This pathology case review focuses on a deep, dissecting spindle cell tumor from the trunk of a 34-year-old female, diagnosed as Dermatofibrosarcoma Protuberans (DFSP). The definitive diagnostic feature highlighted is the highly infiltrative pattern resulting in "fat trapping," where islands of adipocytes are choked and stranded within the tumor mass in the subcutaneous tissue. Cytologically, the tumor cells are bland, elongated, and CD34 positive, distinguishing them from highly pleomorphic sarcomas. The video identifies an area suggestive of fibrous sarcomatous transformation—a higher-grade change marked by increased cellularity and fascicles, which elevates the risk of metastasis. The review concludes by discussing the molecular basis (primarily COL1A1-PDGFB fusion) and the necessity of using FISH testing for definitive confirmation in ambiguous or small biopsy specimens due to the required extensive surgical margins.
Dermatofibrosarcoma Protuberans (DFSP) Case Review
0:09 Case Introduction: The case involves a 34-year-old female presenting with a slow-growing trunk lesion.
0:35 Gross Pattern: The lesion is a deep, dissecting spindle cell proliferation extending significantly below the skin surface into the subcutaneous fat.
1:43 Key Diagnostic Feature – Fat Trapping: DFSP is characterized by the tumor wiping out the normal subcutis, leaving behind "stranded and squished" islands or clusters of adipocytes ("fat trap"). This feature is highly suggestive of DFSP.
3:09 Cytology: The spindle cells in DFSP are thin, elongated, and bland, often resembling neural cells. They typically do not exhibit the pleomorphism seen in other sarcomas.
3:24 Immunohistochemistry (IHC): DFSP cells stain positive for CD34. They are generally negative for S100 (though a rare exception was noted).
4:34 Histological Patterns: DFSP is a translocation sarcoma classically described as having a swirled storiform pattern, though vague fascicles can also be present.
6:27 Infiltration and Margins: The tumor aggressively "snakes out" through the fat, making margin assessment (particularly for Mohs surgery) highly challenging. DFSP is locally aggressive, requiring wide local excision or Mohs surgery, which often results in significant scarring and morbidity.
7:10 Fibrosarcomatous Transformation: An area of increased cellularity, sweeping fascicles, and a mild herringbone pattern is identified, suggesting fibrous sarcomatous transformation. This transformation increases the metastatic risk from ~2% (regular DFSP) to approximately 15%.
9:31 Molecular Abnormality: DFSP is caused by a translocation, most commonly the COL1A1-PDGFB fusion. A rare alternate fusion involving PDGFD also exists.
10:48 Clinical Diagnosis Challenges: DFSP is frequently clinically misdiagnosed as benign lesions (cysts, lipomas, scars). It may present as a flat plaque rather than the classic nodular "protuberance."
11:17 (Takeaway): FISH testing for the specific translocations (COL1A1-PDGFB, and potentially reflexing to PDGFD) is highly recommended for small or ambiguous biopsies where fat trapping may not be visible, to definitively confirm the diagnosis before extensive surgery.