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) pho-ton 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 acquisi-tion passing the angle interval of 90° in 15 seconds. In total, the MV frames needed 8.25 Monitor Units. The kV dose was neg-ligible herein. In order to deduce the missing angle informa-tion, 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 au-tomated. Structures with high contrast interfaces can be rec-ognized 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.