This presentation reviews six challenging and diverse cases in soft tissue pathology, emphasizing diagnostic pitfalls, required immunohistochemical markers, and molecular associations. Key cases include Sclerosing Epithelioid Fibrosarcoma (SEF) and its morphological overlap with Low-Grade Fibromyxoid Sarcoma (LGFMS); the differentiation of benign Lipoblastoma from Myxoid Liposarcoma in pediatric patients; and a detailed exploration of the dramatic morphological heterogeneity within Dedifferentiated Liposarcoma (DDLPS), including a rare meningothelial whirl pattern surrounding skeletal muscle fibers. The lecture also covers vascular tumor recognition challenges, such as an Angiosarcoma masked by an inflammatory background (ruptured cyst) and a Kaposi Sarcoma lacking typical extravasated red blood cells, necessitating ancillary testing like HHV8. Finally, the varied and sometimes atypical morphology of the benign Angiofibroma (Fibrous Papule) of the nose is presented as a diagnostic consideration.
Soft Tissue Pathology Case Review (FedPath 2023)
0:26 Case 1: Sclerosing Epithelioid Fibrosarcoma (SEF):
Details: Large, deep soft tissue mass in an adult. Features monotony, lacks pleomorphism (suggesting a translocation tumor).
Key Pattern: Cords and chains of round/epithelioid cells dissecting dense, sclerotic collagen, often with artifactual clefting.
Diagnosis/Takeaway: SEF is positive for MUC4 (similar to LGFMS). It can show hybrid features blending into LGFMS areas (bland spindle cells, fine collagen). SEF is often more aggressive than LGFMS. Molecularly, it usually involves EWSR1-CREB3L1 fusion.
Details: Soft tissue mass in a young child (5-year-old). Lipomas are uncommon in children.
Key Pattern: Mature fat arranged in distinct lobules divided by dense fibrous septa, often with focal myxoid areas. The myxoid areas contain bland signet-ring lipoblasts, mimicking fetal fat development and potentially resembling Myxoid Liposarcoma (MLS).
Takeaway: If doubt exists, molecular testing is required. Lipoblastoma has PLAG1 rearrangements, while MLS (rare but the most common sarcoma in children) has FUS-DDIT3 fusions.
13:35 Case 3: Dedifferentiated Liposarcoma (DDLPS):
Details: Large retroperitoneal mass in an adult (DDLPS is the top differential for high-grade sarcoma here). Confirmed by the juxtaposition of a high-grade sarcoma component and a Well-Differentiated Liposarcoma (WDLPS) component (atypical cells, sclerotic collagen, variable-sized fat).
Morphological Variation: DDLPS shows extreme heterogeneity, presenting areas that resemble Myxofibrosarcoma or even contain pleomorphic lipoblasts (homologous differentiation).
Rare Feature (19:07): The tumor displays a fascinating meningothelial whirl pattern of spindle cells, and unusually, these whirls developed around central skeletal muscle fibers (20:07).
Takeaway: DDLPS requires testing for MDM2 amplification (FISH preferred).
Details: Papule on the nose, clinically thought to be a ruptured cyst. Histology showed a ruptured cyst and inflammation.
Pitfall/Takeaway: A meticulous examination revealed large, atypical endothelial cells (CD31 positive) forming infiltrative, anastomosing vascular channels within the inflammatory background. This highlights the risk of overlooking a malignancy (Angiosarcoma) when an adjacent benign diagnosis (ruptured cyst) is confirmed.
Details: Multinodular, cellular spindle cell proliferation on the foot of an elderly man (Classic KS). Features streaming fascicles and abundant mitoses.
Diagnostic Challenge (27:29): The first example was tricky because it lacked the expected classic feature of heavy extravasated red blood cells (RBCs) filling slit-like spaces.
Takeaway: Not all vascular tumors contain abundant blood. If highly suspicious, use vascular markers (ERG is highly recommended for clarity) and, definitively, the highly specific HHV8 immunostain. Classic KS often affects the lower extremities in older adults, linked to HHV8 and often associated with lymphedema.
31:12 Case 6: Angiofibroma (Benign Fibrous Papule):
Details: Papule on the nose (common location). Shows bizarre multinucleated giant cells (sometimes Touton-like), stellate spindle cells, dense collagen, and dilated vessels.
Takeaway: This benign lesion has a wide range of atypical morphological findings, including pleomorphism. The key features (dense collagen, dilated vessels, plump stellate cells) should prevent confusion with malignant mimics like Atypical Fibroxanthoma (AFX).