Due to breathing and other motions, tumours, such as breast and lung cancer, as well as their surrounding organs (e.g. the heart), move, which poses a challenge for radiotherapy treatment. Reducing or even stopping breathing, e.g. by irradiating during inspiration, is a way to decrease tumour and organ motion resulting in a reliable target coverage with smaller margins. These smaller margins can result in a better sparing of normal tissues. Furthermore, in some patients, during inspiration, the heart may move away from the target volume making it possible to better spare the heart. Finally, during inspiration, the lung volume is larger and the lung density is lower, which can lead to a lower dose to the surrounding normal lung tissue. Reduction of radiation dose to normal tissues leads to less radiation-induced toxicity. This makes treating breast and lung cancer patients in breath hold (BH) conditions an attractive strategy. Standard BH durations in RT treatment are around 20 seconds, which is not enough to perform a complete CBCT. The health status of lung cancer patients is generally worse compared to breast cancer patients, making it more difficult to treat this patient group during breath hold. Nasal High Flow Therapy (NHFT) is a non-invasive system that provides controlled oxygen concentrations and low levels of positive pressure via a nasal interface. NHFT improves oxygenation in diverse patient groups, and is increasingly used as an alternative to mechanical ventilatory support. It has been shown to be a safe device in several clinical situations and patient populations, such as COPD patients, but also in apneic conditions under general anesthesia.

Due to breathing and other motion, tumours, such as breast and lung cancer, as well as their surrounding organs (e.g. the heart), move, which poses a challenge for radiotherapy treatment. Reducing or even stopping breathing, e.g. by irradiating during inspiration, is a way to decrease tumor and organ motion resulting in a reliable target coverage with smaller margins. These smaller margins can result in a better sparing of normal tissues. Furthermore, in some patients, during inspiration, the heart may move away from the target volume making it possible to better spare the heart. Finally, during inspiration, the lung volume is larger and the lung density is lower, which can lead to a lower dose to the surrounding normal lung tissue. Reduction of radiation dose to normal tissues leads to less radiation-induced toxicity. This makes treating breast and lung cancer patients in breath hold (BH) conditions an attractive strategy. A BH has to be stable and long enough for the duration of a planning CT-scan, cone beam CT (CBCT) scan and treatment delivery. Treatment of left-sided breast cancer with radiotherapy in moderate deep inspiration breath hold (mDIBH) is well established. Also, in MAASTRO clinic, left-sided breast cancer patients are treated in mDIBH, but this is done without any support or control of the breath hold. Standard BH durations in RT treatment are around 20 seconds, which is not enough to perform a complete CBCT. The health status of lung cancer patients is generally worse compared to breast cancer patients, making it more difficult to treat this patient group during breath hold. mDIBH in lung cancer patients is therefore not widely used, and not yet performed in MAASTRO clinic. Ventilation techniques that can support patients in holding their breath might make it a feasible approach in patients with a less favourable performance status, and might increase the duration of a breath hold. Nasal High Flow Therapy (NHFT) is a non-invasive system that provides controlled oxygen concentrations and low levels of positive pressure via a nasal interface. NHFT improves oxygenation in diverse patient groups, and is increasingly used as an alternative to mechanical ventilatory support. It has been shown to be a safe device in several clinical situations and patient populations, such as COPD patients, but also in apneic conditions under general anesthesia. It has however never been used in the context of breath hold support, neither has it been used in radiotherapy practice. The investigators hypothesize that supporting BH with nasal high flow therapy (NHFT) will allow robust radiotherapy treatments of moving targets in a broad patient population allowing for BHs that are long enough, stable and reproducible during a whole treatment course.

Inclusion Criteria:

• Locally advanced lung cancer or oligometastatic patients with "local" stage III, treated with curative or radical intent OR
• Left-sided breast cancer patients treated with radiotherapy with curative intent.
• WHO≤ 2
• ≥ 18 years old
• Able to give informed consent

Exclusion Criteria:

• Patient refusal
• Hypercapnic COPD patient (= arterial carbon dioxide tension PaCO2 > 45 mmHg)