Department of Radiation Oncology, University of Toronto and Radiation Medicine Program, Canada.
J Appl Clin Med Phys. 2012 Sep 6;13(5):3911. doi: 10.1120/jacmp.v13i5.3911.
This study evaluated the dosimetric impact of surface dose reduction due to the loss of backscatter from the bone interface in kilovoltage (kV) X-ray radiation therapy. Monte Carlo simulation was carried out using the EGSnrc code. An inhomogeneous phantom containing a thin water layer (0.5-5 mm) on top of a bone (thickness = 1 cm) was irradiated by a clinical 105 kVp photon beam produced by a Gulmay D3225 X-ray machine. Field sizes of 2, 5, and 10 cm diameter and source-to-surface distance of 20 cm were used. Surface doses for different phantom configurations were calculated using the DOSXYZnrc code. Photon energy spectra at the phantom surface and bone were determined according to the phase-space files at the particle scoring planes which included the multiple crossers. For comparison, all Monte Carlo simulations were repeated in a phantom with the bone replaced by water. Surface dose reduction was found when a bone was underneath the water layer. When the water thickness was equal to 1 mm for the circular field of 5 cm diameter, a surface dose reduction of 6.3% was found. The dose reduction decreased to 4.7% and 3.4% when the water thickness increased to 3 and 5 mm, respectively. This shows that the impact of the surface dose uncertainty decreased while the water thickness over the bone increased. This result was supported by the decrease in relative intensity of the lower energy photons in the energy spectrum when the water layer was with and over the bone, compared to without the bone. We concluded that surface dose reduction of 7.8%-1.1% was found when the water thickness increased from 0.5-5 mm for circular fields with diameters ranging from 2-10 cm. This decrease of surface dose results in an overestimation of prescribed dose at the patient's surface, and might be a concern when using kV photon beam to treat skin tumors in sites such as forehead, chest wall, and kneecap.
本研究评估了千伏 (kV) X 射线放射治疗中因骨界面反向散射丢失而导致表面剂量减少的剂量学影响。使用 EGSnrc 代码进行了蒙特卡罗模拟。使用 Gulmay D3225 X 射线机产生的临床 105 kVp 光子束照射包含薄水层 (0.5-5 毫米) 的不均匀体模，该水层位于厚度为 1 厘米的骨头上。使用 2、5 和 10 厘米直径的射野和源皮距 20 厘米。使用 DOSXYZnrc 代码计算了不同体模配置的表面剂量。根据包括多次交叉在内的粒子评分平面的相空间文件确定了体模表面和骨处的光子能谱。为了进行比较，在体模中用骨代替水重复了所有的蒙特卡罗模拟。当水层位于水下时，发现表面剂量减少。当水层厚度为 1 毫米时，5 厘米直径的圆形射野的表面剂量减少了 6.3%。当水层厚度增加到 3 和 5 毫米时，剂量减少分别减少到 4.7%和 3.4%。这表明，当水层覆盖在骨头上时，表面剂量不确定性的影响减小。当水层覆盖在骨头上时，与没有骨时相比，在能谱中较低能量光子的相对强度减小，证明了这一结果。我们得出结论，当水层厚度从 0.5-5 毫米增加时，2-10 厘米直径的圆形射野的表面剂量减少了 7.8%-1.1%。表面剂量的这种减少导致患者表面处规定剂量的高估，当使用 kV 光子束治疗额头、胸壁和膝盖骨等部位的皮肤肿瘤时，可能会引起关注。
