A Study of Intensity-Modulated Pleural Radiation Therapy (IMPRINT) in People With Thymic Cancer That Has Spread to the Lining of the Lungs and Chest

Inclusion Criteria: * Provide written informed consent to participate on the study * Patients must have a pathologically confirmed diagnosis of thymic malignancy. Thymic carcinoma is allowed. * Patients must have radiologic or pathologic evidence of pleural metastases. Patients may have de novo stage IVA or recurrent disease in the pleura. Gross disease in the fissure is allowed if resected. * No evidence of extrathoracic metastatic disease or contralateral pleural/pericardial disease. * Patient age ≥ 18 years but ≤ 80 years at the time of consent * Karnofsky performance status ≥ 80% * Preoperative or Postoperative Pulmonary Function Tests: DLCO \> 40% predicted (corrected for Hgb) and FEV1 ≥ 35% (corrected for Hgb) * Glomerular filtration rate (GFR): ≥50 mL/min/1.73 m2 (must be calculated using estimated creatinine clearance (CrCl) by the Cockcroft-Gault (C-G) equation (Nephron 1976;16:31-41): CrCl (mL/min) = \[140 - age (years)\] x weight (kg) \[x 0.85 for female patients\] 72 x serum creatinine (mg / dL) ° In cases of concern about decreased renal function and potential high radiation dose to the kidneys, an optional nuclear medicine kidney function scan may be performed prior to radiation therapy to determine the functional contribution of each kidney. Exclusion Criteria: * Continuous oxygen use * Patients with an acute flare of myasthenia gravis requiring addition of new medication in the past 6 months * Prior nephrectomy on the contralateral side of the pleural metastases * Prior thoracic radiation therapy preventing hemithoracic pleural IMRT. Prior thymic bed radiation is allowed. Prior pleural SBRT is allowed. * Patients undergoing extrapleural pneumonectomy. Other surgical resection approaches of the pleural nodules (ex: P/D, debulking/metastasectomy) are allowed. Surgical resection of the primary thymic tumor is allowed. * Acute congestive heart failure requiring hospitalization within the past 30 days. * COPD requiring chronic oral steroid therapy of \> 10 mg prednisone daily or equivalent at the time of registration. Inhaled corticosteroids are allowed * Unstable angina requiring hospitalization and/or transmural myocardial infarction within the last 3 months * History of interstitial lung disease * Pregnant or lactating women * Men or women not using effective contraception