Debnath Sree Bash Chandra, Ferre Marjorie, Tonneau Didier, Fauquet Carole, Tallet Agnes, Goncalves Anthony, Darreon Julien
Aix Marseille Université, CNRS, CINaM UMR 7325, Marseille, 13288, France.
Institut Paoli-Calmettes, Marseille, 13009, France.
Med Phys. 2021 Apr;48(4):1485-1496. doi: 10.1002/mp.14727. Epub 2021 Mar 1.
Brachytherapy (BT) deals with high gradient internal dose irradiation made up of a complex system where the source is placed nearby the tumor to destroy cancerous cells. A primary concern of clinical safety in BT is quality assurance to ensure the best matches between the delivered and prescribed doses targeting small volume tumors and sparing surrounding healthy tissues. Hence, the purpose of this study is to evaluate the performance of a point size inorganic scintillator detector (ISD) in terms of high dose rate brachytherapy (HDR-BT) treatment.
A prototype of the dose verification system has been developed based on scintillating dosimetry to measure a high dose rate while using an Ir BT source. The associated dose rate is measured in photons/s employing a highly sensitive photon counter (design data: 20 photons/s). Dose measurement was performed as a function of source-to-detector distance according to TG43U1 recommendations. Overall measurements were carried out inside water phantoms keeping the ISD along the BT needle; a minimum of 0.1 cm distance was maintained between each measurement point. The planned dwell times were measured accurately from the difference of two adjacent times of transit. The ISD system performances were also evaluated in terms of dose linearity, energy dependency, scintillation stability, signal-to-noise ratio (SNR), and signal-to-background ratio (SBR). Finally, a comparison was presented between the ISD measurements and results obtained from TG43 reference dataset.
The detection efficiency of the ISD was verified by measuring the planned dwell times at different dwell positions. Measurements demonstrated that the ISD has a perfectly linear behavior with dose rate (R = 1) and shows high SNR (>35) and SBR (>36) values even at the lowest dose rate investigated at around 10 cm from the source. Standard deviation (1σ) remains within 0.03% of signal magnitude, and less than 0.01% STEM signal was monitored at 0.1 cm source-to-detector distance. Stability of 0.54% is achieved, and afterglow stays less than 1% of the total signal in all the irradiations. Excellent symmetrical behavior of the dose rate regarding source position was observed at different radiation planes. Finally, a comparison with TG-43 reference dataset shows that corrected measurements agreed with simulation data within 1.2% and 1.3%, and valid for the source-to-detector distance greater than 0.25 cm.
The proposed ISD in this study anticipated that the system could be promoted to validate with further clinical investigations. It allows an appropriate dose verification with dwell time estimation during source tracking and suitable dose measurement with a high spatial resolution both nearby (high dose gradient) and far (low dose gradient) from the source position.
近距离放射治疗（BT）涉及由复杂系统构成的高梯度内照射，该系统将放射源置于肿瘤附近以破坏癌细胞。BT临床安全的一个主要关注点是质量保证，以确保针对小体积肿瘤的实际剂量与处方剂量之间达到最佳匹配，同时保护周围健康组织。因此，本研究的目的是评估点尺寸无机闪烁体探测器（ISD）在高剂量率近距离放射治疗（HDR - BT）中的性能。
基于闪烁剂量测定法开发了一种剂量验证系统原型，用于在使用铱BT源时测量高剂量率。使用高灵敏度光子计数器以光子/秒为单位测量相关剂量率（设计数据：20光子/秒）。根据TG43U1建议，将剂量测量作为源到探测器距离的函数进行。所有测量均在水模体内进行，使ISD沿着BT针放置；每个测量点之间保持至少0.1厘米的距离。通过两个相邻传输时间的差值精确测量计划的驻留时间。还从剂量线性、能量依赖性、闪烁稳定性、信噪比（SNR）和信号背景比（SBR）方面评估了ISD系统的性能。最后，对ISD测量结果与从TG43参考数据集获得的结果进行了比较。
通过测量不同驻留位置的计划驻留时间验证了ISD的探测效率。测量表明，ISD与剂量率具有完美的线性关系（R = 1），即使在距源约10厘米处研究的最低剂量率下，也显示出高SNR（> 35）和SBR（> 36）值。标准偏差（1σ）保持在信号幅度的0.03%以内，在源到探测器距离为0.1厘米时监测到的STEM信号小于0.01%。实现了0.54%的稳定性，并且在所有照射中余辉保持在总信号的1%以下。在不同辐射平面上观察到剂量率关于源位置的出色对称行为。最后，与TG - 43参考数据集的比较表明，校正后的测量结果与模拟数据在1.2%和1.3%以内相符，并且对于源到探测器距离大于0.25厘米有效。
本研究中提出的ISD预计该系统可通过进一步的临床研究得到推广验证。它允许在源跟踪期间通过驻留时间估计进行适当的剂量验证，并在源位置附近（高剂量梯度）和远处（低剂量梯度）进行具有高空间分辨率的合适剂量测量。
