A cone-beam megavoltage CT scanner for treatment verification in conformal radiotherapy Mohammad Amin Mosleh-Shirazi*, Philip M. Evans, William Swindell, Steve Webb, Mike Partridge Joint Department of Physics, Institute of Cancer Research and Royal Marsden NHS Trust, Sutton, Surrey SM2 5PT, UK Received 25 August 1997; revised version received 17 March 1998; accepted 8 April 1998 Abstract Purpose: A prototype scanner for large-volume megavoltage computed tomography (MVCT) in a clinical set-up is described. The ultimate aim is to improve treatment accuracy in conformal radiotherapy through patient set-up error reduction and transit dosimetry. Materials and methods: The scanner consists of a custom-built 2D CsI(Tl) crystal array viewed by a lens and a CCD camera. Image acquisition is synchronized with radiation pulses. The 2D projections resulting from a single continuous 360° gantry rotation are recon- structed using a cone-beam tomography algorithm. Prior to reconstruction, the raw projections are calibrated and corrected for centre of rotation movement and accelerator output fluctuation. The performance of the system has been evaluated by reconstructing projections of open fields, test objects and a humanoid phantom. Results : Hundreds of 2D projections can be acquired with a clinically-acceptable data collection time (about 2 min) and dose (approxi- mately 40 cGy, with a possible four-fold reduction). A maximum density resolution of about 2% is achieved offering some soft tissue discrimination without using image enhancement tools. A spatial resolution of 2.5 mm is obtained. The reconstructed image intensity is linear with electron density over the range of interest. Coronal or sagittal slices through the 3D reconstruction of the humanoid phantom show a better delineation of structures than the corresponding portal images taken at the same orientation. Conclusions: A similar image quality to our current single-slice MVCT scanner is achieved with the advantage of providing tens of tomographic slices for a single gantry rotation. This work demonstrates the feasibility of clinical cone-beam MVCT and indicates how this prototype can be improved.  1998 Elsevier Science Ireland Ltd. All rights reserved Keywords: Megavoltage computed tomography; Cone-beam reconstruction; Electronic portal imaging; Treatment verification; Detector performance; Conformal radiotherapy 1. Introduction Extensive effort has been directed towards improving the accuracy of dose planning and radiation delivery in radio- therapy of cancer. The eventual goal of the work presented here is to improve the accuracy of the delivery of the planned treatment. A large set-up error reduces the benefits gained from improving the accuracy of the other stages leading up to radiation delivery. In conformal radiotherapy, where the dose margin around the tumour in the planning target volume is usually tight and the prescribed dose is sometimes escalated, treatment verification is even more critical. Whilst electronic portal imaging devices (EPIDs) have facilitated and improved treatment verification [3,13,24], a major problem still remains which limits their usefulness. A portal image, being a 2D projection, contains the effects of superimposed structures between the source and the detector. As a result, clinical portal images normally only show large density differences, for example, distin- guishing air and bone from the soft tissue background. The few percent density difference between the various soft tissues is not usually resolved. This poses a problem for treatment verification since most tumours are made of soft tissue. Bony landmarks are usually used for set-up com- parisons. Whilst this is adequate for detecting gross errors and where the tumour does not move relative to the bone, movement of a soft tissue tumour with respect to the bony structures will not be detected normally by portal imaging. For the same reason that diagnostic computed tomogra- Radiotherapy and Oncology 48 (1998) 319–328 0167-8140/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved PII S0167-8140(98)00042-5 * Corresponding author. Department of Medical Physics, Royal Sussex County Hospital, Eastern Road, Brighton BN2 5BE, UK.