Liposarcoma Imaging 

Liposarcoma is a malignant tumor of mesenchymal origin in which the bulk of the tumor differentiates into adipose tissue. Liposarcoma is a common neoplasm of the soft tissues, and it affects middle-aged patients. Commonly affected sites include the thigh, gluteal region, retroperitoneum, leg, and shoulder area.

Liposarcomas rarely arise from a preexisting lipoma. The clinical presentation depends on the site of the tumors, most of which are palpable masses. The radiologic characteristics of liposarcomas are demonstrated in the images below.

In general, the more aggressive tumors demonstrate more radiopacity, whereas well-differentiated tumors have a greater fat content. Liposarcomas are the most radiosensitive soft-tissue tumors.

Preferred examination

Computed tomography (CT) scanning plays an important role in the preoperative evaluation of lipomatous and myxoid tumors of the soft tissue.^[1] Besides providing valuable morphologic information, CT scan findings help to some extent in the differentiation of various types of lipomatous tumors. Well-differentiated liposarcomas may be distinguished from other types of tumors on the basis of their largely lipomatous appearance.

Gadolinium-enhanced magnetic resonance imaging (MRI) is important in differentiating myxoid liposarcomas from benign cystic tumors.

Findings in plain radiography are nonspecific, and the modality usually offers little except perhaps aid in confirming the presence of a soft-tissue mass. Frequently, fat is not detectable radiologically. Ultrasonography is helpful in confirming the presence of a mass when doubt exists. The use of radionuclides has been anecdotal, but imaging with fluorine-18 fluorodeoxyglucose ([18F]FDG) positron emission tomography (PET) scanning holds promise.^{[2, 3]}

Limitations of techniques

Well-differentiated tumors are easily characterized as fat-containing masses, although differentiation between a malignant tumor and a benign one may not be possible. Poorly differentiated tumors may be indistinguishable from other mesothelial tumors, lymphomas, metastases, and inflammatory masses.

Malignant fibrous histiocytoma, leiomyosarcoma, and desmoid tumors may have a CT scan appearance that is indistinguishable from that of liposarcoma, particularly liposarcoma of the myxoid, mixed myxoid and round cell, round cell, or pleomorphic type.

Because myxoid tissue is present in many benign and malignant tumors, MRI may not enable a specific diagnosis of a liposarcoma. Moreover, any lesion consisting of edema, an extracellular matrix with a high level of mucopolysaccharide, hyaline cartilage content, and necrosis may appear cystic on MRI scans.

Radiographic findings are seldom diagnostic, and the images may demonstrate a nonspecific soft-tissue mass. Frequently, no fat is detectable. Rarely, calcification (seen in the image below) is present.

Abdominal radiographs in patients with retroperitoneal tumors may reveal a soft-tissue–displacing, gas-filled structure and effacement of the normal fat planes.

Contrast-enhanced studies of the gastrointestinal tract may show displacement of the stomach, small bowel, or colon, depending on the location of the tumor.

Intravenous urography may show renal or ureteric displacement; rarely, hydronephrosis is demonstrated.

Degree of confidence

The sensitivity and specificity of radiographs are low in liposarcoma.

False positives/negatives

Inflammatory masses and other types of benign and malignant tumors can have a similar appearance.

CT scans demonstrate 3 distinct patterns in liposarcomas, as follows:

• An enhancing, solid, inhomogeneous, poorly defined, infiltrating mass
• A mixed-pattern tumor with foci of fat interspersed in high-attenuating tissue
• A pseudocystic water-density tumor.

Liposarcomas of the myxoid type, the mixed myxoid and round cell type, the round cell type, and the pleomorphic type are usually poorly defined, with attenuation values of 12-38 HU and varying degrees of contrast enhancement.^{[4, 5]} Calcification is detectable in as many as 12% of the tumors.

Occasionally, the mass may appear inhomogeneous, with areas of low-attenuating, fatty components.^{[6, 7]} Fatty components may be demonstrated better with planar tomography.

The CT-scan characteristics of liposarcomas are demonstrated below.

Degree of confidence

CT scanning plays an important role in the preoperative evaluation of lipomatous and myxoid tumors of the soft tissue.^{[8, 9]} Besides providing valuable morphologic information, CT scanning helps to some extent in differentiating various types of lipomatous tumors.^{[10, 11]} CT scanning is useful in the determination of the tumoral response to radiation therapy or chemotherapy; it is also invaluable in the detection of tumor recurrence.

False positives/negatives

Malignant fibrous histiocytoma, leiomyosarcoma, and desmoid tumors may have an appearance that is indistinguishable from that of liposarcoma, particularly liposarcoma of the myxoid, mixed myxoid and round cell, round cell, or pleomorphic type.

Most liposarcomas appear well defined on MRI scans, mostly with lobulated margins.^{[12, 13]} Well-differentiated liposarcomas are made up primarily of fat and have septations or nodules; these neoplasms are hyperintense on T2-weighted images.^{[4, 14, 15]} After the administration of contrast material, well-differentiated liposarcomas may enhance minimally or not at all. (See the images below.)

A spectrum of abnormal MRI findings may occur in the myxoid type, depending on the quantity of fat and myxoid material that is present, on the degree of cellularity and vascularity, and on whether or not necrosis exists.^[16] Most myxoid tumors have linear or lacy, amorphous foci of fat.^[17] Some myxoid tumors may appear cystic on nonenhanced MRI scans, but they are usually enhancing after the administration of contrast agents.^[18]

Pleomorphic tumors show a markedly heterogeneous internal structure and moderate contrast enhancement. The malignancy grade is believed to increase in parallel with tumor heterogeneity and contrast enhancement.

Degree of confidence

Well-differentiated liposarcomas may be distinguished from other types of tumors on the basis of their largely lipomatous appearance.^{[19, 20]} Gadolinium-enhanced imaging is important in differentiating myxoid liposarcomas from benign cystic tumors.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography (MRA) scans.

As of late December 2006, the Food and Drug Administration (FDA) had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see Medscape.

False positives/negatives

Myxoid tissue is present in many benign and malignant tumors, including extraskeletal myxoid chondrosarcomas, intramuscular myxomas, ganglia, and myxoid, malignant fibrous histiocytomas, in addition to myxoid liposarcomas. Any lesion consisting of edema, an extracellular matrix with a high level of mucopolysaccharide, hyaline cartilage content, and necrosis may appear cystic on MRI scans.

Ultrasonography is helpful in confirming the presence of a mass.^{[21, 22]} Liposarcomas are usually hyperechoic. Retroperitoneal liposarcomas are highly reflective, although this feature may be absent when the tumor is poorly differentiated.

The finding of a solid retroperitoneal mass that demonstrates a heterogeneous echo pattern with an echo-poor center usually suggests a sarcoma. The central echo-poor area is usually the result of hemorrhage or necrosis (because the tumors tend to outgrow their blood supply).

A well-differentiated, peripheral liposarcoma is usually hyperechoic and may be indistinguishable from a lipoma; however, Doppler ultrasonography studies reveal that a liposarcoma is more vascular than a lipoma.^[23]

The remaining 3 varieties of liposarcoma appear as a heterogeneous soft-tissue mass with no distinguishing characteristics.

Degree of confidence

Confirming the retroperitoneal origin of a tumor is not always possible with ultrasonography. However, some characteristic features may be helpful in locating the tumor's origin. These include anterior displacement of the pancreas, abdominal aorta, inferior vena cava, kidneys, and ascending or descending colon. The benign or malignant nature of retroperitoneal tumors cannot be determined by using ultrasonography.

False positives/negatives

Vascular tumors, such as hemangiopericytomas, can be highly reflective, presumably because of the numerous tissue interfaces with multiple vascular walls. Distinguishing poorly differentiated liposarcomas from other types of retroperitoneal or peripheral masses is not always possible. A peripheral, well-differentiated liposarcoma may have the appearance of a lipoma.

It appears that gallium-67 (⁶⁷ Ga) scanning can play an important part in the evaluation of patients presenting with a primary or metastatic soft-tissue sarcoma, according to a study by Imaeda et al. In the investigation,⁶⁷ Ga citrate scintigraphy scanning was evaluated in 90 patients with soft-tissue tumors of the limbs. Positive findings were found in 78% of patients with malignant tumors, 25% of patients with benign lesions, and 31% of patients with other types of lesions. Distinguishing liposarcoma from lipoma appeared to be possible by means of a⁶⁷ Ga scan, according to the authors.^[24]

⁶⁷ Ga scanning may also have a role in imaging liposarcoma recurrence.^[25]

A study of 78 patients with malignant soft-tissue sarcoma who were evaluated with thallium-201 (²⁰¹ Tl) chloride revealed a radionuclide sensitivity of 81%.^[26] Another study showed that technetium-99m (^99m Tc) pertechnetate can potentially aid in the localization of malignant soft-tissue tumors and may be useful in their evaluation.^{[27, 28]}

A study demonstrated that^99m Tc bleomycin and^99m Tc pentavalent dimercaptosuccinic acid (^99m Tc[V]-DMSA) scanning can better localize liposarcomas than can⁶⁷ Ga imaging.^[29]

Liposarcomas are usually hypovascular to moderately vascular, and they displace major vessels, particularly the inferior vena cava.^[30] Moderately hypervascular liposarcomas may show irregular, fine tumor vessels and areas of tumor stain (as in the images below). Venous filling may occur early, and the veins may be dilated and tortuous. Displacement of the kidneys and arteries is seen in all except very small retroperitoneal tumors.

Degree of confidence

Angiography may be useful for preoperative planning, intra-arterial infusion, and/or transcatheter embolization.

False positives/negatives

Angiography cannot help in the differentiation of liposarcomas from other types of sarcomas. Benign and malignant retroperitoneal tumors can be avascular.