Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA, USA.
Med Phys. 2011 Oct;38(10):5563-8. doi: 10.1118/1.3633903.
For routine clinical dosimetry of photon beams, it is often necessary to know the minimum thickness of backscatter phantom material to ensure that full backscatter condition exists.
In case of insufficient backscatter thickness, one can determine the backscatter correction factor, BCF(s,d,t), defined as the ratio of absorbed dose measured on the central-axis of a phantom with backscatter thickness of t to that with full backscatter for square field size s and forward depth d. Measurements were performed in SAD geometry for 6 and 15 MV photon beams using a 0.125 cc thimble chamber for field sizes between 10 × 10 and 30 × 30 cm at depths between d(max) (1.5 cm for 6 MV and 3 cm for 15 MV) and 20 cm.
A convolution method was used to calculate BCF using Monte-Carlo simulated point-spread kernels generated for clinical photon beams for energies between Co-60 and 24 MV. The convolution calculation agrees with the experimental measurements to within 0.8% with the same physical trend. The value of BCF deviates more from 1 for lower energies and larger field sizes. According to our convolution calculation, the minimum BCF occurs at forward depth d(max) and 40 × 40 cm field size, 0.970 for 6 MV and 0.983 for 15 MV.
The authors concluded that backscatter thickness is 6.0 cm for 6 MV and 4.0 cm for 15 MV for field size up to 10 × 10 cm when BCF = 0.998. If 4 cm backscatter thickness is used, BCF is 0.997 and 0.983 for field size of 10 × 10 and 40 × 40 cm for 6 MV, and is 0.998 and 0.990 for 10 × 10 and 40 × 40 cm for 15 MV, respectively.
对于常规临床光子束剂量学，通常需要知道背散射体模材料的最小厚度，以确保存在完全背散射条件。
在背散射厚度不足的情况下，可以确定背散射修正因子 BCF(s,d,t)，定义为具有背散射厚度 t 的体模的中心轴上测量的吸收剂量与具有方形射野尺寸 s 和正向深度 d 的完全背散射的吸收剂量之比。在 SAD 几何条件下，使用 0.125 cc 量规室，对 6 和 15 MV 光子束进行了测量，射野尺寸为 10×10 至 30×30 cm，在 d(max)（6 MV 为 1.5 cm，15 MV 为 3 cm）和 20 cm 之间的深度处进行测量。
使用蒙特卡罗模拟点扩散核，针对 Co-60 至 24 MV 之间的临床光子束，通过卷积法计算 BCF。卷积计算与实验测量结果的偏差在 0.8%以内，具有相同的物理趋势。对于较低能量和较大射野尺寸，BCF 的值更偏离 1。根据我们的卷积计算，在正向深度 d(max)和 40×40 cm 射野尺寸下，BCF 的最小值为 0.970（6 MV）和 0.983（15 MV）。
作者得出结论，当 BCF=0.998 时，6 MV 的背散射厚度为 6.0 cm，15 MV 的背散射厚度为 4.0 cm，射野尺寸最大可达 10×10 cm。如果使用 4 cm 的背散射厚度，则 6 MV 的 10×10 和 40×40 cm 射野的 BCF 分别为 0.997 和 0.983，15 MV 的 10×10 和 40×40 cm 射野的 BCF 分别为 0.998 和 0.990。
