Primary rhabdomyosarcoma: An extremely rare and aggressive variant of male breast cancer.

Background: Rhabdomyosarcoma (RMS) of the breast, a mesenchymal neoplasm with skeletal muscle differentiation, is an extremely rare tumour in males, with less than 30 cases published in English-language literature. We report on the first case of a male breast RMS, with an unusual ectomesenchymal/neuroectodermal component.

Case Summary: A 55-year-old, previously healthy male, underwent a radical left mastectomy for an ulcerated tumour mass, occupying the breast and left anterior thoracic wall. The biopsy specimen indicated the presence of a tumour with neural origins, namely a peripheral neuroectodermal tumour (PNET). The surgical specimens identified two components. The rhabdomyosarcomatous component (over 70%) was represented by large pleomorphic cells with positivity for desmin, sarcomeric actin and myogenin. The PNET-like ectomesenchymal component, which was admixed with the RMS cells, and was also revealed during the preoperative biopsy, consisted of small cells which expressed neurofilament, neuron specific enolase and CD99. The microscopic examination, along with the immunohistochemical profile, allowed the diagnosis of an RMS, with unusual ectomesenchymal differentiation. The patient refused the postoperative oncologic therapy and died three months after surgery.

Conclusion: In patients with RMS of the breast, the PNET-like ectomesenchymal component increases the diagnosis difficulty, especially in biopsy specimens. This differentiation can be immunohistochemically proven and might highlight the possible development of high-grade sarcoma of the breast from remnants of the embryological ectodermal layer.