7. The Management of Patient and Organ Motion and its Consequences

One major advantage of ion-beam therapy in comparison to other radiation therapy techniques is the improved conformation of RBE-weighted dose to the clinical target volume (CTV) due to physical and biological characteristics. This is characterized by a absorbed dose vs. depth profile exhibited by a mono-energetic beam with a relatively low absorbed dose in the entrance region (plateau) and a sharp dose peak (Bragg peak) at the end of the finite range (which can be spread out to match the longitudinal extent of the PTV), little lateral scattering, and increased RBE in the target volume in comparison with the entrance region. To maintain the advantage of conformity over a course of ion-beam therapy, in addition to accounting for uncertainties due to beam parameters (e.g., position, spot size) that are discussed in Section 8, the target volume has to be positioned precisely for each treatment fraction. Two types of possible anatomical variations have to be considered to achieve adequate target volume positioning: (1) interfractional organ motion and (2) intrafractional organ motion. Apart from these internal changes of the patient’s anatomy, misalignment of the patient itself is typically constrained by dedicated immobilization equipment. In addition, imaging techniques in the treatment room may be employed to register the actual patient position to the planned position. Interfractional target motion occurs in a time-scale of hours to weeks, e.g., weight loss or radiationinduced effects such as tumor shrinkage, whereas intrafractional target motion occurs in a time-scale of seconds to minutes, e.g., respiration. An overview of organ motion and its management in radiation therapy is given elsewhere (Bert and Durante, 2011; Korreman, 2012; Langen and Jones, 2001). The management of motion in ion-beam therapy depends on the motion type. Ion-beam therapy should not be delivered to patients for whom adequate mitigation of motion and setup errors cannot be established. The following sections cover immobilization and patient positioning techniques (some of which are ion-beam-therapy specific) including imaging for treatment planning and patient position verification that deal with interfractional as well as intrafractional motion.