Cancer Care Department, Provena Saint Joseph Medical Center, Joliet, Illinois 60435, USA.
Med Phys. 2011 Jan;38(1):390-6. doi: 10.1118/1.3531547.
The optically guided target localization had been developed for linear accelerator based stereotactic radiosurgery (SRS). Unlike the traditional laser localization, the optical guided target localization utilizes a digital system to position patient. Although the system has been proven accurate and robust, it takes away the capability of physicist to directly double check the target position prior to irradiation. Any error from system calibration, data transformation, or head ring position maintenance will not be caught. The purpose of this work is to investigate the possibility of using cone-beam CT (CBCT) to double check the optically guided SRS target localization and reposition the patient.
A SRS quality assurance (QA) phantom was used in the study. The phantom mounted with SRS head frame was scanned by computer tomography (CT) and planned according to the SRS radiation treatment planning process. A target isocenter is defined and transferred to the optically guided target localization system. The phantom was then transported to the linear accelerator room and localized at the initial position agreed by the optically guided target localization system and the CBCT system. Tests were conducted by moving/rotating the phantom to a set of preset offsets and taking CBCT images. Shifts detected by CBCT were compared with the preset offsets. Agreements between them were studied to see how well the CBCT was in discovering the optically guided target localization error.
Experiment results demonstrated good agreement between the CBCT detected phantom shift and the preset offset, when the offset is above 1 mm shift or 0.2 degree rotation. Offset less than 1 mm shift or 0.2 degree rotation was not detectable by CBCT.
The study concludes that the CBCT is able to discover the optically guided target localization error due to the system calibration or had ring migration. It is a valuable second check tool for SRS target localization quality assurance. The accuracy of CBCT in estimating patient positioning deviation satisfies the SRS procedures with generous tumor size and margin that can tolerate 1 mm or 0.2 degree accuracy. This avoids sending patient home without treatment. CBCT can be neither used as a primary SRS target localization nor can it be used to reposition the patient that cannot tolerate 1 mm shift or 0.2 degree rotation.
光学引导的靶区定位已经开发用于基于直线加速器的立体定向放射外科（SRS）。与传统的激光定位不同，光学引导的靶区定位利用数字系统来定位患者。虽然该系统已被证明准确可靠，但它剥夺了物理学家在照射前直接检查靶区位置的能力。系统校准、数据转换或头环位置维护过程中的任何误差都不会被发现。本研究的目的是探讨使用锥形束 CT（CBCT）检查光学引导 SRS 靶区定位并重新定位患者的可能性。
本研究使用了 SRS 质量保证（QA）体模。将装有 SRS 头架的体模进行计算机断层扫描（CT）扫描，并根据 SRS 放射治疗计划流程进行计划。定义一个靶区等中心点并将其传输到光学引导的靶区定位系统。然后将体模运送到直线加速器室，并在光学引导的靶区定位系统和 CBCT 系统同意的初始位置进行定位。通过将体模移动/旋转到一组预设偏移位置并拍摄 CBCT 图像来进行测试。通过 CBCT 检测到的移位与预设偏移进行比较。研究它们之间的一致性，以了解 CBCT 发现光学引导的靶区定位误差的能力。
实验结果表明，当偏移量大于 1mm 或 0.2 度旋转时，CBCT 检测到的体模偏移与预设偏移量之间存在良好的一致性。偏移量小于 1mm 或 0.2 度旋转的偏移量无法通过 CBCT 检测到。
该研究得出结论，CBCT 能够发现由于系统校准或头环迁移导致的光学引导的靶区定位误差。它是 SRS 靶区定位质量保证的一种有价值的二次检查工具。CBCT 在估计患者定位偏差的准确性方面满足了 SRS 程序的要求，对于能够耐受 1mm 或 0.2 度精度的较大肿瘤和边界，该精度是足够的。这避免了将患者送回家而不进行治疗。CBCT 既不能用作 SRS 靶区定位的主要工具，也不能用于重新定位不能耐受 1mm 偏移或 0.2 度旋转的患者。
