Department of Radiation Oncology, Hospital of University of Pennsylvania, 3400 Spruce Street, 2 Donner, Philadelphia, Pennsylvania 19104, USA.
Med Phys. 2010 Oct;37(10):5270-8. doi: 10.1118/1.3480480.
A three-parameter semiempirical model for scatter-to-primary dose ratio (SPR) is proposed to fit PDD (or TPR) and S(p) beam data. The SPR formula proposed in this study is more accurate than the previously published formula utilizing two parameters, especially for lower energy megavoltage photon beams, because the effect of backscattered photons is now taken into account.
Monte Carlo (MC) calculated SPR for photon energy spectrum between 60Co and 24 MV are used to evaluate the accuracy of the models. Based on fitting the MC data, the dependence of the SPR parameters (a0, w0,d0) with the attenuation coefficients of the photon beams is obtained and they were incorporated into the authors' optimization routine. The ability of the optimization routine to fit measured clinic data is examined for photon energies ranging from 60Co to 25 MV for all major cobalt and linear accelerator manufacturers.
The authors' model successfully fits the measured photon beam data for field size (E/3-40 cm), where E is photon energy in MV and for clinically usable depths, d(max) to 20 cm for 60Co, d(max) to 30 cm for 4 MV, and d(max) to 40 cm for 6 MV and higher photon energies. The maximum error among these fits is better than 2% for photon energies above 60Co.
The new SPR formula, along with the optimization routine, can serve as an efficient tool for performing quality control of x-ray beam data that conforms to AAPM Radiation Therapy Committee TG40 and Therapy Physics Committee TG142 reports on beam data requirement.
提出了一种三参数半经验散射与初级剂量比（SPR）模型，用于拟合 PDD（或 TPR）和 S(p)束数据。与利用两个参数的先前发表的公式相比，本研究中提出的 SPR 公式更准确，特别是对于较低能量的兆伏级光子束，因为现在考虑了反向散射光子的影响。
使用蒙特卡罗（MC）计算的光子能谱在 60Co 和 24 MV 之间的 SPR 来评估模型的准确性。基于对 MC 数据的拟合，获得了 SPR 参数（a0，w0，d0）与光子束衰减系数的依赖性，并将其纳入作者的优化程序中。作者的优化程序检查了从 60Co 到 25 MV 的所有主要钴和线性加速器制造商的光子能量的临床测量数据的拟合能力。
作者的模型成功地拟合了 60Co 到 25 MV 的光子束数据，用于束场大小（E/3-40 cm），其中 E 是光子能量，用于临床可使用的深度，d（max）到 20 cm 对于 60Co，d（max）到 4 MV 的 30 cm，以及 d（max）到 6 MV 及更高光子能量的 40 cm。在这些拟合中，最大误差优于 2%，对于光子能量高于 60Co。
新的 SPR 公式以及优化程序可以作为一种有效的工具，用于执行符合 AAPM 放射治疗委员会 TG40 和治疗物理委员会 TG142 关于束数据要求的 X 射线束数据质量控制。
