This study aims to investigate the energy response of a Farmer-type ionization chamber for kilovoltage x-ray beams. The chamber response in free air and at depths of 1 to 15 cm in a water phantom was calculated using the Monte Carlo method. The response in the phantom was evaluated using a beam quality conversion factor, $k_{Q_x}$, for x-ray beams Q_x to a ⁶⁰Co beam. The response was expressed as a function of the effective energy or a half-value layer (HVL, mm Al) for x-ray beams with given quality index (QI) values of 0.4, 0.5, and 0.6 and for monoenergetic photon beams. QI is defined as the ratio of the effective energy to the tube voltage. The response for 25 fluence spectra of x-ray beams was calculated for the beam qualities with HVL of 1.39-10.4 mm Al and tube voltages of 50-137.6 kVp, as classified by QI. The chamber response calculated in free air for the given QI values agreed with 1% of that provided by secondary standard laboratories. The $k_{Q_x}$ values depended on depth at the lower energy levels. The depth dependence was less than 1% for an HVL (mm Al) of more than 6 mm. The dose to water obtained from $k_{Q_x}$ and a ⁶⁰Co absorbed dose-to-water calibration coefficient $N_{D,w}^{{}^{60}\text{C}\text{o}}$ was within 1.5% at depths of 1 to 15 cm with that determined by the AAPM TG-61 for HVL of 2.9 to 10.8 mm Al. This method can reduce the uncertainty of doses in the phantom without any correction using P_Q,cham and ${\left[{({\bar{\mu }}_{\text{en}}/\rho )}_{\text{air}}^w\right]}_w$, and can directly determine the absorbed dose to water from the ionization chamber reading for medium-energy x-ray beams, including kilovoltage x-ray CT and image-guided cone beam CT in radiotherapy.

本研究旨在研究农民型电离室对千伏 X 射线束的能量响应。使用蒙特卡罗方法计算自由空气中和水体模中 1 至 15 厘米深度的腔室响应。使用光束质量转换因子评估体模中的响应，${克}_{{问}_X}$，对于 X 射线束Q _x到⁶⁰ Co 束。对于具有 0.4、0.5 和 0.6 的给定质量指数 (QI) 值的 X 射线束和单能光子束，响应表示为有效能量或半值层 (HVL，mm Al) 的函数。QI 定义为有效能量与管电压之比。根据 QI 分类的 HVL 为 1.39-10.4 mm Al 和管电压为 50-137.6 kVp，计算 X 射线束的 25 个注量谱的响应。对于给定的 QI 值，在自由空气中计算的腔室响应与二级标准实验室提供的响应值的 1% 一致。这${克}_{{问}_X}$值取决于较低能级的深度。对于大于 6 mm 的 HVL (mm Al)，深度相关性小于 1%。从获得的水剂量${克}_{{问}_X}$和⁶⁰ Co 吸收剂量-水校准系数$N_{D,瓦}^{{}^{60}\text{C}\text{○}}$AAPM TG-61 在 1 至 15 厘米的深度处对 2.9 至 10.8 毫米铝的 HVL 测定的误差在 1.5% 以内。这种方法可以减少体模中剂量的不确定性，而无需使用P _Q、cham和${\left[{({\bar{\mu }}_{\text{恩}}/\rho )}_{\text{空气}}^{瓦}\right]}_{瓦}$，并且可以从电离室读数中直接确定中能 X 射线束的吸收剂量，包括放射治疗中的千伏 X 射线 CT 和图像引导锥形束 CT。