A 48-year-old woman presents with progressive dyspnea for 1 week. There was no cough or chest pain. She was diagnosed with metastatic cancer of the right breast 1 year ago, for which a radical mastectomy was performed. She completed her 6th cycle of chemotherapy three months ago. On admission, her oxygen saturation was 82%, which improved to 99% with high flow oxygen. A full blood count and chest x-ray are normal.