Respiratory-gated radiotherapy offers a significant potential for improvement in the irradiation of tumor sites affected by respiratory motion such as lung, breast, and liver tumors. An increased conformality of irradiation fields leading to decreased complication rates of organs at risk is expected. Five main strategies are used to reduce respiratory motion effects: integration of respiratory movements into treatment planning, forced shallow breathing with abdominal compression, breath-hold techniques, respiratory gating techniques, and tracking techniques. Measurements of respiratory movements can be performed either in a representative sample of the general population, or directly on the patient before irradiation. Reduction of breathing motion can be achieved by using either abdominal compression, breath-hold techniques, or respiratory gating techniques. Abdominal compression can be used to reduce diaphragmatic excursions. Breath-hold can be achieved with active techniques, in which airflow of the patient is temporarily blocked by a valve, or passive techniques, in which the patient voluntarily breath-holds. Respiratory gating techniques use external devices to predict the phase of the breathing cycle while the patient breathes freely. Another approach is tumor-tracking technique, which consists of a real-time localization of a constantly moving tumor. This work describes these different strategies and gives an overview of the literature. 1. Introduction Taking into account the respiratory movements has always been a major concern of thoracic radiotherapy. The development of conformal radiotherapy using reduced radiation fields, with or without intensity modulation, and above all the growing interest for hypofractionated stereotactic body radiotherapy, further enhanced this concern [1–4]. In 1987, an American team noticed that treatment in deep inspiration spared parts of the lungs, and they suggested a need to develop “Radiotherapy Gated to Respiration” [5]. The term “gating” was subsequently used to designate a variety of different practices. The first technique designed to control respiratory movements was developed in Japan in 1989. It consisted of an airbag system designed to synchronize radiotherapy, but it was not further developed [6]. Five main strategies are currently used to reduce respiratory motion effects: integration of respiratory movements into treatment planning (geometrical or dosimetric), forced shallow breathing with abdominal compression, breath-hold techniques (active or voluntary), respiratory gating techniques, and tracking
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