Département de Physique, Université de Montréal, Pavilion Roger-Gaudry (D-428), 2900 Boulevard Edouard-Montpetit, Montréal, Québec H3T 1J4, Canada.
Med Phys. 2009 Oct;36(10):4654-63. doi: 10.1118/1.3213518.
For the purpose of nonstandard beam reference dosimetry, the current concept of reporting absorbed dose at a point in water located at a representative position in the chamber volume is investigated in detail. As new nonstandard beam reference dosimetry protocols are under development, an evaluation of the role played by the definition of point of measurement could lead to conceptual improvements prior to establishing measurement procedures.
The present study uses the current definition of reporting absorbed dose to calculate ionization chamber perturbation factors for two cylindrical chamber models (Exradin A12 and A14) using the Monte Carlo method. The EGSnrc based user-code EGS_chamber is used to calculate chamber dose responses of 14 IMRT beams chosen to cause considerable dose gradients over the chamber volume as previously used by Bouchard and Seuntjens ["Ionization chamber-based reference dosimetry of intensity modulated radiation beams," Med. Phys. 31(9), 2454-5465 (2004)].
The study shows conclusively the relative importance of each physical effect involved in the nonstandard beam correction factors of 14 IMRT beams. Of all correction factors involved in the dosimetry of the beams studied, the gradient perturbation correction factor has the highest magnitude, on average, 11% higher compared to reference conditions for the Exradin A12 chamber and about 5% higher for the Extradin A14 chamber. Other perturbation correction factors (i.e., Pwall, Pstem, and Pcel) are, on average, less than 0.8% different from reference conditions for the chambers and beams studied. The current approach of reporting measured absorbed dose at a point in water coinciding with the location of the centroid of the chamber is the main factor responsible for large correction factors in nonstandard beam deliveries (e.g., intensity modulated radiation therapy) reported in literature.
To reduce or eliminate the magnitude of perturbation correction factors in nonstandard beam reference dosimetry, two possible ways to report absorbed dose are suggested: (1) Reporting average dose to the sensitive volume of the chamber filled with water, combined with removing the reference field implicit gradient effect when measuring output factors, and (2) reporting average dose to the chamber itself during output factor verifications. The first option could be adopted if clinical beam correction factors are negligible. The second option could simplify quality assurance procedures when correction factors are not negligible and have to be calculated using Monte Carlo simulations.
为了进行非标准射束参考剂量学，详细研究了当前在腔体内代表位置处水中点报告吸收剂量的概念。随着新的非标准射束参考剂量学协议的制定，对测量点定义所起作用的评估可能会在建立测量程序之前带来概念上的改进。
本研究使用当前报告吸收剂量的定义，使用蒙特卡罗方法为两个圆柱形腔模型（Exradin A12 和 A14）计算电离室扰动量因数。使用 EGSnrc 为基础的用户代码 EGS_chamber 计算 14 个调强放射治疗射束的腔室剂量响应，这些射束的选择是为了在腔体内引起相当大的剂量梯度，正如 Bouchard 和 Seuntjens 之前使用的那样["调强放射治疗射束的基于电离室的参考剂量学"，《医学物理学》31（9），2454-2465（2004）]。
该研究明确表明了 14 个调强放射治疗射束非标准射束校正因子中涉及的每个物理效应的相对重要性。在所研究的射束的所有校正因子中，梯度扰动量校正因子的幅度最大，对于 Exradin A12 腔，平均比参考条件高 11%，对于 Extradin A14 腔，平均比参考条件高约 5%。其他扰动量校正因子（即 Pwall、Pstem 和 Pcel）与研究腔室和射束的参考条件相比，平均相差不到 0.8%。当前报告水中与腔室质心位置重合的一点处测量吸收剂量的方法是导致文献中报道的非标准射束（例如调强放射治疗）中较大校正因子的主要因素。
为了减少或消除非标准射束参考剂量学中的扰动量校正因子的幅度，建议报告吸收剂量的两种可能方法：（1）报告充满水的腔敏感体积的平均剂量，同时在测量输出因子时消除参考场隐式梯度效应，（2）在输出因子验证期间报告腔本身的平均剂量。如果临床射束校正因子可以忽略不计，则可以采用第一种选择。如果校正因子不可忽略并且必须使用蒙特卡罗模拟计算，则第二种选择可以简化质量保证程序。
