Sahlgrenska University Hospital, Department of Medical Physics and Biomedical Engineering, Gothenburg, Sweden. Department of Radiation Physics, Sahlgrenska Academy, University of Gothenburg, Sweden.
Biomed Phys Eng Express. 2020 May 6;6(4):045003. doi: 10.1088/2057-1976/ab8b25.
Ionization chamber dosimetry is predominantly used for determination of the absorbed dose to water in Co and high-energy radiotherapy photon beams. The most widespread ionization chambers employed for absolute or reference dose determinations in reference conditions are the Farmer-type cylindrical ionization chambers. The Farmer-type ionization chambers have a variety of constructions and materials and their responses vary in the radiation beam. Clinical accelerators, in addition to conventional photon beams with flattening-filter, can also deliver flattening-filter-free (FFF) photon beams. The responses of five different Farmer-type cylindrical ionization chambers were experimentally examined with reference to absorbed dose determination in reference conditions when using the International Atomic Energy Agency (IAEA) - American Association of Physicists in Medicine (AAPM) Technical Reports Series no. 483 (TRS-483) and the IAEA TRS-398 dosimetry protocol in the present investigation. The irradiations were performed using Co and megavoltage photon beams with 6 MV, 15 MV, 6 MV FFF and 10 MV FFF nominal photon energies. The chamber calibrations were performed at different Secondary Standard Dosimetry Laboratories and are traceable to primary standards at different Primary Standard Dosimetry Laboratories. The chambers were also cross-calibrated at our laboratory using Co γ-beam. The variation found in the data regarding the reference dose determination using the various Farmer-type chambers in the photon beams employed was about 1% at maximum. Thus, the selection of the ionization chamber in reference dose determinations may affect the outcomes. The differences in the absorbed dose values were similar in the conventional as well as in the FFF photon beams. For the FFF photon beams the absorbed dose computations were performed using the IAEA-AAPM TRS-483 dosimetry protocol. Two of the ionization chambers used had identical construction but different central electrodes, i.e. graphite versus aluminium. The results obtained using these two chambers show that, in the photon beams examined, the employed correction for the central electrode (p ) regarding these two chambers is associated with an inaccuracy which is larger than the calculated uncertainty for this correction. The outcomes found in the present experimental investigation using the various ionization chambers also indicate possible inaccuracy in the employed beam quality correction factors (k ) and imply the need for a revision of these factors.
电离室剂量学主要用于确定钴和高能放射治疗光子束中的水吸收剂量。在参考条件下，用于绝对或参考剂量测定的最广泛使用的电离室是圆柱形 Farmer 型电离室。圆柱形 Farmer 型电离室具有多种结构和材料，其在辐射束中的响应也有所不同。除了具有平坦化滤波器的常规光子束之外，临床加速器还可以提供无平坦化滤波器（FFF）光子束。本研究中，使用国际原子能机构（IAEA）-美国医学物理学家协会（AAPM）技术报告系列第 483 号（TRS-483）和 IAEA TRS-398 剂量学协议，对五种不同的圆柱形 Farmer 型电离室的响应进行了实验研究，以确定在参考条件下的吸收剂量。在本研究中，使用 Co 和兆伏光子束，能量分别为 6 MV、15 MV、6 MV FFF 和 10 MV FFF。在不同的二级标准剂量学实验室进行了腔室校准，并可追溯到不同一级标准剂量学实验室的一级标准。还在我们的实验室中使用 Co γ 射线对腔室进行了交叉校准。在使用不同 Farmer 型腔室在光子束中进行参考剂量测定时，数据中发现的差异最大约为 1%。因此，在参考剂量测定中选择电离室可能会影响结果。在常规和 FFF 光子束中，吸收剂量值的差异相似。对于 FFF 光子束，使用 IAEA-AAPM TRS-483 剂量学协议进行吸收剂量计算。使用的两个电离室具有相同的结构，但中央电极不同，即石墨与铝。使用这两个腔室获得的结果表明，在所检查的光子束中，对于这两个腔室，用于中央电极（p）的校正与该校正的计算不确定度相关的不准确性大于该校正的计算不确定度。使用各种电离室在本实验研究中获得的结果也表明，所使用的束质量校正因子（k）可能存在不准确性，并暗示需要对这些因子进行修订。
