Das I J
Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA 19111, USA.
Br J Radiol. 1998 Jan;71(841):68-73. doi: 10.1259/bjr.71.841.9534701.
In kilovoltage X-ray treatment, beam shaping and shielding normal tissue are accomplished by thin sheets of lead cutout, the thickness of which is selected based upon either published data or measurements. Available broad beam attenuation (BBA) data are found to be unsatisfactory and are the subject of this investigation. BBA is defined as the ratio of intensity with (I) and without (I0) attenuating medium for a large field in a phantom. BBA = I(x,t,E)/I0(x,0,E), where x is the depth of measurement, t is the thickness of attenuator, and E is the beam energy. The depth x should be zero for kilovoltage beams and dmax for megavoltage beams. Unfortunately, x is limited by the window thickness which is the core of this study. A Farmer-type cylindrical ion chamber and three parallel plate ion chambers (Capintec, PS-033; Markus; and Holt) were used to measure BBA for kilovoltage beams from a Siemens Stabilipan unit. Results indicate that attenuation is strongly dependent on the window thickness. For the 240 kVp beam, the thickness of lead for 5% and 1% transmissions are 3.1 mm, and 5.2 mm, respectively, with the Capintec chamber. The corresponding values of lead thickness for the Markus chamber are 2.3 mm and 4.0 mm; for the Holt chamber the values are 1.1 mm and 2.2 mm; and for the cylindrical chambers the values are 1.1 mm and 2.3 mm, respectively. Similar variabilities in lead thickness with ion chambers were also noted for the other kilovoltage beams. The large differences in lead thicknesses produce enormous clinical errors, especially for shielding eye and other critical structures. For small thickness of lead (< 0.1 mm), a 20-fold increase in surface dose could be observed instead of usual beam attenuation. This is due to intense low energy photoelectrons liberated from lead sheets in the contact with tissue. It is concluded that the lead thickness required to shield normal tissue varies with ion chamber. Until national or international guidelines for broad beam transmission measurements are established, the shielding materials in contact with skin should be coated with a thin (> or = 0.3 mm) low atomic number medium. In such a situation, transmission measurements will be independent of the choice of an ion chamber.
在千伏级X射线治疗中，射野塑形和对正常组织的屏蔽是通过铅制挡块薄片来实现的，其厚度根据已发表的数据或测量结果来选择。现有的宽束衰减（BBA）数据并不理想，因此开展了本研究。BBA定义为体模中一个大射野在有（I）和无（I0）衰减介质时的强度之比。BBA = I(x,t,E)/I0(x,0,E)，其中x是测量深度，t是衰减器厚度，E是射束能量。对于千伏级射束，深度x应为零；对于兆伏级射束，深度x应为最大剂量深度（dmax）。不幸的是，x受限于窗口厚度，而窗口厚度正是本研究的核心。使用一个 Farmer 型圆柱形电离室和三个平行板电离室（Capintec，PS - 033；Markus；以及 Holt）来测量西门子 Stabilipan 装置产生的千伏级射束的BBA。结果表明，衰减强烈依赖于窗口厚度。对于240 kVp的射束，使用Capintec电离室时，5%和1%射束透射率对应的铅厚度分别为 3.1 mm和5.2 mm。Markus电离室对应的铅厚度值分别为2.3 mm和4.0 mm；Holt电离室对应的铅厚度值分别为1.1 mm和2.2 mm；圆柱形电离室对应的铅厚度值分别为1.1 mm和2.3 mm。对于其他千伏级射束，也注意到了电离室导致的铅厚度存在类似的差异。铅厚度的巨大差异会产生巨大的临床误差，尤其是在对眼部和其他关键结构进行屏蔽时。对于薄铅片（< 0.1 mm），观察到表面剂量增加了20倍，而不是通常的射束衰减。这是由于与组织接触的铅片中释放出强烈的低能光电子。得出的结论是，屏蔽正常组织所需的铅厚度随电离室而变化。在建立国家或国际宽束透射率测量指南之前，与皮肤接触的屏蔽材料应涂覆一层薄（≥ 0.3 mm）的低原子序数介质。在这种情况下，透射率测量将与电离室的选择无关。
