Primary Embryonal Rhabdomyosarcoma of the Breast: A Case Report and Literature Review.

[BACKGROUND] Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma in children and adolescents, but primary RMS of the breast is exceptionally rare and diagnostically challenging. Imaging findings are nonspecific and can mimic benign adolescent lesions (e.g., fibroadenoma), making timely histopathologic confirmation crucial. Immunohistochemistry for muscle markers - particularly desmin and myogenin - supports definitive diagnosis of embryonal RMS (ERMS).

[CASE PRESENTATION AND CASE DISCUSSION] A 14-year-old Arab female presented with a rapidly enlarging left-breast mass and ipsilateral axillary lymphadenopathy. Core biopsy showed small round blue cells with rhabdomyoblastic differentiation; tumor cells were desmin- and myogenin-positive, consistent with ERMS. She received 6 cycles of mesna-doxorubicin-ifosfamide-dacarbazine (MAID) chemotherapy, followed by local recurrence; one cycle of ifosfamide-carboplatin-etoposide (ICE) achieved an approximately 50% partial response but was complicated by cystitis and rapid radiologic progression. After three cycles of vincristine-dactinomycin-cyclophosphamide (VAC), she underwent total mastectomy with lymph-node dissection. Restaging fluorodeoxyglucose positron emission tomography/computed tomography demonstrated local recurrence and nodal metastases (bilateral axillary, subpectoral, and internal mammary) with pulmonary nodules. Despite multimodal therapy, the disease remained refractory and the patient ultimately died from complications of metastatic disease. This case highlights the aggressive biology of primary breast ERMS in adolescents and the risk of early recurrence and dissemination despite intensive therapy. While standard management of RMS is multimodal - systemic chemotherapy with surgical resection and/or radiotherapy - responses can be transient, and treatment interruptions (e.g., toxicity-related delays) may jeopardize disease control. The diagnostic value of myogenin (highly specific for rhabdomyoblastic differentiation) and Desmin was pivotal here, while the clinical course underscores the limitations of currently available regimens (MAID, ICE, VAC) for refractory disease at this uncommon site.

[CONCLUSION] This case illustrates the need for early biopsy of rapidly enlarging breast masses in adolescents, the central diagnostic role of muscle-specific immunohistochemical markers, and the importance of uninterrupted multimodal therapy in primary breast ERMS. It also underscores the limitations of currently available regimens in refractory disease and highlights the need for collaborative, guideline-based management, and clinical-trial enrollment when feasible.