A 68-year-old man with a history of left-sided breast cancer treated with chemotherapy 8 years ago presents with progressive dyspnea over 2 weeks. Chest X-ray shows a large right-sided pleural effusion. Thoracentesis is performed and pleural fluid analysis reveals: LDH 180 IU/L (serum LDH 280 IU/L), protein 2.8 g/dL (serum protein 6.5 g/dL), glucose 95 mg/dL, pH 7.4, ADA 8 U/L, CEA 12 ng/mL (normal <2.5). Which of the following is the MOST likely diagnosis?