This pathology presentation details a case of Dedifferentiated Liposarcoma (DDLPS) originating as a large mass in the retroperitoneum of an adult. The diagnosis is based on the combination of a high-grade, hypercellular atypical spindle cell sarcoma component (often resembling Undifferentiated Pleomorphic Sarcoma) and an adjacent component of well-differentiated liposarcoma. The video emphasizes the significant morphological variation within DDLPS, including areas with a mixoid background and pleomorphic lipoblasts ("homologous lipogenic differentiation"). The most unique and fascinating feature of this specific case is the presence of "meningothelial whirls"—swirling arrangements of spindle cells—which are observed wrapped around internal fibers of skeletal muscle. Furthermore, the tumor background contains multivacuolated lipoblasts resembling the pattern seen in hibernoma (brown fat). The video concludes by stressing the importance of considering DDLPS in retroperitoneal masses and utilizing MDM2 amplification testing (preferably FISH) for definitive diagnosis.
Dedifferentiated Liposarcoma (DDLPS) with Unique Morphologies
0:08 Initial Diagnosis & Location: The tumor is a hypercellular mass of atypical spindle cells with mixoid background and necrosis, found in the retroperitoneum of an adult. DDLPS is the top differential diagnosis for a high-grade sarcoma in this location.
1:04 Defining DDLPS: The definitive diagnosis is established by the presence of both a high-grade sarcoma component and an adjacent component of Well-Differentiated Liposarcoma (WDLPS).
1:11 WDLPS/ALT Criteria: WDLPS (or Atypical Lipomatous Tumor [ALT] in extremities) is characterized by mature fat cells of variable sizes, fibrous/sclerotic bands, and scattered hyperchromatic atypical cells.
2:04 Morphological Variability: The dedifferentiated component can vary widely, often resembling Undifferentiated Pleomorphic Sarcoma (UPS). Importantly, a small subset of DDLPS may contain pleomorphic lipoblasts, a feature now recognized as "homologous lipogenic differentiation."
3:30 Key Takeaway on Mixoid Change: The presence of a mixoid, blue background in a liposarcoma does not exclusively confirm it as Myxoid Liposarcoma; DDLPS, WDLPS, and Pleomorphic Liposarcoma can all exhibit mixoid changes.
5:22 Unique Morphological Pattern: The case features rare "meningothelial whirls"—swirling areas of spindle cells that resemble the pattern seen in meningioma. This pattern is well-described but uncommon in DDLPS.
6:33 Highly Unusual Finding: A bizarre observation is that each meningothelial whirl is centered around a fiber of skeletal muscle. This is suspected to be due to the tumor infiltrating and swirling around existing muscle rather than tumor differentiation.
7:16 Brown Fat/Hibernoma Pattern: The background spaces around the whirls are filled with bland, multivacuolated lipoblasts, mimicking the appearance of brown fat or a hibernoma.
7:41 Diagnostic Confirmation: DDLPS must always be considered in retroperitoneal tumors. MDM2 amplification testing (Fluorescence In Situ Hybridization/FISH is preferred over Immunohistochemistry) can be used to support the diagnosis, especially in small biopsies.