A CT acquisition during a free breathing examination generates images of poor quality. It creates an uncertainty on the reconstructed gross tumour volume and dose distribution. The aim of this study is to test the feasibility of a breath hold method applied in all preparation and treatment days. Five patients received a thoracic radiotherapy with the benefit of this procedure. The breathing of the patient was measured with a spirometer. The patient was coached to reproduce a constant level of breath-hold in a deep inspiration. Video glasses helped the patients to fix the breath-hold at the reference level. The patients followed the coaching during preparation and treatment, without any difficulty. The better quality of the CT reconstructed images resulted in an easier contouring. No movements of the gross tumour volume lead to a better coverage. The deep breath hold decreased the volume of irradiated lung. This method improves the reproducibility of the thoracic irradiation. The decrease of irradiated lung volume offers prospects in dose escalation and intensity modulation radiotherapy.