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Myxoid Liposarcoma

Myxoid liposarcoma (MLPS) is a malignant tumor of uniform round to oval cells with small lipoblasts in a jelly-like stroma and a branching capillary network. It typically has FUS-DDIT3 or, less commonly, EWSR1-DDIT3 fusion transcripts. High-grade, more cellular tumors previously called round cell liposarcoma are included in this category.

Symptoms & Causes

Introduction

Myxoid liposarcoma is a type of cancerous tumor that develops in fat tissue. It’s characterized by the presence of round or oval-shaped cells, sometimes with small lipoblasts (immature fat cells), embedded in a jelly-like substance called myxoid stroma. The tumor also has a distinctive network of branching capillaries (tiny blood vessels). Specific genetic changes, like translocations involving the FUS and DDIT3 genes, are typically found in these tumors.

Reference
WHO Classification of Tumours Editorial Board. Soft tissue and bone tumours [Internet]. Lyon (France): International Agency for Research on Cancer; 2020 [cited 2024 09 11]. (WHO classification of tumours series, 5th ed.; vol. 3). Available from: https://tumourclassification.iarc.who.int/chapters/33.

Related Terminology
Not recommended: round cell liposarcoma

Subtype(s)
None

Symptoms

MLPSs typically present as large, painless masses. Retroperitoneal MLPS most often represents a metastasis. Multifocal disease, either synchronous or metachronous, represents distant soft tissue metastases of monoclonal origin. Surgical wide excision is the current treatment mainstay. MLPS is extremely radiosensitive and relatively sensitive to anthracyclines and trabectedin compared with other sarcomas.

Localization

MLPSs typically occur within the deep soft tissues of the extremities, most often the thigh. Very rarely, they arise as primary tumors in the subcutis (the layer of fat beneath the skin) or retroperitoneum.

Epidemiology

MLPS accounts for approximately 20-30% of liposarcomas and 5% of adult soft tissue sarcomas, with no significant difference in occurrence between males and females. Although the peak incidence is in the fourth to fifth decades of life, MLPS is the most common liposarcoma subtype in children and adolescents.

Etiology

Unknown

Diagnosis & Treatment

Staging

The American Joint Committee on Cancer (AJCC) and Union for International Cancer Control (UICC) TNM staging systems can be applied.

Pathogenesis

MLPS and its hypercellular subtype (formerly known as round cell liposarcoma) share the same genetic abnormality. Most cases are characterized by a translocation between chromosomes 12 and 16, resulting in the fusion of the FUS and DDIT3 genes. This fusion creates a chimeric protein that disrupts normal cell processes and prevents fat cells from maturing properly.

Macroscopic appearance

MLPSs are typically large (>10 cm), well-defined, multinodular tumors within muscles. The cut surface is smooth, gelatinous, and glistening. Higher-grade tumors have a firmer, fleshy tan surface. Visible tissue death (necrosis) is uncommon. Adequate sampling to assess the degree of hypercellularity (increased cell density) is crucial, as this is a significant factor in determining prognosis.

Histopathology

Under a microscope, MLPSs appear as moderately cellular tumors with a lobulated structure and increased cellularity at the periphery. They consist of uniform, small, oval cells without clear fat cell features, along with varying numbers of small lipoblasts. The tumors have abundant myxoid stroma (a jelly-like substance) with a distinctive network of branching capillaries, often surrounded by tumor cells.

Cytology

Cytological examination (studying cells under a microscope) shows varying proportions of small round cells in a myxoid matrix containing thin-walled branching vessels.

Prognosis and prediction

Local recurrence (the tumor returning at the original site) occurs in 12-25% of cases. Distant metastases (spread to other parts of the body) develop in approximately 30-60% of cases, sometimes years after the initial diagnosis, and may progress slowly. Unlike most sarcomas, MLPSs often metastasize to other soft tissue sites and can also spread to bone (especially the spine), rather than the lungs. Tumors with a high histological grade (>5% hypercellularity) have a significantly higher risk of metastasis or death from the disease. The presence of necrosis (tissue death) and alterations in the TP53 and CDKN2A genes are associated with a worse prognosis. The prognostic significance of transitional areas with more limited hypercellularity is less certain. There is no association between different FUS-DDIT3 transcript isoforms and tumor grade or prognosis.

Clinical Features

Diagnostic molecular pathology

Demonstrating the presence of the translocation or fusion transcript can help distinguish MLPS from other myxoid sarcomas and high-grade MLPS from various round cell sarcomas.

Essential and desirable diagnostic criteria

Essential: myxoid matrix containing delicately arborizing capillaries; bland round to oval cells; variable number of small non-pleomorphic lipoblasts, often adjacent to capillaries; hypercellularity, diminished myxoid matrix, obscured capillaries, and elevated nuclear grade and mitotic activity in high-grade MLPS.

Desirable: demonstration of DDIT3 rearrangement (FUS-DDIT3 or EWSR1-DDIT3 fusion genes).

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