ABSTRACT
Considerable geometrical uncertainties such as setup error, organ motion, shape change, and treatment response limit the precision and accuracy of radiation therapy (RT; Langen and Jones 2001; van Herk 2004). Consequently, the actually delivered dose does not equal the planned dose (what you see is not what you get). Generally, generous safety margins (ICRU Report 50; ICRU Report 62) are applied around the target and optionally for organs-at-risk (OARs), such that undertreatment and overtreatment due to geometrical uncertainties can be avoided with an acceptable probability (van Herk et al. 2000). Image-guided RT (IGRT) is the process of (1) acquiring an image of the patient’s anatomy in the treatment room (Figure 15.1) generally with the patient in treatment position; (2) comparing the treatment position with planned position of the tumor, OARs, or some surrogate; and (3) correcting the treatment position. This reduces the required safety margin and allows dose escalation without compromising nearby OARs. A large variety of in-room imaging modalities have been proposed for IGRT. In this chapter, an overview of these imaging modalities will be given.
