用于国产6711型125I粒籽源空气比释动能强度测量的外推电离室初步研究
Preliminary Study on Extrapolation Ionization Chamber for Measurement Air Kerma Strength of Domestic 6711 125I Seed
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摘要: 近距离放射治疗用125I粒籽源具有低能、低剂量率的特点，基于绝对测量原理，利用现有的自由空气电离室无法实现空气比释动能强度的准确测量，因此设计了测量125I粒籽源的空气比释动能强度专用的外推电离室。电离室的设计需要满足带电粒子平衡条件，通过蒙特卡罗模拟计算与理论分析对电离室的关键部件结构设计进行了研究，包括光阑、高压极、收集极、栅极及分压电阻等。设计的专用电离室采用锥形束照射方式，增加电离室的有效测量体积占比，相比自由空气电离室有效测量体积占比由0.3%提升至12%。采用电荷积分法对活度标称值为3.404×107 Bq的6711型125I粒籽源空气比释动能强度进行了测量，测量结果为1.250 μGy·m2·h-1（Urel=1.6%，k=2）。通过进一步优化设计，该外推电离室可用于粒籽源空气比释动能强度的量值复现，为建立近距离放疗剂量的量值传递体系提供技术基础。Abstract: A new physical quantity, air kerma strength, was proposed to replace the apparent activity of the brachytherapy source to characterize the source strength, which is reported by the American Association of Physicists in Medicine (AAPM) task group No.32 report. 125I seeds are widely used for interstitial brachytherapy treatment. It is very important to establish the primary standard to realize the traceability and transmission of the physical quantity. The existing primary standard free air chamber (FAC) can’t measure the air kerma strength of 125I in a high accuracy, because of the characteristics of low energy and low dose rate of 125I seeds. Therefore, a special ionization chamber was designed and manufactured for the realization of the absolute measurement of air kerma strength from a single 125I seeds. In order to meet the basic requirement of ionization chamber design, that is charged particle balance, the structure design of key components of ionization chamber was studied by Monte Carlo and theoretical calculation, which included diaphragm, high voltage electrode, collecting electrode, potential rings, divider resistors, et al. A large conical beam irradiation was designed to increase the coverage of the measurement volume to increase the deposited energy of the secondary electrons. The distance between the high voltage electrode and the collecting electrode is 3-200 mm. The response of the ionization chamber with different thicknesses of high voltage electrode was determined by utilizing the MCNP code. The high voltage electrode was made of Al-coated polyethylene terephthalate (PET) foil. Two types of the collecting electrode of the ionization chamber were compared with the deposited energy in measurement volume. The type of the PET foil has a 0.07% effect of the deposited energy in the ionization chamber, and the type of the 1 cm PMMA sheet causes a 7.5% energy deposited change. Finally, the collecting electrode consists of a radius of 5 cm Al-coated PET foil with the width of 9 cm potential guard. The effect measurement volume varies from 235 cm3 to 1 570 cm3 and the collection volume varies from 1 847 cm3 to 12 315 cm3. Compared with the primary standard FAC, the ratio of effective measurement volume to collection volume increases from 0.3% to 12%. Charge integration method was used to measure the air kerma strength of 125I seed. And the measurement result of the air kerma strength is 1.250 μGy·m2·h-1（Urel=1.6%, k=2） of the nominal value of activity of 3.404×107 Bq. By further optimizing the design, this extrapolation ionization chamber will be applied to realize the quantity of air kerma strength. This work provides a technical basis for value dissemination of brachytherapy.
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Keywords:
- 125I seed ,
- extrapolation ionization chamber ,
- air kerma strength
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