Superficial Rhabdomyosarcomas: A Review of Subtypes, Diagnostic Features, and Differential Diagnoses.

Superficial rhabdomyosarcomas (RMS) represent a rare and diagnostically challenging group of malignant neoplasms that may arise in, involve, or metastasize to the skin. Although RMS is the most common soft tissue sarcoma of childhood, cutaneous and superficial presentations are uncommon and encompass a heterogeneous spectrum of clinicopathologic scenarios. These include primary cutaneous RMS of conventional subtypes, newly recognized molecularly defined entities such as TFCP2-rearranged RMS, rhabdomyosarcomatous transdifferentiation of primary cutaneous malignancies, and metastatic RMS to the skin. Each of these entities may closely mimic a variety of benign and malignant cutaneous tumors, leading to potential diagnostic pitfalls. This review provides a comprehensive overview of superficial RMS, with emphasis on histopathologic features, immunophenotypic profiles, molecular alterations, and key differential diagnoses relevant to dermatopathologists and surgical pathologists. Conventional primary cutaneous RMS subtypes-including alveolar, embryonal, pleomorphic, and spindle cell/sclerosing variants-are discussed alongside emerging entities such as FUS/EWSR1::TFCP2-rearranged RMS, which frequently demonstrate epithelial marker expression and ALK overexpression. In addition, rhabdomyosarcomatous transdifferentiation of melanoma and squamous cell carcinoma is reviewed, highlighting the importance of clinicopathologic correlation and molecular testing to avoid misclassification. Cutaneous metastases of RMS, though exceedingly rare, are also addressed. Recognition of these diverse presentations requires integration of morphologic assessment, immunohistochemistry, and increasingly, molecular diagnostics. Continued clinicopathologic studies and molecular characterization will be essential to refine classification, improve diagnostic accuracy and enhance understanding of the biologic behavior of superficial RMS.