Chow James C L, Grigorov Grigor N
Radiation Medicine Program, Princess Margaret Hospital and Department of Radiation Oncology, University of Toronto, Toronto, ON M5G 2M9, Canada ; Department of Physics, Ryerson University, Toronto, ON M5B 2K3, Canada.
Department of Medical Physics, Grand River Regional Cancer Center, Kitchener, ON N2G 1G3, Canada.
Rep Pract Oncol Radiother. 2011 Nov 15;17(1):38-43. doi: 10.1016/j.rpor.2011.09.001. eCollection 2011.
In orthovoltage radiotherapy, since the dose prescription at the patient's surface is based on the absolute dose calibration using water phantom, deviation of delivered dose is found as the heterogeneity such as bone present under the patient's surface.
This study investigated the dosimetric impact due to the bone heterogeneity on the surface dose in orthovoltage radiotherapy.
A 220 kVp photon beam with field size of 5 cm diameter, produced by a Gulmay D3225 orthovoltage X-ray machine was modeled by the BEAMnrc. Phantom containing water (thickness = 1-5 mm) on top of a bone (thickness = 1 cm) was irradiated by the 220 kVp photon beam. Percentage depth dose (PDD), surface dose and photon energy spectrum were determined using Monte Carlo simulations (the BEAMnrc code).
PDD results showed that the maximum bone dose was about 210% higher than the surface dose in the phantoms with different thicknesses of water. Surface dose was found to be increased in the range of 2.5-3.7%, when the distance between the phantom surface and bone was increased in the range of 1-5 mm. The increase of surface dose was found not to follow the increase of water thickness, and the maximum increase of surface dose was found at the thickness of water equal to 3 mm.
For the accepted total orthovoltage radiation treatment uncertainty of 5%, a neglected consideration of the bone heterogeneity during the dose prescription in the sites of forehead, chest wall and kneecap with soft tissue thickness = 1-5 mm would cause more than two times of the bone dose, and contribute an uncertainty of about 2.5-3.7% to the total uncertainty in the dose delivery.
在深部X线放射治疗中，由于患者体表的剂量处方是基于使用水模体的绝对剂量校准，当患者体表下存在诸如骨骼等不均匀性时，会发现所交付剂量出现偏差。
本研究调查了深部X线放射治疗中骨骼不均匀性对表面剂量的剂量学影响。
使用BEAMnrc对Gulmay D3225深部X线机产生的直径5 cm射野的220 kVp光子束进行建模。用220 kVp光子束照射置于骨骼（厚度 = 1 cm）上方含有水（厚度 = 1 - 5 mm）的模体。使用蒙特卡罗模拟（BEAMnrc代码）确定百分深度剂量（PDD）、表面剂量和光子能谱。
PDD结果表明，在不同水厚度的模体中，最大骨骼剂量比表面剂量高约210%。当模体表面与骨骼之间的距离在1 - 5 mm范围内增加时，表面剂量增加了2.5 - 3.7%。发现表面剂量的增加不随水厚度的增加而变化，并且在水厚度等于3 mm时表面剂量增加最大。
对于公认的深部X线放射治疗总剂量不确定性5%而言，在前额、胸壁和膝盖部位软组织厚度为1 - 5 mm的剂量处方过程中，若忽略骨骼不均匀性的影响，会导致骨骼剂量超过两倍，并给剂量交付的总不确定性贡献约2.5 - 3.7%的不确定性。
