Gerbi B J, Khan F M
University of Minnesota, Department of Therapeutic Radiology-Radiation Oncology, Minneapolis 55455.
Med Phys. 1990 Jan-Feb;17(1):17-26. doi: 10.1118/1.596522.
Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed-separation plane-parallel ionization chambers are most commonly used for this purpose. Recent papers have re-emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed-separation plane-parallel ionization chambers. Data for Co-60, 6-, 10-, 18-, and 24-MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane-parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of greater than 19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.
准确测量模体表面和建成区的剂量是一项艰巨的任务，但对于患者的恰当治疗而言却至关重要。这些测量的首选仪器是外推电离室，但很少有机构能够使用这些仪器。因此，固定间距的平行板电离室最常用于此目的。最近的论文再次强调了使用固定间距平行板电离室测量正常入射光子束建成区剂量时的不准确性。给出了钴 -60、6、10、18 和 24 兆伏光子束的数据，这些数据显示了几种市售平行板电离室在建成区的这种过度响应程度与使用外推电离室和氟化锂热释光探测器所获结果的对比情况。对于其中一个电离室，发现模体表面的百分深度剂量差异超过了 19%。基于其内部尺寸，所有电离室在建成区都有一定程度的过度响应。评估了已发表的针对这些电离室的校正的适用性，并给出了在建成区准确测量剂量的指导原则。
