Surface guidance compared with ultrasound-based monitoring and diaphragm position in cone-beam computed tomography during abdominal stereotactic radiotherapy in breath-hold oleh Lena Kaestner, Lara Streb, Svetlana Hetjens, Daniel Buergy, Dwi S.K. Sihono, Jens Fleckenstein, Iris Kalisch, Miriam Eckl, Frank A. Giordano, Frank Lohr, Florian Stieler, Judit Boda-Heggemann