Thrapsanioti Zoi, Peppa Vasiliki, Hourdakis Costas J, Karaiskos Pantelis, Lekatou Aristea, Pantelis Evaggelos
Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, Athens, Greece.
Ionizing Radiation Unit, Greek Atomic Energy Commission (EEAE), Agia Paraskevi, Greece.
J Appl Clin Med Phys. 2025 Jan;26(1):e14575. doi: 10.1002/acm2.14575. Epub 2024 Nov 27.
To provide beam quality correction factors ( ) for detectors used in Ir brachytherapy dosimetry measurements.
Ten detectors were studied, including the PTW 30013 and Exrading12 Farmer large cavity chambers, seven medium (0.1-0.3 cm) and small (< 0.1 cm) cavity chambers, and a synthetic microdiamond detector. The k correction factors were calculated at distances from 1 to 10 cm away from an Ir source, using the EGSnrc Monte Carlo (MC) code. All detectors were calibrated in a Co 10 × 10 cm reference field provided by standard calibration laboratories. The impact of the central electrode, stem and wall on the detectors' responses in Ir photon energies was investigated. An experimental procedure was additionally applied for dose measurements around a microSelectron-v2 Ir high dose rate (HDR) brachytherapy source using a motorized water phantom.
Farmer chambers underestimated the dose near the source due to signal volume averaging effects, resulting in k values ranging from 1.177 and 1.317 at 1 cm, decreasing with distance to between 0.980 and 1.005 at 10 cm. Small cavity volume detectors should be used close to the source. The k for the studied small and medium cavity volume detectors were found to be close to unity (within 1.3%), showing also a small dependence on source-to-detector distance, except for ion chambers containing high-Z materials in their construction. The presence of high-Z materials caused an overresponse in these detectors, resulting in k values ranging from 0.950 at 1 cm to 0.729 at 10 cm away from the source. A dose rate constant of (1.114 ± 0.023)cGyhU was found in agreement with the literature (within 0.5%).
k values were calculated for dose measurements around Ir brachytherapy sources. Farmer chambers should be preferred for measurements at increased distances, whereas small or medium cavity volume detectors, not containing high-Z materials, should be used close to the source.
为铱近距离放射治疗剂量测量中使用的探测器提供射束质量校正因子（ ）。
研究了10种探测器，包括PTW 30013和Exrading12 Farmer大腔室电离室、7种中腔（0.1 - 0.3厘米）和小腔（<0.1厘米）腔室以及一种合成微金刚石探测器。使用EGSnrc蒙特卡罗（MC）代码在距铱源1至10厘米的距离处计算k校正因子。所有探测器均在标准校准实验室提供的钴10×10厘米参考场中进行校准。研究了中心电极、杆和壁对探测器在铱光子能量下响应的影响。另外还应用了一种实验程序，使用电动水模体围绕microSelectron-v2铱高剂量率（HDR）近距离放射治疗源进行剂量测量。
由于信号体积平均效应，Farmer腔室在源附近低估了剂量，导致1厘米处的k值范围为1.177至1.317，随着距离增加到10厘米时降至0.980至1.005之间。小腔室体积探测器应在靠近源处使用。研究发现，所研究的小腔室和中腔室体积探测器的k值接近1（在1.3%以内），除了构造中含有高Z材料的电离室外，对源到探测器距离的依赖性也较小。高Z材料的存在导致这些探测器响应过度，导致距源1厘米处的k值范围为0.950至10厘米处的0.729。发现剂量率常数为（1.114±0.023）cGy/hU，与文献一致（在0.5%以内）。
计算了铱近距离放射治疗源周围剂量测量的k值。在距离增加时进行测量应首选Farmer腔室，而靠近源处应使用不包含高Z材料的小腔室或中腔室体积探测器。
