A 72-year-old man with a significant smoking history presents to the emergency room with complaints of dyspnea and truncal, arm, and facial swelling for one week. Physical examination is remarkable for facial erythema and facial, truncal, and arm edema with prominence of thoracic and neck veins. On chest x-ray, there is a mass in the right mediastinum with adenopathy. Which of the following is the MOST likely diagnosis?