Comparison of LINAC equipped with either OBI/CBCT or CT-on-rails with a 6D robotic couch-top for IGRT

Almon Shiu, Ph.D.

The University of Texas, M.D. Anderson Cancer Center

Houston, Texas 77030

Introduction: Clinical outcome of radiotherapy can potentially be improved by increasing the precision of tumor localization and dose delivery during the treatment. Various image-guided radiotherapy delivery systems are now available to achieving the goal. The purpose of this study is to investigate the relative advantages and limitations of the OBI/CBCT (Trilogy) vs. Varian LINAC/CT-on-rails unit with a 6D robotic couch top for image-guided radiotherapy (IGRT).

Method and Materials: Optimal image-guide radiotherapy (IGRT) delivery system should be capable of handling the auto setup of the updated isocenter with 6D corrections, the adaptive radiotherapy, and treating multiple lesions simultaneously. The CT image quality should be suitable for designing a treatment plan. The planning CT and the daily CT are acquired in the treatment setup condition, minimizing any systematic errors coiled into the image-data. A 60-cm diameter of maximum field-of-view (FOV) is crucial to scan almost all the patient population in US. The scanning-range in superior-inferior direction should cover the maximum treatment-field length available on the radiotherapy delivery system. It should have KVp and MV portal-imaging capabilities for setup verification. Monitor the target-isocenter shift due to couch rotation. A thoracic phantom was used to evaluate the IGRT setup accuracy using either the OBI/CBCT or the CT from LINAC/CT-on-rails unit. The planning CT images were acquired from the LINAC/CT-on-rails unit. A treatment plan was generated from a clinical treatment planning system and is shown in Figure 1. Then, the thoracic phantom was setup at Trilogy and LINAC/CT-on-rails with similar conditions; the shifts and rotations were applied to the phantom to test the accuracy of aiming the target accurately on Trilogy and on LINAC/CT-on-rails unit. The in-house image-registration software was used for both tests to ensure a fair comparison in our study.

Results: the current CT image quality of CBCT is suitable only for image registration. However, the relative large intra- and inter-variation of CT numbers for the same tissue inhibits the use of CBCT for treatment planning and the adaptive radiotherapy. The single scanning-range of CBCT is limited to 14 cm. Both scanners need to increase the FOV to 60-cm diameter. The results of image registration between the planning DRRs and the CBCT DRRs are shown in Figure 2. It shows the phantom had 0.75° CW roll (viewing from the foot), the 1° CW yaw (viewing from the beam direction) and 2.5° tilt down towards the gantry side. Figure 3 shows the verification of the AP and RT LAT planned DRRs and the KVp AP and RT LAT portal images in horizontal and vertical split screens. It aimed isocenter correctly but could not handle the rotations in the setup (No mechanism is available now to correct the rotations except the roll). However, the similar study on the CT-on-rails unit, we could apply 6D corrections and the verification portals vs. DRRs displayed in the split screen are shown in Figure 4. It indicated that the isoceter was aiming correctly and the tilt of the spine above and below the isocenter were also aligned very well. The phantom study indicated that both units could setup the daily isocenter correctly based on the image registration. One of the findings suggests that the rotation corrections are essential for the crucial treatment to avoid treating the critical structures. It should be corrected either through a 6D robotic couch-top or software.

Conclusions: Based on the image registration, LINACs equipped with either OBI/CBCT or CT-on-rails perform the IGRT treatment nicely. If we plan to fully use the advantages of IGRT to improve the outcome of tumor control, the issues were brought up in this study are needed to address for the CBCT and the CT from the LINAC/CT-on-rails unit.