Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

cytoma is extremely rare. These tumors present with a palpable mass or simply with an enlarging liver. Most are large at initial presentation. Some contain calcifications.

These are poorly marginated tumors showing inhomogeneous contrast-enhancement. Their differential diagnosis includes hepatocellular carcinoma, angiosarcoma, and a hypervascular metastasis.

hypointense on both T1and T2-weighted MR images.

Some appear solid at US but more cystic at CT and MRI, a finding useful in suggesting the diagnosis. T1-weighted MRI reveals a hypointense tumor, modified by scattered high signal intensities from hemorrhage,while T2-weighted MRI shows them to be mostly hyperintense. Heterogeneous, irregular enhancement, often

Undifferentiated Carcinoma/Sarcoma

Undifferentiated Embryonal Cell

Carcinoma/Sarcoma

Also called mesenchymal sarcoma, undifferentiated sarcoma, and malignant mesenchymoma, an undifferentiated embryonal cell carcinoma is a rare, highly malignant tumor found mostly in older children and young adults. These tumors grow rapidly, patients have few symptoms, and often the first sign is an abdominal tumor.

These tumors are unique in that they are mostly solid, yet imaging often suggests the presence of a cystic component—an appearance secondary to their high water content within a myxoid stroma (Fig. 7.41). Superimposed hemorrhage and necrosis are common. Internal calcifications and a surrounding fibrous capsule are evident in some. Imaging identifies a wellencapsulated multilocular, often heterogeneous tumor. Some have peripheral contrast enhancement of a fibrous rim, with the rim being

Figure 7.41. Undifferentiated embryonal sarcoma. Contrastenhanced CT reveals a liver cyst-like tumor with a fluid–fluid level. These are mostly solid tumors, but due to their high water content CT often shows them to be of almost water density. (Courtesy of Luann Teschmacher, M.D., University of Rochester.)

more peripheral in location, is evident on postcontrast images.

Some of these tumors are initially misdiagnosed as hepatobiliary cysts, with a correct diagnosis made only after recurrence.

Undifferentiated (Rhabdoid) Tumor

A rare undifferentiated malignant primary liver tumor containing rhabdoid features develops in infants. Superficially, tumor cells resemble cells of muscle origin. Some contain neuroepithelial differentiation. The a-fetoprotein level is normal or only slightly elevated.

Metastases

Clinical

In a patient with a known nonhepatic cancer, what is the risk of a small hepatic tumor, discovered by CT, being a metastasis? A study of almost 3000 patients with cancer found small hepatic tumors (1cm or less in diameter or too small to characterize by CT) in 13%, among these tumors interval growth was identified in 12% of patients and these tumors presumably represent metastasis (133); further analysis revealed that small hepatic tumors were metastatic in 4% of patients with lymphoma, 14% of those with colorectal carcinoma, and 22% of breast cancer patients.

Patients undergoing systemic chemotherapy for breast cancer metastatic to the liver develop an appearance similar to cirrhosis. Computed tomography reveals liver capsule retraction, an irregular lobular margin, segmental volume loss, and caudate lobe enlargement, a finding also seen in cirrhosis; retraction occurs at sites of metastases. Pathologic findings suggest regenerative nodules. An indented capsule and fibrous septa can extend into liver parenchyma, an appearance presumably due to chemother- apy-induced tumor regression followed by healing and scar formation. The overall appearance has been called hepar lobatum, a term of old used to describe liver syphilis.

Septicemia is a recognized presentation of an occult malignancy, especially a colorectal cancer. Likewise, some colorectal cancer liver metastases become infected and the patient presents with a liver abscess. Whether some anaerobic bacteria tend to grow selectively in

ADVANCED IMAGING OF THE ABDOMEN

tumor nodules rather than normal liver parenchyma is conjecture.

A solitary liver tumor detected synchronously with a gastric cancer is usually presumed to be a metastatic focus. In high-risk regions, such as East Asia, a concomitant hepatocellular carcinoma should also be considered in the differential diagnosis.

Detection

Most metastases have a nodular appearance and are sharply demarcated from adjacent liver parenchyma, at times by inflammation or fibrosis; diffuse tumor infiltration, as occasionally seen with primary liver carcinomas and leukemia, is unusual; extensive intrahepatic obstruction of lymphatic flow by metastases is seen on postcontrast CT as hypodense regions throughout the liver.

Calcifications develop in metastatic welldifferentiated mucinous adenocarcinomas. Occasionally visible with conventional radiography, CT reveals them as fine, speckled calcifications. They are echogenic and have posterior shadowing with US.

Division between hypoand hypervascular metastases (Table 7.15) is not absolute, and overlap occurs. Thus an occasional hypovascular lymphoma and hepatocellular carcinoma and an occasional isovascular metastasis are encountered. Breast carcinomas, in particular, although usually somewhat hypervascular, exhibit a wide range of imaging appearances. Some liver metastases, especially neuroendocrine and ovarian tumors, have a cystic component (Fig. 7.42).

Table 7.15. Vascular appearance of liver metastases

LIVER

Figure 7.42. Multiple cystic liver metastases from a cystic pancreatic acinar carcinoma.

Comparison Studies

A number of imaging comparison studies are difficult to place in perspective because they contain an unknown number of false positive foci. In general, combining contrast enhanced CT, US and MRI allows detection of false positive tumors and distinguishing them from metastases.

A number of earlier studies concluded that MRI is superior to CT in detecting liver metastases. In particular, early postcontrast breath-hold T1-weighted and T2-weighted fat-suppressed images are very useful. Whether the introduction of multislice CT has changed these conclusions is not yet clear.

Pulse inversion harmonic US using a contrast agent identifies a similar number of metastases as helical CT (134); it detects more and smaller metastases than conventional US. In one study pulse inversion harmonic US detected 90% of metastases found on ferumoxides-enhanced MRI (135).

CT arterial portography and MRI (including arterial dominant phase, gadolinium-enhanced and spoiled gradient-echo imaging) appear similar in metastasis detection (136). Earlier studies had suggested that SPIO-enhanced MR sensitivity for metastases detection was inferior to CT arterial portography, but more recent studies show these two techniques to be similar in accuracy.

Imaging Techniques

In the United States, Canada, and parts of Europe, CT is the preferred screening modality for suspected liver metastases. In some regions US is more often performed. For hypovascular hepatic metastases, unenhanced helical CT detected 66%, arterial-dominant phase CT 74% and portal-dominant phase CT 92% of focal tumors (137); portal phase imaging depicted significantly more of these tumors than did the other two phases. Whether all three phases need be obtained for suspected hypovascular metastases continues to be debated. MRI, especially using various contrast agents, continues to gain ground.

Liver metastases range from hypoechoic to hyperechoic. In general, tumor echogenicity is related to tumor vascularity. Thus the more vascular metastatic neuroendocrine tumors tend to be hyperechoic, while less vascular metastases are more hypoechoic. No Doppler signal is observed in avascular or necrotic metastases.

Contrast-enhanced harmonic US modifies the late-phase appearance of liver parenchyma. Thus with pulse inversion harmonic US normal liver parenchyma appears hyperechoic relative to focal metastases (Fig. 7.43). Compared to unenhanced US, a contrast agent increases metastases detection sensitivity and specificity by improving metastases conspicuity. Some metastases develop a rim-like enhancement after contrast injection.

Intraoperative US is an accurate tool for detecting unsuspected metastases and is used as an aid for intraoperative biopsies and when resecting metastatic foci. Nevertheless, compared to eventual pathological study, even intraoperative US detects only about 80% of metastases. Laparoscopic US also identifies biopsy sites.

Precontrast MRI reveals most hypovascular metastases to be hypointense on T1and isointense to slightly hyperintense on T2-weighted images, findings similar to many other liver lesions. Most metastases are poorly marginated. Nevertheless, considerable variability in appearance exists. A hyperintense T1-weighted image is seen with hemorrhage. Mucincontaining tumors also tend toward a hyperintense signal on T1-weighted images. Tumor necrosis leads to a hypointense center on T2weighted images. Suggesting a metastasis is a

LIVER

Rim Enhancement

Metastases reveal varying postcontrast perilesional (rim) enhancement, best seen during the arterial phase. Some tumors compress adjacent liver parenchyma and a thick tumor border represents surrounding inflammation, desmoplastic reaction, small vessel proliferation, and eventual atrophy, although this finding explains perilesional enhancement only partly. In some tumors rim enhancement represents viable tumor surrounding a necrotic center. Aside from an abscess and an occasional hepatocellular carcinoma, few other common conditions have a similar appearance. Peripheral enhancement is also seen with cavernous hemangiomas, but their enhancement is more irregular and nodular in appearance.

Ultrasonography of some metastases reveals a hypoechoic halo or doughnut appearance, with the hypoechoic rim representing proliferating tumor cells rather than surrounding inflammation.

Similar to CT, during arterial-phase MRI, some larger hypovascular metastases reveal a peripheral enhancing rim; this enhancement gradually extends centrally, and the overall appearance superficially mimics a hemangioma. These metastases are poorly identified on capillary phase imaging. An occasional metastasis has a hypointense rim on delayed postgadolinium MRI, called the peripheral washout sign, described as being specific for a malignancy.

Hypovascular Metastases

Size of colorectal carcinoma metastases differ on precontrast and postcontrast CT images. Thus during tumor follow-up not only the same modality but also a similar technique should be employed.

About a third of colorectal metastases <1cm in diameter enhance homogeneously. Larger ones tend to develop a cauliflower-like appearance and CT arteriography often detects rim enhancement. Most metastases are detected with portal-phase imaging, but arterial-phase imaging does increase tumor conspicuity and diagnostic confidence. Postcontrast CT identified 85% of tumors found at resection in patients with liver metastases from colorectal cancer and is useful in selecting patients for resection (138); false-positive findings included

hemangiomas, hemorrhage, periportal fibrosis, and even normal parenchyma.

Computed tomography arterial portography is more sensitive than biphasic CT or US in detecting metastatic colorectal cancer, especially for lesions <10mm in diameter. Sensitivities of colorectal cancer metastases detection for CT arterial portography, CT hepatic arteriography, and both procedures combined were 80%, 83%, and 87%, respectively (139); combined CT arterial portography and CT hepatic arteriography significantly improved metastases detectability, and the authors recommend both procedures for pretherapy evaluation. A practical limitation of CT arterial portography is a high false-positive rate due to perfusion anomalies and the presence of nonneoplastic hypervascular tumors. Computed tomography arterial portography also tends to overestimate tumor size.

In patients with known colorectal metastases, Regardless of tumor size, the accuracy of SPIO-enhanced T2-weighted sequences is significantly greater than with precontrast sequences (140). Also, SPIO-enhanced MR sensitivity appears to be greater than with contrastenhanced CT.

In patients treated for colorectal cancer and suspected of harboring a recurrence, a number of studies have confirmed the superiority of FDG-PET imaging over CT; FDG-PET detects more unsuspected metastases.

Bone scintigraphy with Tc-99m–hydrox- ymethylene diphosphonate (HMDP) occasionally reveals ring-like tracer activity around a liver metastasis from colon carcinoma. The reason for such extraosseous radionuclide accumulation is not known.

An occasional colorectal carcinoma exhibits somatostatin receptors. The somatostatin analogue octreotide used to treat neuroendocrine tumors might be useful in this subgroup of cancers, yet little data exists on this topic.

In women with known or suspected liver metastases from breast cancer, prospectively evaluated with triple phase (precontrast, arterial phase, and portal venous phase) CT, portal venous phase is most accurate, and the routine use of precontrast and arterial phases does not appear warranted when the clinical question is simply the presence or absence of liver metastases (141).