Design and Deployment of a Proton Therapy Cone-Beam CT

Rosa Seabra, José Carlos [UCL] Teo, Kevin [UPenn, USA] Brousmiche, Sébastien [UCL] Labarbe, Rudi [UCL] Wikler, David [IBA] Maughan, Richard [Upenn, USA] Lee, John Aldo [UCL] et al.[show all]

Objective : Cone-Beam CT (CBCT) has been successfully used for image-guided radiation therapy for about 10 years. The use of proton instead of photon beams for radiation therapy enables an optimal distribution of the prescribed dose delivered to the tumor, provided that the target region has been accurately defined. If a CBCT can be directly acquired in the treatment position, this will allow much better spatial registration with the planning CT and precise assessment of anatomical modifications. Methods : A design relying on an upgrade of an already installed rotating gantry orthogonal stereoscopic X-Ray system is presented. Additional fast radiographic High-Voltage generator and new high capacity heating units tube will be attached to the gantry structure in order to provide high currents required by 2.8m long Source to Axis Distance (SAD). A turn-table based prototype has been built in order to test various models of high sensitivity CsI or Gadox Flat Panels for evaluation of specifications for signal, noise, resolution, gain/offset calibration and ghosting. A software platform for image acquisition and fast GPU-based reconstruction (FDK) has been implemented. This platform integrates validated state-of-the-art artifact correction techniques More specifically, geometric distortions of the proton gantry have been characterized by on-site measurements in order to assess reproducibility of equipments deformations and a calibration was implemented using inverse projection method of a phantom onto the detector. Results : Projections images of phantoms, from both quality assurance and anthropomorphic phantoms (head, thorax and pelvis), were acquired and reconstructed in order to evaluate performances of flat panels and reconstruction software. A CsI high sensitivity (DQE < 0.2 above 1mm spatial frequency) flat panel providing excellent ghosting correction (<10% relative signal after 10s) was selected. The long SAD leads to some advantages in particular reduced scatter, decreased penumbra, FDK cone-beam axial artifact improvement and larger field of view for fixed detector geometry. Reproducible geometric deformations of the imaging components (tubes and flat panel detectors) of the order of a few mm for the detector and up to 20 mm for the X-Ray tube were measured and successfully fitted with an analytical model. Figure 1 presents results for deformations along IEC Fixed Y axis according to gantry angle for the CBCT kV imager. These deformation models have been used to improve reconstruction in CBCT. Conclusions : We have designed a CBCT system for image-guided proton therapy. Communication with the hardware components, CBCT image acquisition and GPU-based reconstruction are performed through a dedicated software. Data was collected to assess the radiographic imaging performance of the flat panel and to compensate for geometric deformations of the CBCT geometry. The use of CBCT in a proton therapy room will contribute to a more accurate and adapted radiation therapy which takes into account the position of the patient during treatment and the change of tumor size.