This pathology case review focuses on the diagnosis of a 12 cm subcutaneous back mass in a middle-aged woman, which initially presented morphologic features suggestive of Well-Differentiated Liposarcoma (WDLPS). However, the crucial finding of negative MDM2 amplification via FISH rules out WDLPS.
The tumor, which exhibits marked heterogeneity (mature fat, sclerotic collagen, myxoid/spindled areas), contains hallmark features of Pleomorphic Lipoma, including thick, rope-like collagen bundles and atypical cells with wreath-like or floret-like hyperchromatic nuclei. These tumors belong to the Spindle Cell/Pleomorphic Lipoma spectrum, characterized by deletion of the 13q chromosome (RB1 gene loss).
The video discusses the appropriate terminology for this large, atypical presentation, noting the proposed term "Atypical Pleomorphic Lipomatous Tumor" (APLT), which is recognized in the 5th edition WHO soft tissue tumor book. Despite its alarming appearance, APLT is considered benign with no risk of metastasis or de-differentiation. The discussion concludes with immunohistochemistry pearls, including a suggestion for using HMGA2 staining to distinguish APLT from WDLPS.
Atypical Pleomorphic Lipomatous Tumor (APLT) Case Review
0:16 Case Presentation: A 12 cm subcutaneous back mass in a middle-aged woman is analyzed.
0:24 Key Molecular Finding: MDM2 amplification by FISH was negative, ruling out the vast majority of Well-Differentiated Liposarcomas (WDLPS).
0:38 Morphological Heterogeneity: The tumor is highly heterogeneous, showing areas of mature fat and dense sclerotic collagen, alongside less fatty regions that are spindled and myxoid.
1:25 Initial Differential: The presence of mature fat, sclerotic collagen, and large pleomorphic/hyperchromatic cells strongly mimics WDLPS/Atypical Lipomatous Tumor (ALT).
2:27 WDLPS/ALT Distinction: WDLPS and ALT are the same tumor, named differently based on location (ALT for superficial/extremity tumors, WDLPS for retroperitoneal tumors, which have a significantly worse prognosis).
3:16 Diagnostic Features: The tumor differs from WDLPS by displaying thick, rope-like collagen bundles (characteristic of Spindle Cell/Pleomorphic Lipoma) and atypical cells with wreath-like or floret-like nuclei (the classic feature of Pleomorphic Lipoma).
3:45 Molecular Mechanism: Spindle cell and pleomorphic lipomas are associated with loss or deletion of the 13q chromosome (where the RB1 gene resides). Loss of nuclear RB1 staining is often seen.
5:01 Terminology Debate: The proposed name for these large, ugly tumors at the upper end of the Pleomorphic Lipoma spectrum is Atypical Pleomorphic Lipomatous Tumor (APLT). This term is considered confusing but is included in the new 5th edition WHO soft tissue tumor book.
6:11 Clinical Behavior (Key Takeaway): APLT is fundamentally benign. It does not metastasize and carries no known risk of de-differentiating into high-grade sarcoma. Simple excision is usually curative, although negative margins may be recommended for these large, alarming tumors.
8:51 MDM2 Testing Preference: The speaker expresses hesitation regarding the use of MDM2 immunostain due to the risk of false positives from histiocyte staining in areas of fat necrosis, preferring to proceed directly to MDM2 FISH.
11:41 Alternative IHC (French Method): A guest suggests using HMGA2 immunostaining as a helpful tool. HMGA2 is typically positive in Lipomas/WDLPS but negative in Pleomorphic Lipoma/APLT, avoiding the histiocyte issues associated with MDM2 IHC.