Low D A, Li Z, Drzymala R E
Mallinckrodt Institute of Radiology, Saint Louis, Missouri, USA.
Med Phys. 1995 Apr;22(4):443-8. doi: 10.1118/1.597475.
The stereotactic radiosurgery system used at the Mallinckrodt Institute of Radiology is patterned after that developed at the Joint Center for Radiation Therapy (Brigham & Women's Hospital, Boston, MA) and uses the Brown-Roberts-Wells computed tomography (CT) stereotactic system. The patient's head is attached to a stand that rotates with the treatment couch. The irradiation is conducted using a set of converging arcs of irradiation. Because of mechanical limitations, no accelerator or treatment couch is capable of placing the center of the radiation beam at precisely the same point for all gantry and couch angles and a compromise must be made when locating the nominal isocenter. The stand settings are checked by placing a radiopaque QA sphere at the desired target location. The QA sphere is imaged using a series of eight films exposed at a set of couch and gantry angles that encompass the treatment angles. The distances between the QA sphere image and the center of the radiation field indicate if the correct coordinates were set on the stand and if the radiation beam converges to a sufficiently small region (< 0.1-cm diameter) for treatment. A mathematical procedure has been developed to use the film-measured position errors to determine a stand offset that will minimize the distance between the accelerator isocenter and the target. The technique is capable of reducing the average placement error, as measured by imaging the QA sphere, to 0.035 cm with a maximum deviation of 0.07 cm.
马林克罗特放射研究所使用的立体定向放射外科系统是以联合放射治疗中心（马萨诸塞州波士顿市布里格姆妇女医院）开发的系统为蓝本，采用布朗-罗伯茨-韦尔斯计算机断层扫描（CT）立体定向系统。患者头部固定在一个与治疗床一起旋转的支架上。使用一组汇聚的照射弧进行照射。由于机械限制，没有加速器或治疗床能够在所有机架和床角度下将辐射束中心精确地置于同一点，因此在确定标称等中心时必须做出妥协。通过将一个不透射线的质量保证（QA）球放置在期望的目标位置来检查支架设置。使用一系列八张在一组涵盖治疗角度的床和机架角度下曝光的胶片对QA球进行成像。QA球图像与辐射野中心之间的距离表明在支架上是否设置了正确的坐标，以及辐射束是否汇聚到足够小的区域（直径<0.1厘米）用于治疗。已经开发出一种数学程序，利用胶片测量的位置误差来确定一个支架偏移量，该偏移量将使加速器等中心与目标之间的距离最小化。通过对QA球成像测量，该技术能够将平均放置误差降低到0.035厘米，最大偏差为0.07厘米。
