When using highly conformal radiotherapy techniques, such as proton therapy, a controlled breathing pattern and a minimal breathing amplitude could greatly benefit the treatment of mobile tumors. This reduction in tumor motion may be achieved with the use of a ventilator that is able to regulate and modulate the breathing pattern. CPAP provides a constant level of positive airway pressure. Compared to spontaneous breathing, the use of CPAP increased lung volume and can result in a significant decrease in tumor movement and a significant decrease in both mean lung and mean heart radiation dose. These results were found in patients treated for limited stage disease, it is not clear if this approach is feasible for patients with more advanced stage of disease that undergo radiotherapy with curative intent. With Bilevel Positive Airway Pressure (BiPAP), tidal volume excursions are determined by the pressure difference between the set inspiratory positive airway pressure (IPAP) and the set expiratory positive airway pressure (EPAP). This mode of ventilation increases lung volume comparable to CPAP, but also to control tidal volumes and breathing frequency. However, BiPAP has never been studied in the setting of motion mitigation during radiotherapy and BiPAP might be more difficult to adjust to for patients compared to CPAP. Therefore, the current study is proposed to evaluate whether or not CPAP or BiPAP is of benefit in patients that undergo radiotherapy for larger intra-thoracic tumor volumes.