Q. A 56-year-old man presents to the emergency room with a 2-week history of facial swelling, headache, and shortness of breath. He is a heavy smoker with a 30 pack-year history. His vital signs are: temperature 98.2°F (36.8°C), blood pressure 138/84 mmHg, heart rate 96 bpm, respiratory rate 20 breaths/min, and oxygen saturation 94% on room air. On examination, you notice facial plethora, engorged neck veins, and bilateral arm swelling. A chest X-ray is ordered. Based on the clinical presentation and history, what is the most likely diagnosis?