osteoid that calcifies. These tumors readily metastasize. One of the roles of imaging is to detect metastases and determine resectability. Recurrence after therapy, including other organ involvement, is common in adults.

Imaging

A hepatoblastoma detected in an adult has no specific defining imaging findings. In children, from an imaging viewpoint a hepatoblastoma is similar in appearance to a hepatocellular carcinoma (Fig. 7.39). It readily invades adjacent vessels. When extensive, a hepatoblastoma involves almost the entire liver. Some contain both cystic and solid components. Necrosis and calcifications develop in some. An occasional one contains ossified tissue.

Precontrast CT reveals a hypodense mass. Variable contrast enhancement is seen postcontrast.

A typical US pattern is a hyperechoic lesion with poorly defined margins. The degree of echogenicity reflects hemorrhage and necrosis within the tumor.

Similarly to many other liver tumors, a hepatoblastoma is hypointense on T1and hyperintense on T2-weighted MRI, although

Figure 7.39. Hepatoblastoma with pulmonary metastases in a 2-year-old girl. The tumor was discovered incidentally. Contrastenhanced CT reveals a large slightly enhancing tumor (arrows) containing central necrosis and calcifications. (Courtesy of Luann Teschmacher, M.D., University of Rochester.)

considerable heterogeneity is often evident. Magnetic resonance, especially 3D MRA, is useful in defining vascular invasion prior to partial hepatectomy.

Angiography reveals a hypervascular tumor containing neovascularity.

Occasionally a hepatoblastoma shows uptake of sulfur colloid; a hepatoblastoma, however, does not have uptake of both Tc-99m–sulfur colloid and HIDA, thus differentiating these tumors from focal nodular hyperplasia.

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all appearance can mimic a cavernous hemangioma.

These tumors decrease in size after chemotherapy, gradually becoming isointense to liver and enhancing negligibly postcontrast.

Fibrosarcoma

Fibrosarcomas and its benign counterpart are rare in the liver. Solitary fibrous liver tumor in three patients, two benign and one malignant, presented as well-marginated, heterogeneously enhancing tumors (132).

Sarcoma

Almost any type of sarcoma can develop in the liver, including a rare primary liver osteosarcoma.

Angiosarcoma

Hepatic angiosarcomas are rare, yet still are the most frequent primary mesenchymal liver tumor. Most develop in elderly men and most have a very poor prognosis. Metastasis is common, with survival measured in months.

A distinct association exists with prior exposure to certain chemical carcinogens, such as thorium dioxide (Thorotrast), vinyl chloride, arsenic salt therapy, cyclophosphamide treatment, and anabolic steroids. Many of these tumors, however, develop with no known predisposing factors. An angiosarcoma can be associated with disseminated intravascular coagulopathy.

Most angiosarcomas consist of multifocal but well-marginated nodules throughout the liver, seen with precontrast CT as hypodense tumors. Commonly superimposed hemorrhage modifies their imaging appearance considerably. Some display hypointense central regions on T2-weighted images and postcontrast peripheral nodular enhancement, and superficially mimic a hemangioma, but central hemorrhage is common and their heterogeneous, asymmetrical enhancement usually allows differentiation between these entities. Occasional massive tumor invasion is not detected by CT or US but is identified by MRI.

Not uncommonly, angiosarcomas are hypointense on T1and markedly hyperintense on T2-weighted images, with hypointense septa identified on T2-weighted images; the over-

Leiomyosarcoma

A primary hepatic leiomyoma or leiomyosarcoma is uncommon. Some of these patients have had another previous malignancy, and a genetic predisposition to neoplasms is postulated. Some leiomyosarcomas originate in the adjacent ligamentum teres. Most metastatic leiomyosarcomas are from the gastrointestinal tract, with an occasional one being from the uterus, vena cava, or other extraperitoneal structures.

Most patients present with right upper quadrant pain and hepatomegaly.

Imaging shows similar findings for both primary and metastatic tumors. A large, irregular tumor is common. Computed tomography appearance ranges from homogeneous with little enhancement to heterogeneous and peripheral enhancement. Some contain cystic regions. Heterogeneous contrast enhancement is common, with enhancement ranging from central to peripheral. An internal low-density region suggests fluid.

Most of these tumors are homogeneous and hypointense on T1-weighted MR images, while T2-weighted images reveal most to be sharply outlined, homogeneous, and markedly hyperintense, mimicking a hemangioma; metastases heterogeneous on T1or T2-weighted images typically contain varying degrees of necrosis and hemorrhage (Fig. 7.40).

Percutaneous biopsy should be diagnostic of a leiomyosarcoma.

Histiocytoma

Although not uncommon in extremity soft tissues, primary malignant liver fibrous histio-