This presentation, "Soft Tissue Pathology in 30 Minutes," provides a rapid, case-based review of several complex and rare soft tissue tumors, focusing on diagnostic morphology, key differentials, and molecular findings. The featured cases include Sclerosing Epithelioid Fibrosarcoma (SEFS), highlighting its connection to Low-Grade Fibromyxoid Sarcoma (LGFMS), and Lipoblastoma, contrasting it with the rare pediatric Myxoid Liposarcoma. The discussion also explores the extreme morphological heterogeneity of Dedifferentiated Liposarcoma (DDLPS), including a unique case exhibiting meningothelial whirls. Finally, the presentation addresses crucial diagnostic pitfalls, demonstrating an Angiosarcoma masked by a ruptured cyst and a Kaposi Sarcoma lacking typical hemorrhage, concluding with a common, yet highly variable, benign entity: Angiofibroma.
Soft Tissue Pathology in 30 Minutes: A Case-Based Review
0:48 Case 1: Sclerosing Epithelioid Fibrosarcoma (SEFS)
Morphology: Large, deep mass with cellular nodules of monotonous spindle cells. The key finding is cords and chains of round/epithelioid cells dissecting through a dense, sclerotic collagen background (1:42).
Diagnosis: SEFS is positive for MUC4 (2:48) and is often considered to have a hybrid relationship with Low-Grade Fibromyxoid Sarcoma (LGFMS).
Molecular: Usually involves an EWSR1-CREB3L1 fusion (5:55), though a subset can have the FUS-CREB3L2 fusion seen in LGFMS.
Takeaway: SEFS can be aggressive and the cord-like pattern on a small biopsy may mimic metastatic carcinoma (4:36).
Patient Context: Mass in a 5-year-old child (lipomas are rare in children, necessitating caution).
Morphology: Mature fat arranged in striking lobules divided by dense fibrous septa (8:34). Mixoid areas contain Bland signet-ring-like lipoblasts and small capillaries (9:52)—resembling fetal fat development.
Differential: Myxoid Liposarcoma (MLPS), which is the most common liposarcoma in children.
Molecular Distinction: Lipoblastoma is characterized by PLAG1 rearrangements (12:06); MLPS features FUS-DDIT3 fusions (11:52).
Takeaway: Recognize the lobulated structure and bland lipoblasts. Molecular testing may be needed if mixoid change is extensive.
13:37 Case 3: Dedifferentiated Liposarcoma (DDLPS)
Context: Large retroperitoneal mass in an adult (DDLPS is the top differential for high-grade sarcomas in this location).
Components: Adjacent areas of well-differentiated liposarcoma (WDLPS—mature fat, fibrous bands, scattered atypical cells) and a high-grade sarcoma component (dedifferentiated area) (14:32).
Morphologic Variation: DDLPS shows extreme heterogeneity, including mixoid fibrosarcoma-like areas (16:36) and areas with bizarre, pleomorphic lipoblasts (homologous lipogenic differentiation) (16:58).
Key Rare Feature: The tumor contains meningothelial whirls (19:07) with the unusual finding of entrapped fibers of skeletal muscle in the center of the whirls (20:08).
Diagnosis Support: Confirm DDLPS using MDM2 amplification testing (21:20).
Context: Lesion on the nose, clinically suspected as a ruptured cyst.
Pitfall: The slide showed an obvious ruptured cyst and inflammation (22:04). However, closer inspection revealed large, atypical cells in the background inflammatory infiltrate (22:43).
Context: Multinodular cellular proliferation on the foot of an elderly man. Cells are spindled, arranged in streaming fascicles, with relatively abundant mitotic figures (25:26).
Key Pitfall: The presented case had very minimal hemorrhage (27:29). The speaker provides a comparison to a more typical KS case showing extravasated red blood cells/slit-like spaces (26:47).
Diagnosis: Diagnosis is confirmed using the highly reliable HHV8 immunostain (29:16).
Takeaway: Not all vascular tumors have abundant blood. If a lesion is unclassifiable or stains negative on standard panels, use vascular markers (ERG, CD31, CD34) and specifically HHV8 for suspected KS (28:08).
31:12 Case 6: Angiofibroma / Benign Fibrous Papule
Context: Papule on the nose of an adult.
Morphology: Features dense collagen bundles, dilated vessels (33:17), and plump/stellate spindle cells. The key variation in this example is the presence of bizarre, sometimes foamy, multinucleated giant cells (31:40).
Takeaway: Angiofibroma (Fibrous Papule) has a vast array of benign unusual morphologies (giant cells, clear cells, pleomorphism) but is consistently found on the face/nose. It should not be confused with malignant pleomorphic tumors (33:49).