The set-up deviations and required correction are displayed as couch displacements in X, Y, and Z directions, and couch rotation. Each displacement can be selected individually; the rotation of the couch was not considered. The position displacements can be applied using the remote table control. The total positioning errors, combining both set-up and internal organ motion errors after initial positioning according to the laser marks, have been quantified and analysed. The largest position error is due to the internal prostate motion (see Figure 2). The uncorrected results for 12 patients with prostate cancer are shown in Table 1, where moverall is the overall mean systematic deviation, ?set-up is the random set-up error, and ?set-up is the set up systematic error. However, correcting AP, lateral, superior-inferior, pitch, and roll will not completely correct anatomical deformation. The majority of the patients treated using OBI were patients with prostate and gynaecological tumours. Other target volumes have also been positioned with OBI such as lung, head, and neck cancers, and brain tumors. The additional workload due to online registration is approximately two minutes added to the time slot booked for the patient.

CB kV CT

The CBCT function of OBI was installed in March 2005 in the author’s department. It focuses on localizing tumors based on internal anatomy and not just on the conventional external marks or tattoos.The CBCT system provides the capacity for soft-tissue imaging in the treatment position and realtime radiographic monitoring during treatment delivery.

Using the 3-D manual image registration, a match between the planning scan and verification CBCT was achieved and verified using the split view. This gave a good correlation between the two data sets. Currently, the author is using manual online registration tools for clinical set-up correction protocol. The image quality of the CBCT scans is slightly poorer than that of conventional CT scans and it also includes some artifacts from the tabletop. An example of a successful online registration of lung cancer can be seen in Figure 3, off-line registration of the prostate is shown in Figure 4a, and the head and neck in Figure 4b.The dose to the patient during CT acquisition is approximately 45mGy for full-fan geometry. The image quality is better for full-fan mode (the imager is centered with X-ray axis) than half-fan mode (the imager shifted a side to cover larger patient volume) due to more projections covering the whole field of view (FOV). The use of CBCT functions with an on-board imaging system in adjusting and verifying the position of the tumor and soft tissues before irradiation is a work in progress.

Future Work

Adaptive radiotherapy (ART) is the next step from IGRT and will be a valuable new tool for evaluating online and off-line treatment strategies to account for set-up uncertainty and anatomical changes. The adaptive framework should support different adaptation scenarios, including deformable target, tumor shrinkage, and organ at-risk modelling, re-planning and compensating for residual dose errors. In this treatment process multiple CT scans are acquired for each or few treatment fractions and are used as feedback to form a new planning target volume and modify the treatment plan accordingly. The clinical implementation of ART is now possible in some centers. The off-line adaptive planning approach used the daily scanned CT images during the first week of treatment delivery to shape an average over the planning target volume (PTV), the clinical target volume (CTV), and the critical surrounding organs in the new modified treatment plan. This technique has been implemented in different institutions especially for re-planning the prostate cancer where the target is contoured on each CBCT scan. The online adaptive automatic re-planning and target deformation will be the main issues to be solved in the very near future.