Mega Voltage Cone Beam Computed Tomography (MV- CBCT) using a Standard Medical Linear Accelerator and EPID: A feasibility study.

Introduction: The success of radiotherapy cancer treatment delivery depends on the accuracy of patient positioning for each treatment session. A number of kilovoltage x-ray volumetric imaging modalities with an additional source and detector have been developed to allow patient set-up verification based on the internal anatomy, but a significant portion of medical linacs are only equipped with electronics portal imaging device (EPID) as a planar imaging system. The aim of this study is investigating the feasibility of megavoltage cone beam computed tomography (MV-CBCT) using medical linac and EPID to obtain volumetric data for 3D representation of patient in treatment position. Materials and Methods: The MV-CBCT system consists of an amorphous silicon EPID with an active area of 41×41 cm2 attached to an Elekta Precise linac. Initially, the quality control tests were performed to evaluate the safety features, mechanical stability and quality of the megavoltage imaging system. The MV-CBCT scanning protocol was divided into three major parts. In the first part, Step and Shoot delivery mode was used to acquire projections of an anthropomorphic head phantom and quality assurance phantom in increments of 30 with 3 MU and x-rays of 6 MV energy over 3600 of gantry rotation. Afterwards, noise reduction was conducted by applying 2D adaptive wiener filter to projections. Furthermore, it is found that scatter produces cupping artifacts and increase uniformity variations across the reconstructed tomograms. Therefore, a scatter correction method which is based on a superposition of Monte Carlo generated scatter kernels was applied to the portal images. In the latest part of the research, the Feldkamp cone-beam algorithm used to reconstruct the 2D portal images to a 3D image, then, image quality characteristics such as; uniformity, contrast-to-noise ratio (CNR) and spatial resolution were investigated using the Gammex phantom Results: The safety interlocks were found to be functional. The EPID response uniformity was within 97% across the detector. Contrast resolution of the MV imaging system was found to agree with the recommended tolerance; and the value of f50 for spatial resolution was 0.41lp/mm for 6 MV. The supporting arm deviation was within ±1 mm, and corrections are required to realign the projections. Reconstructed slices from the spatial and contrast resolution modules of the Gammex phantom shown that the 4 lp/cm section was clearly resolvable, and also bone, air, polyethylene, and acrylic inserts can be clearly resolved, with bone and air having the greatest and polyethylene having the lowest contrast, which improved after scatter correction. Furthermore, the maximum uniformity variation was determined of 15.75%-pixel intensity units which reduced significantly after scatter correction. Conclusion: The result of this study has demonstrated that EPID can be used to acquire volumetric images with high enough quality to improve patient alignment prior to dose   delivery.