Mirzakhanian Lalageh, Bassalow Rostem, Zaks Daniel, Huntzinger Calvin, Seuntjens Jan
Medical Physics Unit, McGill University, Montreal, Quebec, H4A 3J1, Canada.
RefleXion Medical, 25841 Industrial Blvd, Hayward, California, 94545, USA.
Med Phys. 2021 Apr;48(4):1884-1892. doi: 10.1002/mp.14631. Epub 2021 Mar 4.
The purpose of this study is to provide data for the calibration of the recent biology-guided radiotherapy (BgRT) machine (Hayward, CA, USA) following the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS-483 code of practice (COP) (Palmans et al. International Atomic Energy Agency, Vienna, 2017) and (Mirzakhanian et al. Med Phys, 2020).
In RefleXion BgRT machine, reference dosimetry was performed using two methodologies described in TRS-483 and (Mirzakhanian et al. Med Phys, 2020) In the first approach (Approach 1), the generic beam quality correction factor was calculated using an accurate Monte Carlo (MC) model of the beam and of six ionization chamber types. The is a beam quality factor that corrects (absorbed dose to water calibration coefficient in a calibration beam quality ) for the differences between the response of the chamber in the conventional reference calibration field with beam quality at the standards laboratory and the response of the chamber in the user's A field with beam quality . Field A represents the reference calibration field that does not fulfill msr conditions. In the second approach (Approach 2), a square equivalent field size was determined for field A of and . Knowing the equivalent field size, the beam quality specifier for the hypothetical field size was derived. This was used to calculate the beam quality correction factor analytically for the six chamber types using the TRS-398. (Andreo et al. Int Atom Energy Agency 420, 2001) Here, TRS-398 was used instead of TRS-483 since the beam quality correction values for the chambers used in this study are not tabulated in TRS-483. The accuracy of Approach 2 is studied in comparison to Approach 1.
Among the chambers, the PTW 31010 had the largest correction due to the volume averaging effect. The smallest-volume chamber (IBA CC01) had the smallest correction followed by the other microchambers Exradin-A14 and -A14SL. The equivalent square fields sizes were found to be 3.6 cm and 4.8 cm for the and field sizes, respectively. The beam quality correction factors calculated using the two approaches were within 0.27% for all chambers except IBA CC01. The latter chamber has an electrode made of steel and the differences between the correction calculated using the two approaches was the largest, that is, 0.5%.
In this study, we provided the values as a function of the beam quality specifier at the RefleXion BgRT setup ( and ) for six chamber types. We suggest using the first approach for calibration of the RefleXion BgRT machine. However, if the MC correction is not available for a user's detector, the user can use the second approach for estimating the beam quality correction factor to sufficient accuracy (0.3%) provided the chamber electrode is not made of high Z material.
本研究的目的是依据国际原子能机构（IAEA）和美国医学物理学家协会（AAPM）的TRS - 483实践规范（COP）（帕尔曼斯等人。国际原子能机构，维也纳，2017年）以及（米尔扎哈尼安等人。医学物理，2020年），为校准近期的生物引导放射治疗（BgRT）机器（美国加利福尼亚州海沃德）提供数据。
在RefleXion BgRT机器中，参考剂量测定采用TRS - 483和（米尔扎哈尼安等人。医学物理，2020年）中描述的两种方法进行。在第一种方法（方法1）中，使用束流和六种电离室类型的精确蒙特卡罗（MC）模型计算通用束流质量校正因子。 是一个束流质量因子，用于校正 （校准束流质量 下的水吸收剂量校准系数），以考虑标准实验室中具有束流质量 的常规参考校准场中电离室的响应与用户A场中具有束流质量 的电离室响应之间的差异。A场代表不满足msr条件的参考校准场。在第二种方法（方法2）中，确定了 和 场A的等效方形场尺寸。知道等效场尺寸后，推导出假设 场尺寸的束流质量指定器。这用于使用TRS - 398（安德烈奥等人。国际原子能机构420，2001年）为六种电离室类型解析计算束流质量校正因子。此处使用TRS - 398而非TRS - 483，因为本研究中使用的电离室的束流质量校正值未列入TRS - 483。将方法2的准确性与方法1进行比较研究。
在这些电离室中，由于体积平均效应，PTW 31010的 校正最大。最小体积的电离室（IBA CC01）的校正最小，其次是其他微电离室Exradin - A14和 - A14SL。对于 和 场尺寸，等效方形场尺寸分别为3.6厘米和4.8厘米。除IBA CC01外，使用两种方法计算的束流质量校正因子在所有电离室中相差0.27%以内。后一种电离室的电极由钢制成，使用两种方法计算的校正差异最大，即0.5%。
在本研究中，我们提供了六种电离室类型在RefleXion BgRT设置（ 和 ）下作为束流质量指定器函数的 值。我们建议使用第一种方法校准RefleXion BgRT机器。然而，如果用户的探测器无法进行MC校正，只要电离室电极不是由高Z材料制成，用户可以使用第二种方法以足够的精度（0.3%）估计束流质量校正因子。
