A Case of Breast Metastasis from Primary Rhabdomyosarcoma of the Lower Leg

Rhabdomyosarcoma (RMS) is the most common malignant soft tissue tumor in children, with the alveolar subtype associated with an aggressive clinical course and poor prognosis. Metastasis to the breast from RMS is extremely rare, particularly in preadolescent patients, and optimal local treatment strategies have not been established.

We report the case of a 9-year-old girl with alveolar RMS originating in the left lower extremity. The patient underwent wide resection of the primary tumor and dissection of the regional lymph node, followed by multimodal therapy, including chemotherapy and proton beam radiotherapy, and achieved complete remission. Three months after the completion of treatment, she developed a palpable mass in the right breast. Imaging studies revealed multiple breast lesions without evidence of local recurrence at the primary site. Histopathological examination confirmed the presence of metastatic RMS. Preoperative chemotherapy resulted in a limited tumor response, and local control was pursued. An endoscopic-assisted nipple-areola-sparing mastectomy with partial resection of the pectoralis major muscle was performed, followed by adjuvant chemotherapy and radiotherapy. Immunohistochemical findings supported the diagnosis of metastatic RMS. Comprehensive genomic profiling of the metastatic lesion identified a TP53 pathogenic variant. However, germline testing was not performed, and the clinical significance of this finding remains uncertain. Despite aggressive multidisciplinary treatment, the patient developed widespread metastatic disease and died approximately 4 years after the initial diagnosis.

Breast metastasis from alveolar RMS is rare but should be considered in pediatric patients who present with breast masses after treatment. Surgical resection may contribute to local control and QOL in selected cases. Additional cases are needed to clarify the role of surgery and to establish optimal treatment strategies, particularly in patients with suspected hereditary cancer predisposition syndromes.