Laboratory Studies
Cytogenetics may be of value when diagnosing lipomatous tumors because different tumor types have different more or less specific chromosomal abnormalities. [30] The lipoblastoma, for example, often exhibits rearrangements of bands 8q11-13, and the gene PLAG1 has been implicated as the target of these chromosomal changes. Atypical lipomatous tumors may display gene amplifications within the 12q13-15 region, including HMG2A (high-mobility group protein 2a), YEATS4 (YEATS domain containing 4), and CPM (carboxypeptidase M). [2]
Imaging Studies
CT scanning (see image below) is superior to MRI in detailing cortical bone erosion and tumor mineralization, whereas MRI is useful in providing views of the long axis of the limb and in depicting the fatty nature of the tumor.
Computed tomography (CT) scan of a left thigh shows a huge mass (arrows) with predominant fat attenuation. The central soft-tissue component (asterisk) and thick, internal septations are consistent with liposarcoma.
Most liposarcomas have well-defined and mostly lobulated margins. The well-differentiated liposarcomas are composed of mainly fat with septa or nodules. These tumors are hyperintense on T2-weighted images, and they demonstrate faint enhancement or no enhancement after the intravenous administration of contrast material.
Myxoid liposarcomas are homogeneous or mildly heterogeneous, and a pseudocapsule can be present. Pleomorphic types have a markedly heterogeneous internal structure. Both myxoid and pleomorphic lesions have moderate or marked heterogeneous enhancement after the administration of contrast material. Well-differentiated liposarcomas may be distinguished from the other types by their largely lipomatous appearance. The malignancy grade increases with the degree of tumor heterogeneity and contrast enhancement.
Angiography may demonstrate tumor malignancy on the basis of prominent vascularity; thus, angiography may be of value in planning surgical resection.
Chest radiography may be used as an initial screening for pulmonary metastases; however, the definitive test for detection of pulmonary metastases is chest CT scanning.
An early-phase bone scan may show a marked increase of radioisotopic uptake.
Risk assessment in liposarcoma patients can be based on [(18)F]fluorodeoxyglucose (FDG) PET imaging. [31] Although tumor grade and subtype are considered standard parameters for risk assessment in patients with liposarcoma, pretherapy tumor standardized uptake values obtained by FDG PET imaging was found to be a more useful parameter for risk assessment in liposarcoma compared with tumor grade or subtype. A maximum standardized uptake value of more than 3.6 was associated with significantly reduced disease-free survival and identified patients at high risk for developing early local recurrences or metastatic disease.
Also see Liposarcoma Imaging.
Procedures
Preoperative biopsy is important in planning therapy. [32] The diagnostic procedure of choice for liposarcoma is open biopsy. With superficial, small, fatty tumors, excisional biopsy is recommended for diagnosis. In large (>3 cm) and deep tumors, diagnosis and treatment may involve open incisional biopsy followed by definitive resection.
Fine-needle aspiration or biopsy should be followed by histologic and immunohistochemical examination. Adjunctive tests for MDM2 (murine double minute 2) may be helpful to distinguish liposarcoma from benign fatty neoplasms, but cutaneous and subcutaneous pleomorphic liposarcoma is less likely to demonstrate amplification. [33, 34] Immunohistochemical examination aids in excluding other sarcomas. Lipid staining may be helpful, although Sudan black or oil red O stains are generally insufficient for diagnosis. Helpful stains include the following:
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S-100 - Positive results in fat cells and lipoblasts
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Alpha-1-antitrypsin - Positive results in malignant fibrous histiocytomas
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Desmin - Positive results in leiomyosarcomas
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Myoglobin - Positive results in rhabdomyosarcomas
Histologic Findings
The recognition of lipoblasts is the key finding in the diagnosis of liposarcoma. A lipoblast has the ability to produce and accumulate non–membrane-bound lipid within its cytoplasm. The key morphologic features are well-demarcated cytoplasmic lipid that shifts, causes indentations in an irregular hyperchromatic nucleus, and creates a characteristic scalloping of the nuclear membrane.
The stage and further differentiation into 1 of the 4 major types affect the prognosis.
Well-differentiated liposarcomas usually contain a predominance of mature fat cells with relatively few, widely scattered lipoblasts. A misdiagnosis of lipoma can result from inadequate sampling. In the sclerosing subtype of a well-differentiated liposarcoma, collagen fibrils that encircle fat cells and lipoblasts make up a prominent part of the matrix.
Myxoid liposarcoma, the most common type, is diagnosed by the observation of a delicate plexiform capillary network that is associated with both primitive mesenchymelike cells and a variable number of lipoblasts. The stroma contains a large proportion of myxoid ground substance (ie, hyaluronic acid), in which numerous microcysts may form.
In the round-cell type, lipoblasts are interspersed among sheets of poorly differentiated round cells.
Poorly differentiated pleomorphic liposarcoma is recognized by a mixture of bizarre, often multivacuolated lipoblasts and atypical stromal cells, many of which contain highly abnormal mitotic figures. Hemorrhagic and necrotic areas are common. Lipoblasts are present.
Staging
The Enneking oncologic staging system defines the biologic behavior of primary tumors. This system has proven to be effective in planning surgery for limb lesions (eg, intralesional, marginal, wide, radical) and in evaluating its results. Note the following:
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The Enneking staging system divides benign tumors into 3 stages: S1, S2, and S3.
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Localized malignant tumors are divided into 4 stages: IA, IB, IIA, and IIB.
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Two other stages include metastatic high-grade intracompartmental tumors, or stage IIIA tumors, and extracompartmental malignant tumors, or stage IIIB tumors.
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This classification scheme was formerly used to describe long-bone tumors.
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This staging system is based on a complete preoperative workup that includes an assessment of the clinical features; the radiographic pattern and CT and MRI data regarding the extension of the tumor; the peculiar imaging features of the tumor and its relationship to the neighboring tissues; the findings from isotopic scanning, which provides information about local aggression and systemic diffusion; and the histologic findings obtained at biopsy.
Surgical staging is appropriate only after the diagnosis is established and the oncologic stage is determined.
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Computed tomography (CT) scan of a left thigh shows a huge mass (arrows) with predominant fat attenuation. The central soft-tissue component (asterisk) and thick, internal septations are consistent with liposarcoma.
