This presentation focuses on the histopathological differentiation of Dermatofibrosarcoma Protuberans (DFSP) from other spindle cell lesions that can exhibit similar features, particularly the presence of CD34 positivity and fat-trapping phenomenon. The speaker illustrates how fat trapping is not unique to DFSP by showing comparative slides of diffuse neurofibroma, fibrous hamartoma, and true DFSP. Key diagnostic tips include using S100 or SOX10 staining alongside CD34, as DFSP is typically negative for these markers, although a rare exception is noted. The speaker emphasizes the utility of Fluorescence In Situ Hybridization (FISH) testing for the COL1A1/PDGFB fusion (or reflexing to PDGFA/D) in ambiguous small biopsies to avoid unnecessary repeat procedures and expedite patient care, even when the clinical suspicion for DFSP is low.
Differentiating DFSP in Diagnostic Pathology: Staining and Molecular Tips
0:00:06 Fat Trapping Caution: The presentation begins by illustrating that the "fat trapping" phenomenon, often associated with DFSP, is present in other entities like diffuse neurofibroma and fibrous hamartoma, emphasizing it is not pathognomonic for DFSP.
0:00:35 CD34 Staining Nuances: CD34 stains many lesions, including most neurofibromas. If differentiating between diffuse neurofibroma and DFSP, staining for S100 or SOX10 alongside CD34 is recommended, as DFSP is typically negative.
0:00:54 Rare S100/SOX10 Positive DFSP: The speaker notes having seen one confirmed DFSP case (by translocation) that showed patchy but definite S100 expression, though this is considered exquisitely rare.
0:01:18 FISH for Definitive Diagnosis: When dealing with small biopsies of bland, CD34-positive spindle cell lesions where the diagnosis lies between leaving the lesion alone (benign) or recommending major surgery, Fluorescence In Situ Hybridization (FISH) is recommended.
0:01:21 FISH Testing Strategy: The standard procedure is to run FISH for the COL1A1/PDGFB translocation; if negative, reflex to test for PDGFA/D.
0:01:52 Rationale for Early FISH: Performing FISH on the initial biopsy, even when suspicion is low, is often preferred over recalling the patient for a larger sample, as it can avoid delaying care if the diagnosis remains uncertain.