Case 130

A 45-year-old female with a large ankle mass:

1.If the tumor did not show a MDM2 amplification, what's the best interpretation?

A: Spindle cell lipoma

B: Desmoid fibromatosis

C: Giant cell fibroblastoma

D: Atypical spindle cell / pleomorphic lipomatous tumor

E: Pleomorphic liposarcoma

Answer

The correct answer is D. The key is the low power infiltrative/irregular border. The lesion is composed of bland spindle cells with abundant myxoid stroma, ropy collagen, and scant admixed fat. The lesion has a somewhat irregular border and does not appear to be encapsulated. Overall, the findings are of an atypical spindle cell/pleomorphic lipomatous tumor.

The tumor displays the following key features:

• Ill-defined Margins: The tumor has poorly defined borders, making it harder to delineate from surrounding tissues (referenced in multiple studies).

• Cellular Composition:

  • Spindle Cells: Mild to moderate atypia present in spindle cells.

  • Adipocytes: Predominantly mature but with variations in size and shape.

  • Lipoblasts: These vary in appearance, from small univacuolated/bivacuolated to larger, pleomorphic, multivacuolated forms.

  • Multinucleated Cells: Bizarre, hyperchromatic, and pleomorphic multinucleated cells often appear scattered within the spindle or adipocytic components.

• Adipocytic Atypia: Focal mild to moderate atypia with features like nuclear enlargement and chromatin coarsening can be observed.

• Extracellular Matrix: The tumor contains either a myxoid or collagenous matrix, with ropy collagen bundles.

• Mitotic Figures: These are present, though generally scarce.

• Tumor Necrosis: No necrosis observed.

• Heterologous Differentiation: Rarely, smooth muscle, cartilage, or bone elements may be seen within the tumor.

Case credit: UCSD Pathology

Author: Wangpan Jackson Shi, MD