Atrial involvement is an uncommon feature of advanced non-small-cell lung cancer, occurring in up to 10% of patients with bronchogenic carcinoma. Additionally, cardiac metastases from other sources are documented in up to 7% of cancer patients at autopsy. Because atrial invasion can lead to systemic embolisation and/or outflow obstruction, it is treated regardless of the overall prognosis. While the gold standard treatment has historically been surgical resection, advances in radiotherapy allow for the safe treatment of cardiac disease. Here we present the case of a woman with pulmonary adenocarcinoma of the left lower lobe that progressed to invade the pulmonary vein and left atrium while maintained on standard chemotherapy. She was treated with intensity-modulated radiotherapy and had a complete response in terms of her atrial disease within 3 months. She suffered no acute toxicity or complications as a result of the radiation therapy.