Breath-hold target localization with simultaneous kilovoltage/megavoltage cone-beam CT and fast reconstruction

Hypofractionated high dose radiotherapy of small lung tumors is very effective and was based on stereotaxy until now. It has recently become possible to achieve a high patient positioning precision based on on-line imaging with cone-beam CT (CBCT) and breath-hold techniques. The CBCT acquisition time of roughly 60 seconds, however, is too long for one breath-hold, resulting in image degradation by respiratory motion artifacts. By using megavoltage (MV) an kilovoltage (kV) photon source (mounted perpendicularly on the Linac gantry) for volume reconstruction, we could reduce the acquisition time to 15 seconds. An Elekta Synergy 6MV linear accelerator, iViewGT as an MV- and XVI as a kV-imaging device were used. In 'movie mode' both systems were used for continuous image acquisition passing the angle interval of 90° in 15 seconds. In total, the MV frames needed 8.25 Monitor Units. The kV dose was negligible herein. In order to deduce the missing angle information, a marble with high absorption coefficient was put next to the thorax phantom. After preprocessing the data a GPU was used for the reconstruction of 100 projections generating a 512 x 512 x 512 volume in 6 seconds. The full acquisition and reconstruction workflow was automated. Structures with high contrast interfaces can be recognized easily in the reconstructed volume. These results show that fast on-board imaging based on combined kV-MV CBCT is promising for patient positioning. The scan duration of 15 seconds is short enough to perform single rotation breath-hold imaging for most patients. © 2009 Springer-Verlag.