As part of standard patient set-up directly before each radiotherapy treatment, kilovoltage (KV) cone-beam computed tomography (CBCT) will be acquired. Treatment positioning will be based on the regular clinical decision protocols for treatment of rectal cancer and not on the markers. During treatment potential fiducial movement will be assessed using an externally positioned ultra-sound probe (this probe will be positioned against the abdominal or perineal skin of the patients depending on de position of the tumor/fiducials in the rectum). Moreover, for patients in this study 2 orthogonal kV images will be made immediately after each radiation using the electronic portal imaging (EPI) device.