Image-guided radiotherapy (IGRT) can be used to measure and correct target and critical structure positional errors immediately prior to or during treatment delivery. Single-slice megavoltage computed tomography (MVCT) using 50MV for the racetrack microtron (RTME) was first utilised at Karolinska University Hospital in 1987.1 There were some technical and clinical limitations with this system.

Some of the most recently available methods of target localization are transabdominal ultrasound, implanted markers with in-room MV or kilovoltage (kV) Xrays, optical surface tracking systems, implantable electro-magnetic markers, in-room CT, such as kVCT on rail, kV or cone beam (CB) MVCT and helical MVCT.

The image-guided systems provide an increasing amount of information about daily target localization and set-up errors during the course of fractionated treatment. This information can be compared online with the treatment planning system and utilized for the set-up adjustment of the treatment fraction. The verification of the accurate treatment position in conjunction with detailed anatomical information before every fraction is therefore essential for the outcome of the treatment.

The on-board imager (OBI) has been in routine clinical use at the Karolinska University Hospital since June 2004. The OBI consists of a diagnostic X-ray tube and a kV flat panel imager, which are both mounted on robotic arms and designed for three main functions—orthogonal radiographs for threedimensional (3-D) patient set-up, CB kVCT and realtime tumour tracking, and fluoroscopy. The availability of high-quality tomographic images and automatic tools for online 3-D image registration will lead to the introduction of new clinical applications and protocols. The most beneficial application is high-precision hypofractioned treatment of lung and liver metastases with online tumor position correction (see Figure 1).

Radiographic Online Set-up Verification of Postate PAQtients Implanted with Gold Markers

The OBI has been used for the online set-up correction of patients using internal gold markers since June 2004. Displacements of these markers can be monitored radiographically during the treatment course and the displacements of the markers act as a surrogate for prostate motion. For this purpose, onboard kV-kV seems to be an ideal system in terms of image quality. Prostate patients are treated in the supine position and are given three gold markers (diameter = 0.9mm; length = 3mm) implanted under transrectal ultrasound control. Individually fabricated vacuum cushions are produced to immobilize the legs and to ensure stable positioning of the patient during the application of treatment fields. In the treatment room the patient is positioned according to the laser marks on the skin. Before each treatment, fraction orthogonal kV-kV images from the anterior-posterior (AP) and lateral directions are acquired using the OBI. Digitally reconstructed radiographs (DRRs) were obtained from planning CT data, and used as reference images, and the marker positions were contoured and displayed. The automatic 2-D matching procedure is based on bony structures used to match the two orthogonal images with reference images (DRRs) followed by a subsequent final manual alignment of the fiducial markers. The correction is applied on these gold markers rather than the bony structures, in order to correct the actual target position (see Figure 2).