This pathology presentation focuses on the diagnosis of Sclerosing Epithelioid Fibrosarcoma (SEF), a rare and often aggressive soft tissue tumor. The video analyzes a large, deep soft tissue mass in an adult, highlighting the key histological features that distinguish SEF: monotonous round or epithelioid tumor cells arranged in cords and chains, dissecting through a background of dense sclerotic collagen, frequently resulting in characteristic artifactual clefting. Diagnosis is strongly supported by MUC4 immunostaining, a marker shared with the related Low-Grade Fibromyxoid Sarcoma (LGFMS). The presentation discusses the challenges of diagnosing SEF on small biopsies (where it may mimic metastatic carcinoma) and differentiates its typical molecular signature (EWSR1-CREB3L1 fusion) from that of LGFMS (FUS-CREB3L2), while noting the existence of hybrid tumors.
Sclerosing Epithelioid Fibrosarcoma (SEF): Pathological Features and Diagnosis
0:08 Case Presentation: The case involves a large, deep soft tissue mass in an adult, showing high variability, including areas of cellular nodules with monotonous spindle cells (0:29). The monotonous appearance suggests a translocation tumor.
0:54 Key Diagnostic Pattern: The essential pattern is identified in areas with dense sclerotic collagen exhibiting artifactual clefting (1:08) around the tumor cells.
1:32 Classic SEF Morphology: The tumor cells are round or epithelioid and are arranged in distinct cords and chains that dissect through the very dense sclerotic collagen background.
2:03 Immunostain Confirmation (MUC4): The diagnosis of SEF is confirmed by positive MUC4 staining (2:07). MUC4 positivity is shared by SEF and Low-Grade Fibromyxoid Sarcoma (LGFMS), indicating a relationship between the two entities.
2:21 Relationship to LGFMS: SEF is related to LGFMS (Evans tumor). Some tumors are "hybrid," merging bland spindle cell areas resembling LGFMS (3:04) with classic SEF patterns. SEF is typically more rapidly aggressive than LGFMS (3:20).
3:51 Diagnostic Pitfall: On a small needle biopsy, the infiltrative cord-like growth pattern in sclerotic collagen can be mistaken for metastatic carcinoma (e.g., lobular breast carcinoma), emphasizing the importance of recognizing this specific pattern.
5:10 Molecular Genetics (SEF): SEF generally involves gene rearrangement of EWSR1, usually fused with CREB3L1.
5:24 Molecular Genetics (LGFMS Comparison): LGFMS typically involves FUS fused with CREB3L2, although a subset of SEF can share this LGFMS fusion.
6:16 Second Case Example: A separate patient example illustrates the heterogeneity of SEF, showing variable cellularity, a mixoid background, and a small round blue cell appearance (6:39), but still retaining the characteristic cord-like pattern in a sclerotic background.