5B02a-Near Simultaneous Computed Tomography Image-Guided
Stereotactic Spinal Radiotherapy
Almon S. Shiu, Ph.D
Associate Professor
Departments of Radiation Physics,
The University of Texas M. D. Anderson Cancer Center,
Houston, TX
The implementation of the treatment setup from an emerging technique using a near-simultaneous computed tomography (CT) image-guided stereotactic radiotherapy (IMSRT) for the treatment of spinal and paraspinal tumors is the focus of this presentation. In addition, the treatment setup accuracy and a comparison of planned and delivered dose distributions using IMSRT will also be discussed.
A targeting system that integrates a CT-on-rails scanner with a linear accelerator (LINAC) was evaluated in the lead-in portion of a Phase I/II protocol for treating patients with paraspinal metastases. Patients were immobilized in the supine position by a moldable body cushion vacuum wrapped with a plastic fixation sheet. Planning CT and immediately repeated CT were performed on the LINAC/CT-on-rails unit to assess respiratory-related vertebral body motion. Coplanar intensity modulated radiotherapy (IMRT) using 7-9 beams was used to deliver 30 Gy in five fractions to the target volume, while limiting the spinal cord dose to <10 Gy. Pretreatment CT scans were fused with the planning CT scans to determine the correct target isocenter by accounting for any translational and roll (axial) rotational discrepancies from the planning CT. The reproducibility of the treatment isocenter as compared with the planned isocenter was measured with digitally reconstructed radiographs (DRRs), portal film imaging, and immediate post-treatment verification with CT scans. Phantom measurements were taken for dose verification for each IMRT plan. An assessment of dose coverage to the clinical target volume (CTV) for each patient was evaluated based on the delivered dose distributions represented on the post-treatment CT scans.
Based on a total of 180 CT scans (fifteen for planning, fifteen for respiration study, 75 pretreatment, and 75 post-treatment) from fifteen patients, no respiration-associated vertebral body motion was seen. A comparison of the corrected daily anterior-posterior (AP) and lateral (LAT) digital portal images with the planning AP and LAT DRRs confirmed that the isocenter setup accuracy for the 90 treatments (3 patients had 2 isocenters) was within 1 mm of the planning isocenter. The results from the immediate post-treatment CT scans reconfirmed the findings from the portal images and verified the absence of spinal movement during the treatment. The ion-chamber measurement for the high-dose region was within 2% of the planning dose for fifteen patient treatment plans. Film dose measurement in an IMRT QA phantom demonstrated good agreement from 90% to 30% isodose lines between the planned and measured results.
The recalculated dose distributions on the post-treatment CT revealed that the CTV received the desired dose coverage as originally planned while meeting the dose constraints to the spinal cord (<10 Gy). On the other hand, the use of orthogonal DRRs alone without taking axial rotation (up to 3°) into account was found that the maximum cord dose is approximate 2 Gy greater than that of original plan, even though the CTV coverage is reasonable. The actual delivered DVH versus the planned DVH will be presented in the meeting.
Preliminary experience suggests that near-simultaneous CT image-guided verification technique can be used as a new platform technology for extracranial applications of stereotactic radiotherapy and radiosurgery to spinal and paraspinal tumors.
5B02a－近同步CT图像引导的脊柱肿瘤的立体定向放射治疗
Almon S. Shiu, Ph.D
Associate Professor
Departments of Radiation Physics,
The University of Texas M. D. Anderson Cancer Center,
Houston, TX
本报告将主要讨论使用近同步CT图像引导这一新技术用于脊柱和椎旁肿瘤立体定向放射治疗（IMSRT）病人摆位的实施过程。另外也将讨论使用图像引导立体定向放射治疗（IMSRT）治疗摆位的准确性和计划与实际剂量分布的比较。
在治疗椎旁转移病人的I/II期临床实验方案开始部分评估了直线加速器（LINAC）和CT（CT-on-rails）共属于一个治疗室的靶区定位系统。患者体位用真空袋和体罩固定。通过CT扫描来评估呼吸引起的椎体运动。使用7－9个共面射野进行调强放射治疗（IMRT），靶区剂量为30 Gy/5次，脊髓剂量限制在10 Gy以下。通过治疗前CT图像和计划CT图像融合来确定正确的靶区等中心，同时考虑计划CT扫描时发现的移位和旋转差异。通过数字重建射线影像（DRR）、射野胶片成像和治疗后立即进行CT扫描验证等方式来测量治疗等中心的重复性，并与计划等中心比较。对于每个调强放射治疗（IMRT）计划通过模体测量进行剂量验证。对于每个病人，基于治疗后扫描的CT图像获得的实际剂量分布评估临床靶区（CTV）的剂量覆盖情况。
根据15例病人的180套CT扫描图像（15套用于治疗计划，15套用于呼吸运动的研究，75套是治疗前扫描，75套是治疗后扫描），没有观察到呼吸运动引起的椎体运动。比较校正的每天前后位（AP）和侧位（LAT）数字射野图像和计划的AP、LAT DRR表明90次（3例病人有2个等中心）治疗等中心都在计划等中心的1毫米以内。治疗后立即进行CT扫描的结果再次确认了射野图像的发现，并且表明治疗过程中没有脊柱运动。对于15例病人的治疗计划，高剂量区电离室测量结果在计划剂量的2%以内。在调强治疗质量保证模体中胶片测量结果表明在90%到30%等剂量线区域内计划和测量剂量分布具有很好的一致性。
在治疗后CT图像上重新计算的剂量分布表明CTV收到计划的剂量分布，同时满足脊髓的剂量约束（<10 Gy）。另外，使用不考虑轴位旋转（最大3°）的正交DRR分析表明即使CTV的覆盖很好，脊髓最大剂量比原来的计划大大约2 Gy。会上将提供实际的DVH和计划的DVH。
初步经验表明近同步CT图像引导的验证技术可以用作颅外立体定向放射治疗和脊柱与椎旁肿瘤立体定向放射治疗的新的技术平台。