SHORT COMMUNICATION The use of megavoltage cone-beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement M AUBIN, MSc, O MORIN, BSc, J CHEN, PhD, A GILLIS, MD, B PICKETT, MSc, J F AUBRY, MSc, C AKAZAWA, CMD, J SPEIGHT, MD, M ROACH III, MD and J POULIOT, PhD Department of Radiation Oncology, University of California San Francisco, Comprehensive Cancer Center, San Francisco, CA 94143, USA ABSTRACT. In Europe and the USA combined, over half a million people had a hip joint replaced in 2005, contributing to the increasing number of radiotherapy patients with metallic hip prostheses. The treatment plan for external beam radiation therapy is based on the delineation of the anatomy in the planning CT scan. When implanted objects of high atomic number (Z) material are present, however, severe image artefacts are generated in conventional CT, strongly hindering the ability to delineate some organs. This is particularly the case for the planning of prostate patients with hip prostheses. This short communication presents the use of a new imaging modality, megavoltage cone-beam CT, to complement the regular CT for target definition of prostate cancer treatment of patients with hip replacements. Received 20 January 2006 Revised 22 June 2006 Accepted 4 July 2006 DOI: 10.1259/bjr/19559792 ’ 2006 The British Institute of Radiology Treatment planning for external beam radiation therapy is based on the delineation of the anatomy visualized in the planning CT scan. However, image artefacts caused by the presence of a hip replacement often render CT images useless for prostate delineation (Figure 1, left) and preclude precise dose calculation. It was recently suggested that CT-MR image registra- tion could facilitate target definition for a prostate patient with hip replacements [1]. Effectively, MR images provide an accurate definition of the clinical target volume (CTV) and better visualization of normal structures. A number of factors, however, were found to affect image quality and/or accuracy of target definition. The standard MR couch, different from a CT or linac treatment couch, might result in different patient position, and the presence of the metallic implants may create significant distortion. Finally, in the presence of a hip replacement, neither the regular CT nor the MR can provide accurate electron density information for hetero- geneous dose calculation. Promising artefact reduction techniques on regular CT are being developed to minimize the impact of streaking artefacts [2]. At that time, they may require manual image post-processing and most CT scanners available in radiation oncology departments are not equipped with these features. We report on the use of megavoltage cone-beam CT (MV CBCT) to complement the conventional CT for target organ definition in seven patients treated for prostate cancer with external beam radiation therapy. In this study, we exploited the predominantly Compton scattering of high-energy photons delivered in MV CBCT systems to obtain 3D images of the anatomy in the presence of unilateral or bilateral hip replacements and complement the planning CT during the target delinea- tion process. Method A cone-beam CT image is reconstructed from a large set of projection images acquired at various angles around a patient in a process similar to that of conventional CT. In cone-beam CT, a two-dimensional (2D) array of detectors, in our case a portal imager attached to the linear accelerator (linac), is used to reconstruct a three-dimensional (3D) image. For MV CBCT, the 6 MV treatment beam of the accelerator, containing photons primarily in the MeV range, is used for the imaging. A MV CBCT system [3, 4] integrated onto an ONCOR clinical Linac (Siemens Oncology Care Systems, Concord, CA) was used to acquire 3D images in treatment position for seven prostate patients with unilateral or bilateral hip prostheses. MV CBCT acquisi- tions were performed by rotating the linac in a continuous 200˚ arc (270˚ to 110˚ , clockwise) acquiring one portal image for each degree. Because MV CBCT uses the treatment beam, the treatment planning system (Pinnacle, Phillips, Best, The Netherlands) was used to evaluate the dose delivered during an MV CBCT Address correspondence to: Dr Jean Pouliot , Department of Radiation Oncology, University of California San Francisco, 1600 Divisadero Street, Suite H1031, San Francisco, CA 94941-1708, USA. E-mail: pouliot@radonc17.ucsf.edu. This research was supported by Siemens Oncology Care Systems. The British Journal of Radiology, 79 (2006), 918–921 918 The British Journal of Radiology, November 2006