Multimodality imaging is one of the fundamental ingredients of modern radiotherapy (RT). Although kilovoltage x-ray computed tomography (CT) imaging continues to be the primary imaging modality used in RT, the use of magnetic resonance imaging (MRI) and nuclear medicine imaging with single photon emission computed tomography (SPECT) and especially positron emission tomography (PET) is common in treatment planning for many patients. Optimal design of RT treatments oen requires anatomical and biological information about target, volumes, and no single imaging modality provides all of the information that can be gathered and used. Each of the abovelisted imaging modalities oers unique information, and complementary use of two or more modalities is needed. e use of multimodality imaging in RT has been extensively described in the literature (Kessler et al. 1991; Ling et al. 2000; Mutic 2006). Multimodality imaging is used for improved detection, diagnosis, staging, therapy selection, target denition, prognosis, response evaluation, and follow-up.