Kilovoltage Imaging

A solution to the inherent limitation of EPIDs on image quality has been offered with the introduction of stereoscopic kV imaging devices.17,20–23

While the use of diagnostic X-rays for verifying treatment set-up is not new, it offers several advantages.24 Image quality (a well-documented problem in EPIDs) is no longer an issue, especially in combination with amorphous silicon (AmSi) detectors.6 Patient dosage becomes less important compared with daily megavoltage images acquired with EPIDs. Dose measurements performed at the Academisch Ziekenhuis van de Vrije Universiteit Brussel (AZ-VUB) with an appropriate ionisation chamber resulted in 0.513mSv per image for a typical clinical setting using a ceiling-mounted, dual kV source-detector system (Novalis Body, BrainLAB).17 The combination with realtime monitoring of patient positioning, independent of linac gantry position, is not limited to target observation, but also offers the possibility of controlling the treatment beam based on that information for breathing synchronised irradiation.

In principle, two approaches exist. One uses image guidance to align the target volume with respect to the treatment beam using a remote couch control,4,18,25,26 while the other uses the imaging information to guide the treatment beam using a robotic linac (CyberKnife”, Accuray Oncology).20,21 The latter has the potential of true realtime tumour-tracking compared with the former; this can be used to gate the treatment in case of organ motion. An example of the gated treatment is given in Figure 3, using the Novalis Body system. Millimetre accuracy has been reported for both approaches.18,20 This ceiling-mounted, dual source-detector configuration has the advantage that it allows for realtime imaging during treatment, but it still requires a surrogate to identify the target (either bony structures or implanted radio-opaque markers). To overcome the two-dimensional (2-D) limitation of planar detection systems in assessing 3-D localisation problems, as with MV, the use of kV CT-scanning has been proposed. This allows the direct comparison of pre-treatment CT data with the planning CT data. kV-CBCT is based on an additional kV source-detector system that is mounted to the treatment machine perpendicular to the treatment beam and allows for volumetric imaging with soft-tissue contrast. The approach originally proposed by Jaffray et al. was to integrate a kV X-ray source and a large-area flat panel detector on a standard linac, allowing fluoroscopy, radiography and kV-CBCT.27 The kV-CBCT allows a volumetric CT image to be reconstructed from data collected during a single gantry rotation. Image quality is generally considered to be superior to the MV solutions, but this is counterbalanced by the fact that the HU-electron density relationship is not straightforward. This hampers its use for dose-calculation purposes.

A final solution in this kV-based approach is the introduction of a stateof- the-art CT scanner in the treatment room. Court et al., reporting mechanical precision and alignment uncertainties for this integrated CT/linac system, give an illustration of the in-room CT system.28 The system described integrates a high-speed CT scanner on rails and a linac. The couch top can be rotated to position the patient for either treatment or scanning, without having to move the patient from the treatment table to the CT couch. These kV-based solutions, as they are added to the treatment machine, require careful calibration to align the axis of the imaging system with the axis of the treatment beam. Both the MV-CT and kV-CT solutions can be used as an IGRT tool only prior to treatment (or near realtime), whereas the dual source-detector solutions can be used during treatment (realtime).