New model for explaining the over-response phenomenon in percentage of depth dose curve measured using inorganic scintillating materials for optical fiber radiation sensors.

Zhuang Qin,Xinyu Dai,Xu Zhang,Song Li,Ihsan Ullah Khan,Ziyin Chen,Weimin Sun,Elfed Lewis,Kun Feng,Daxin Zhang,Yu Ma,Wenhui Zhao,Tianci Xie,Bin Zhang,Jun Xu,Lina Xing,Haopeng Li,Yongji Yan,Xiaokang Hu

doi:10.1364/oe.27.023693

Abstract

Inorganic scintillating material used in optical fibre sensors (OFS) when used as dosimeters for measuring percentage depth dose (PDD) characteristics have exhibited significant differences when compared to those measured using an ionization chamber (IC), which is the clinical gold standard for quality assurance (QA) assessments. The percentage difference between the two measurements is as high as 16.5% for a 10 × 10 cm2 field at 10 cm depth below the surface. Two reasons have been suggested for this: the presence of an energy effect and Cerenkov radiation. These two factors are analysed in detail and evaluated quantitatively. It is established that the influence of the energy effect is only a maximum of 2.5% difference for a beam size 10 × 10 cm2 compared with the measured ionization chamber values. And the influence of the Cerenkov radiation is less than 0.14% in an inorganic scintillating material in the case of OFS when using Gd2O2S:Tb as the luminescent material. Therefore, there must be other mechanisms leading to over-response. The luminescence mechanism of inorganic scintillating material is theoretically analysed and a new model is proposed and validated that helps explain the over-response phenomenon.

Highlights

The ever-increasing sophistication of radiotherapy techniques, including those such as stereotactic radiosurgery, intensity-modulated radiation therapy, intraoperative radiation therapy, and intravascular brachytherapy, introduce new challenges to existing dosimetry systems
ionization chamber (IC) are a range of instruments that provide absolute radiation dose measurement, but the sizes of ICs generally mean that they cannot provide high spatial resolution and this combined with the fact that they rely on an electrical input for operation mean that they are unsuitable for in-vivo measurement
The over-response phenomenon has previously been attributed to the influence of the photoelectric effect caused by the high equivalent atomic number and the Cerenkov radiation received by the fiber optic sensor

Summary

Introduction

The ever-increasing sophistication of radiotherapy techniques, including those such as stereotactic radiosurgery, intensity-modulated radiation therapy, intraoperative radiation therapy, and intravascular brachytherapy, introduce new challenges to existing dosimetry systems. Inorganic scintillation systems have some severe drawbacks including non-adherence to the curve of depth-dose and off-axis ratio which exhibit deviations with the calibrated quality assurance (QA) results measured using standard ICs [19]. They exhibit a clear over-response phenomenon which has been attributed to Cerenkov radiation and the high effective atomic (Zeff) number which cause additional absorption (and emission) due to a combination of the Compton Scattering and the photoelectric effect, collectively referred to as an energy effect in this article [20]. The luminescence mechanism of the scintillating material is considered as the main reason for the over-response phenomenon, being separate from the Cerenkov radiation and the high Z number effect

Methodology

Effect of Cerenkov radiation on PDD curve formation

New physical model explains the over-response phenomenon

Verification of analysis

Findings

Conclusion