- Conventional immunohistochemistry is insufficient. All tumors expressed myogenin and desmin while being negative for S100, SOX10, Melan A, and HMB45, meaning standard staining panels would reliably produce a false diagnosis in this setting.
- DNA methylation profiling was the most decisive tool. Seven of nine evaluable cases clustered with desmoplastic melanoma, providing lineage evidence that mutational data alone could not.
- The diagnosis carries real treatment consequences. At least one patient in the cohort responded to pembrolizumab, a therapy that would not typically be offered for rhabdomyosarcoma — underscoring the stakes of misclassification in the immunotherapy era.
- Not every case resolved neatly. One tumor matched TFCP2-rearranged rhabdomyosarcoma and another partially matched squamous cell carcinoma by methylation, reminding clinicians that histologic mimicry runs in multiple directions.
- Sun-damaged skin in elderly patients should raise suspicion. The cohort's median age of 83, predominance of head and neck tumors, and universal UV signatures collectively suggest that any poorly differentiated malignancy with rhabdomyosarcomatous features in this demographic warrants melanoma workup before a sarcoma diagnosis is finalized.
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