ABSTRACT
One of the most critical steps in the radiotherapy process is the delineation of the tumor volume and the surrounding healthy tissues. In fact, with the introduction of in-room image-guided radiotherapy (IGRT) and its aim of reducing planning target volume (PTV) margins (ICRU 1999; Verellen et al. 2007), one can argue that the delineation process currently has the highest contribution to the geometric uncertainties encountered in radiotherapy. It has been shown in the literature that intra-and interobserver variation has become the weakest link in the target localization process as the imaging system is only as good as the skills of the reader who interprets the images (Engels et al. 2009; Verellen et al. 2009; Van de Steene et al. 1998; Steenbakkers et al. 2005; Van de Steene et al. 2002). Currently, the combination of CT, MRI, and PET is widely used to delineate the gross tumor volume (GTV), which consists of all clinically macroscopic disease based on what is visible on these imaging modalities (ICRU 1999). e clinical target volume (CTV) is created by adding a certain margin to account for microscopic disease and, nally, a safety margin is added for geometric uncertainties such as setup errors and tumor motion, to create the so-called PTV. Uncertainties in target delineation, historically, were largely covered by the generous PTV margins encountered in conventional radiotherapy. With the aim of maximizing outcome while minimizing complications, the latest developments in radiotherapy have been focusing on dose sculpting and reduction of PTV margins, the latter being realized by IGRT. e high-dose gradients encountered in dose sculpting and dose-paintingby-numbers (Ling et al. 2000) are unforgiving with respect to geographical miss, explaining the synergistic developments in IMRT and IGRT. ese eorts are, of course, pointless without accurate knowledge of the target that is to be treated.
