Diagnostic Pitfalls of Morphological Mimicry and CD34 Heterogeneity in Recurrent or Fibrosarcomatous Dermatofibrosarcoma Protuberans: A Clinicopathological Analysis of Three Molecularly Confirmed Cases With Literature Review.

To investigate the clinicopathological features, CD34 heterogeneity, diagnostic value of molecular detection, and targeted therapy in recurrent or fibrosarcomatous dermatofibrosarcoma protuberans (DFSP) with sarcomatous transformation showing unusual morphology. Three molecularly confirmed cases were analyzed and integrated with seven published cases. The 10 patients (6 females, 4 males; age: 27-62 years) showed a broad morphological spectrum (e.g., undifferentiated pleomorphic sarcoma-like, myxofibrosarcoma-like). CD34 expression was diffusely positive (6 cases), partially positive (1), or completely negative (3). Complete CD34 loss is a well-documented phenomenon in fibrosarcomatous DFSP. All cases harbored COL1A1-PDGFB fusion or PDGFB rearrangement. Treatments included surgery, radiotherapy, and imatinib. Follow-up revealed high risks of local recurrence and distant metastasis. Of the two imatinib-treated patients, one responded, the other was resistant. Recurrent or fibrosarcomatous DFSP with sarcomatous transformation can exhibit areas of unusual morphology and loss of CD34 expression, which may create significant diagnostic difficulty. Molecular detection of the COL1A1-PDGFB fusion remains the gold standard, as all cases retained the fusion despite morphological or immunophenotypic heterogeneity. Routine molecular testing is recommended for atypical cases to confirm diagnosis and guide targeted therapy.