ABSTRACT
Intrafraction motion refers to organ motion during a single fraction within the course of the radiation therapy treatment. Improvements in staging, imaging, and precision radiotherapy delivery by means of three-dimensional (3D) conformal or intensity-modulated radiation therapy (IMRT) permit a higher dose to the target, and/or smaller dose to the organs at risk by conforming the dose tightly to the target. Ÿis may result in better therapeutic gain (Fang et al. 2006). Ÿere is, however, a concern related to the increased risk of geometrical misses due to intrafraction motion. Target motion can occur due to bony and internal organ movement. Prostate motion, for example, occurs due to rectal and bladder £lling as well as to leg motion and clenching of pelvic ²oor muscles (Boda-Heggemann et al. 2008). However, as treatment fractions get shorter with the use of new technologies, the most relevant intrafraction motion is that due to respiration. Tumors in the thorax and abdomen move with breathing. It has been shown that the magnitude of the motion can be clinically signi£- cant (approximately 2-3 cm), depending on tumor location and individual patients (Keall et al. 2006). Even the motion of pelvic tumors due to respiration has been observed (Malone et al. 2000; Kitamura et al. 2002; Weiss et al. 2003). Tumor motion a¢ects imaging, treatment planning, and treatment delivery. Motion
poses a number of special problems, including geometry uncertainty (a moving target may appear with distorted shapes and in the wrong locations during treatment planning scans; Chen, Kung, and Beaudette 2004; Yamamoto et al. 2008) and increased irradiation of normal tissues (large margins are o«en used to ensure that the tumor is not missed; ICRU 1999; Ramsey, Cordrey, and Oliver 1999; Langen, and Jones 2001). Ÿese sources of uncertainty may compromise the e¢ectiveness of conformal radiotherapy for the management of such lesions. Although treatment e¢ectiveness might depend on the technique and number of treatment fractions, it is of major concern when the treatment is done in a hypofractionated (few treatments) or single-fraction manner (Bortfeld et al. 2002). One consequence of respiration is that treatment volume has to be increased, thus unnecessarily irradiating a larger normal tissue volume with higher doses. Many breathing compensation strategies have been investigated, and continue to be under investigation. Recommendations for the use of di¢erent imaging and treatment techniques for motion management have been provided in a report by the American Association of Physicists in Medicine (AAPM) Task Group 76 (Keall et al. 2006).
