Imaging Studies
CT scanning and MRI of the primary tumor may provide complementary findings in patients with liposarcoma. [18] CT scanning is superior to MRI in detailing cortical bone erosion and tumor mineralization.
MRI is useful in providing views of the long axis of the limb and can sometimes depict the fatty nature of the tumor. Different subtypes exhibit varying signal characteristics. [19] For example, myxoid liposarcomas have low signal on T1W, high signal on T2W, and thickened septa on postcontrast images. Round-cell liposarcomas and pleomorphic liposarcomas lack characteristic fat signal and look similar to other soft tissue sarcomas.
Angiography may reveal malignancy based on prominent vascularity and, thus, may be of value in planning surgical resection.
Fluorodeoxyglucose (FDG) positron emission tomography (PET) imaging may also reveal malignancy, based on intensity of FDG activity. [20] It is also useful to diagnose bone metastases and might be more sensitive than bone scanning. [21]
Chest radiography may be used as initial screening for pulmonary metastases; however, the definitive modality used to detect pulmonary metastases is chest CT scanning. CT scanning of the chest and abdomen may be needed to evaluate for extrapulmonary metastatic spread, which may be seen with myxoid liposarcoma.
Procedures
The diagnostic procedure of choice is open biopsy. Minimally invasive procedures have been advocated, such as fine needle aspiration; however, experience with these modalities is limited, and results have been equivocal.
For superficially located small fatty tumors, excisional biopsy is recommended for diagnosis.
For larger (>3 cm) and deeper tumors, diagnosis and treatment may involve open incisional biopsy followed by definitive resection.
Histologic Findings
Gross appearance can widely vary from tumor to tumor. Usually, the tumor displays smooth, lobulated, or nodular masses that are both encapsulated and freely movable. Upon section, fine fibrous septa can be observed dividing the tumor into smaller lobules. Areas of necrosis and recent and old hemorrhage are not infrequent. Calcification and ossification are uncommon. [22]
Several histologic subtypes are recognized; the 4 primary subtypes are well-differentiated, myxoid, round cell, and pleomorphic. Myxoid is the most common subtype. In a series of 82 patients younger than 22 years, myxoid liposarcomas accounted for 70-90% of tumors analyzed. This histologic finding is the very important since most myxoid subtypes are low-grade tumors with an excellent prognosis. [12]
Well-differentiated liposarcomas are low-grade lesions composed of a heterogeneous organization of univacuolate adult lipocytes, chronic inflammatory cells, and fibrous connective tissue. This fibrous matrix harbors an arrangement of fat-storing stellate-shaped cells. Neoplastic cells with large hyperchromatic and pleomorphic nuclei are scattered within the region. Fat necrosis and mitosis usually are not observed. However, 10% are dedifferentiated, with a pattern of a densely cellular spindle-shaped sarcoma without fat accumulation and a clinically aggressive course.
Myxoid liposarcomas occur in various forms based on the degree of cellularity. Basic features of this subtype include (1) proliferating lipoblasts in various stages of differentiation, (2) delicate plexiform capillary patterns, and (3) a myxoid matrix containing abundant hyaluronidase-sensitive acid mucopolysaccharides. The least cellular pattern is a low-grade tumor harboring small spindle-shaped cells. These cells surround a plexiform vascular pattern that contrasts with a background of extracellular acid mucopolysaccharide. The most cellular pattern is that of a round-cell liposarcoma containing densely packed uniformly round cells that overcrowd the plexiform vasculature. In reality, most myxoid-type lesions have a mixture of the above features.
Round-cell liposarcomas are associated with an aggressive clinical course and a high frequency of metastases. These are high-grade tumors, and their primary feature is an excessive proliferation of uniform and rounded cells with little intercellular myxoid material. Occasionally, cells are arranged in branching rows and strands along abortive capillaries, resulting in a trabecular and adenoid appearance. Large pale polyhedral cells often give a hypernephroid appearance to the tumor.
Pleomorphic liposarcomas are high-grade lesions and display a disorderly growth pattern, a large degree of cellular pleomorphism, and several bizarre giant cells. Large giant cells may be observed harboring numerous lipid droplets of varying size, giving a moruloid appearance to the cells. Nucleoli vary in occurrence. Numerous small polygonal, round, and spindle-shaped lipoblasts can be observed intermingled with giant cells.
Staging
Staging for childhood soft tissue sarcomas follows the standard American Joint Committee on Cancer (AJCC) system, which is of limited value. Liposarcomas do not spread to regional lymph nodes, and lymph node dissection is not indicated. Careful imaging is crucial in staging to assess size and presence or absence of metastases. Considering the possibility that a retroperitoneal or chest wall lesion may have a primary in the lower limb is important.
Grade or histologic subtype impacts significantly on prognosis, and an experienced pathologist needs to be involved. Myxoid liposarcomas that have more than 5-20% round-cell component have a worse outcome.
The AJCC staging system is as follows:
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G1 - Low grade
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G2 - Intermediate grade
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G3 - High grade
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T1a - Noninvasive (< 5 cm)
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T1b - Noninvasive (>5 cm)
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T2a - Invasive (< 5 cm)
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T2b - Invasive (>5 cm)
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N1 - Regional nodes involved
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M1 - Distant metastases
