Primary cardiac sarcomas are rare tumours carrying poor prognosis. Postradiation sarcoma has been reported in patients with breast, cervical and head and neck cancers. We report a case of a 56-year-old woman with stage IIA breast cancer diagnosed in 1997, submitted to mastectomy, adjuvant chemotherapy, radiotherapy and hormonotherapy. Pulmonary metastasis were detected in 2008 and treated with chemotherapy and hormonotherapy, being in complete remission since August 2009. She was admitted in December 2009 with a 3-week history of fever, dyspnoea, polyarthralgias and leg oedema. An echocardiography showed a mass in the left atrium. She was submitted to a surgical tumour resection and the histology revealed a sarcoma of intermediate degree of differentiation. Chemoradiation therapy was started and she remains alive after 3 years, without tumour regrowth or metastasis. This case is a therapeutic challenge, because the previous therapies for breast cancer hampered the options for extra chemoradiation therapy.