Rice R K, Hansen J L, Svensson G K, Siddon R L
Joint Center for Radiation Therapy, Harvard Medical School, Boston, MA 02115.
Phys Med Biol. 1987 Sep;32(9):1087-99. doi: 10.1088/0031-9155/32/9/002.
Dose distributions produced by small circular beams of 6 MV x-rays have been measured using ionisation chambers of small active volume. Specific quantities measured include tissue maximum ratios (TMR), total scatter correction factors (St), collimator scatter correction factors (Sc) and off-axis ratios (OAR). Field sizes ranged from 12.5 to 30 mm diameter, and were defined by machined auxiliary collimators with the movable jaws set for a 4 cm x 4 cm field size. Due to the lack of complete lateral electronic equilibrium for these small fields, the accuracy of the measurements was also investigated. This was accomplished by studying dose response as a function of detector size. Uncertainties of 2.5% were observed for the central axis dose in the 12.5 mm field when measuring with an ionisation chamber with a diameter of 3.5 mm. The total scatter correction factor exhibits a strong field size dependence for fields below 20 mm diameter, while the collimator scatter correction factor is constant and is defined by the setting of the movable jaws. Off-axis ratio measurements show larger dose gradients at the beam edges than those achieved with conventional collimator systems. Corrected profiles measured with an ionisation chamber are compared with measurements made with photographic film and LiF thermoluminescent dosemeters.
已使用小有效体积的电离室测量了由6兆伏X射线的小圆形射束产生的剂量分布。测量的特定量包括组织最大剂量比（TMR）、总散射校正因子（St）、准直器散射校正因子（Sc）和离轴比（OAR）。射野尺寸范围为直径12.5至30毫米，由加工的辅助准直器定义，活动钳口设置为4厘米×4厘米的射野尺寸。由于这些小射野缺乏完全的横向电子平衡，还研究了测量的准确性。这是通过研究剂量响应作为探测器尺寸的函数来实现的。当使用直径为3.5毫米的电离室进行测量时，在12.5毫米射野中观察到中心轴剂量的不确定度为2.5%。对于直径小于20毫米的射野，总散射校正因子表现出强烈的射野尺寸依赖性，而准直器散射校正因子是恒定的，由活动钳口的设置定义。离轴比测量显示，射束边缘的剂量梯度比传统准直器系统的更大。将用电离室测量的校正后的剂量分布图与用摄影胶片和LiF热释光剂量计进行的测量结果进行了比较。
