Radiotherapy is a non-invasive and painless treatment that uses ionizing radiation to destroy tumor tissue. Around 60% of cancer patients undergo radiotherapy during their treatment. Precision is crucial in radiotherapy to target only the tumor cells while sparing healthy tissue. One challenge is the uncertainty caused by variations in the patient’s breathing, which can affect the accuracy of treatment. To address this, movement mitigation strategies are used, such as controlled breathing techniques and mechanical ventilation, to minimize the risk of harming healthy tissue. There are studies in the literature on invasive ventilation techniques during radiotherapy, such as High-Frequency Ventilation (HFV). Other studies have shown the effectiveness of non-invasive positive pressure mechanical ventilation techniques during the radiotherapy cycle. In this case, ventilation was performed with the aid of a face mask, and it was well tolerated by the patient without the need for anesthesia. Biphasic Cuirass Ventilation (BCV) is a non-invasive negative pressure ventilation carried out with the aid of a cuirass. Nevertheless, while cuirass ventilation is already used for radiological procedures that require good coordination with breathing, such as cardiac MRI, there are fewer documented cases in the literature regarding the use of cuirass ventilation during radiotherapy. This chapter presents the first case report about using BCV during definitive radiation therapy. The article describes a patient with obstructive sleep apnea syndrome (OSAS) and oncological issues, who had a right-sided hemi-diaphragm eventration and was unable to breathe while lying down without ventilatory support. The use of BCV allowed the patient to complete the entire radiotherapy program without needing tracheostomy surgery or invasive ventilation. However, since a single case report is not sufficient in the era of evidence-based medicine, further studies are required.