Tölli H, Johansson K A
Department of Radiation Physics, University of Göteborg, Sahlgrenska University Hospital, Sweden.
Phys Med Biol. 1998 Nov;43(11):3171-81. doi: 10.1088/0031-9155/43/11/001.
This paper presents experimentally determined correction factors for Farmer-type chambers for absorbed dose determination in 60Co and 192Ir brachytherapy dosimetry. The correction factors were determined from measurements made in a PMMA phantom and calculation of ratios of measured charges. The ratios were corrected for the different volumes of the ionization chambers, determined in external high-energy electron beams. The correction factors for the central electrode effect and the wall material dependency in 60Co brachytherapy dosimetry agree with those used in external 60Co beam dosimetry. In 192Ir dosimetry, the central aluminium electrode increases the response of an NE2571 chamber compared with that of a chamber with a central graphite electrode. The increase is 1.1 and 2.1% at 1.5 and 5.0 g cm(-2) distance, respectively. Similar values are obtained with an NE2577 chamber. The wall correction factor in 192Ir dosimetry for a chamber with an A-150 wall has been determined to be 1.018, independent of the measurement distance. For a graphite walled chamber, the correction factor is 0.996 and 1.001 at 1.5 and 5.0 g cm(-2) distance, respectively. The values of the wall correction factors are evaluated by a theory presented. If the chamber is used according to the 'large cavity' principle, the correction factor to account for the replacement of the phantom material by the ionization chamber was determined to be 0.982 for an NE2571 chamber when used with a Delrin cap, and 0.978 for an NE2581 when used with a polystyrene cap. The correction factors for the 'large cavity' principle are valid at both 60Co and 192Ir qualities.
本文介绍了通过实验确定的用于60Co和192Ir近距离治疗剂量测定中Farmer型电离室吸收剂量测定的校正因子。校正因子由在聚甲基丙烯酸甲酯（PMMA）模体中进行的测量以及测量电荷比的计算确定。这些比值针对在外部高能电子束中确定的电离室不同体积进行了校正。60Co近距离治疗剂量测定中中心电极效应和壁材料依赖性的校正因子与外部60Co束剂量测定中使用的校正因子一致。在192Ir剂量测定中，与具有中心石墨电极的电离室相比，中心铝电极会增加NE2571电离室的响应。在1.5和5.0 g/cm²距离处，增加量分别为1.1%和2.1%。使用NE2577电离室可获得类似值。对于具有A - 150壁的电离室，192Ir剂量测定中的壁校正因子已确定为1.018，与测量距离无关。对于石墨壁电离室，在1.5和5.0 g/cm²距离处的校正因子分别为0.996和1.001。壁校正因子的值通过所提出的理论进行评估。如果根据“大腔”原理使用电离室，当与Delrin帽一起使用时，对于NE2571电离室，考虑用电离室替代模体材料的校正因子确定为0.982；当与聚苯乙烯帽一起使用时，对于NE2581电离室，该校正因子为0.978。“大腔”原理的校正因子在60Co和192Ir两种射线质下均有效。
