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Abstract

The primary objective of the present study is to demonstrate that a unique computed tomography (CT)-linear accelerator combination can be used to reduce uncertainties caused by organ motion and setup inaccuracy. The acceptance, commissioning, and clinical implementation of a sliding gantry CT scanner installed in an existing linear accelerator room are reported in this paper. A Siemens CT scanner was installed directly opposite to an existing accelerator. The scanner is movable on a pair of horizontal rails mounted parallel to the longitudinal axis of the treatment couch replaced with a carbon fiber tabletop. Acceptance and commissioning of the CT scanner were verified with phantom studies. For clinical implementation, quality assurance (QA) procedures have been instituted to ensure the integrity of the CT gantry axis alignment and the accuracy of its movement using a phantom designed in house. A clinical example employing the CT-Linac combination to correct the isocenter positioning caused by organ motion and setup inaccuracy was presented for a prostate irradiation. Dose calculations were performed to study the effects on tumor coverage without the adjustments of the isocenter. A summary of the isocenter adjustments for the first 30 patients is also presented. The geometric accuracy of the CT scanner is < or =1 mm. An isocenter deviation of > or =2 mm from the original plan can be detected. For the clinical example of a prostate patient, the average movement of the prostate gland was found to be approximately 3mm in the anterior-posterior (AP/PA) direction and 5 mm in the cephalic-caudal direction. Variations in the isocenter position may result in underdosage of the PTV if correction is not made for the change in the isocenter position. Our experience with the first 30 patients indicates that while the left-right adjustment of the isocenter is minimal, in the AP/PA direction, about 33% of treatments required an adjustment of 3-5 mm, and about 18% required a 5.1-mm to 10-mm adjustment. In the caudal-cephalic direction, about 26% required an adjustment of 3-5 mm, and 8% required a 5.1-mm to 10-mm adjustment. Retrofitting a CT scanner in an existing linear accelerator room requires careful planning and well-coordinated efforts from all personnel involved. Special QA procedures are needed to ensure the mechanical integrity and imaging accuracy of the CT scanner. A CT scan of the patient prior to irradiation provides valuable information on organ motion. Any deviations from treatment plan can be corrected before dose delivery. Significant deviation from the planning isocenter may occur due to daily variations in the rectal filling. The CT-Linac combination has significant implications for the treatment of prostate cancer.

Imaging in Radiation Oncology: A Perspective

“…Ultrasound [25–30] and in-room CT scanners [31–33] were the first volumetric imaging strategies used in the treatment room (e.g., Primatrom; Siemens, Concord, CA and ExaCT; Varian Medical Systems, Palo Alto, CA) [34, 35]. Helical MV CT scans can be obtained using an integrated treatment unit (Tomotherapy, Madison, WI) that allows the MV treatment beam to rotate around the patient while the couch moves, creating volumetric MV images [36–40].…”

Cone-beam CT using a mobile C-arm: a registration solution for IGRT with an optical tracking system

“…Various approaches to image-guided radiation therapy (IGRT) have been proposed. Amongst these are implanted radio opaque markers (Balter et al 1995, Litzenberg et al 2002, Sandler et al 1993, implanted magnetic markers (Lennernas 1997, Lennernas et al 1999, Lennernas and Nilsson 1995, ultrasound systems (Bouchet et al 2001, Lattanzi et al 1999, Morr et al 2002, Tome et al 2002, CT/LINAC (Cheng et al 2003, Court et al 2003, MVCT (Mackie et al 1993, Swindell 1983, MV cone-beam CT (Ford et al 2002, Midgley et al 1998, Mosleh-Shirazi et al 1998, Pouliot et al 2005, Seppi et al 2003 and kV cone-beam CT (Berbeco et al 2004, Jaffray et al 1999, 2002, Jaffray and Siewerdsen 2000. Each system has advantages and disadvantages and no one method has emerged as clearly superior.…”