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An inorganic scintillating material plastic optical fiber (POF) dosimeter for measuring ionizing radiation during radiotherapy applications is reported. It is necessary that an ideal dosimeter exhibits many desirable qualities, including water equivalence, energy independence, reproducibility, dose linearity. There has been much recent research concerning inorganic dosimeters. However, little reference has been made to date of the depth-dose characteristics of dosimeter materials. In the case of inorganic scintillating materials, they are predominantly non water-equivalent, with their effective atomic weight (Zeff) being typically much greater than that of water. This has been a barrier in preventing inorganic scintillating material dosimeter from being used in actual clinical applications. In this paper, we propose a parallel-paired fiber light guide structure to solve this problem. Two different inorganic scintillating materials are embedded separately in the parallel-paired fiber. It is shown that the information of water depth and absorbed dose at the point of measurement can be extracted by utilizing their different depth-dose properties.
© 2016 Optical Society of America
1. Introduction
The ever-increasing sophistication of radiotherapy techniques, including techniques such as stereotactic radiosurgery, intensity-modulated radiation therapy, intraoperative radiation therapy, and intravascular brachytherapy, has driven an urgent demand for dosimeters which can provide accurate measurements in real time. In this way, it would lead to more conformal dose distribution, thus precisely targeting the tumor cells and avoiding potential damage to surrounding normal tissue [1,2]. Moreover, due to the occurrence of a number of major radiotherapy incidents, the importance of in-vivo dosimetry has been further highlighted in recent years [3–5]. However, traditional dosimeters, such as thermoluminescent dosimeters (TLDs), silicon diodes, radiochromic films, metal-oxide field-effect transistors (MOSFETs) and Ionization Chambers (ICs) [6–15], cannot satisfy all or in some cases any of these demands. None of them possess the ability to be used simultaneously for real time and in-vivo dosimetry in the tissue and in close proximity to or even within the tumor. Among them, the Ionisation Chamber (IC) is considered to be the ‘gold standard’ instrument for Quality Assurance (QA purposes), which can be calibrated to provide a near absolute dose measurement. However a major disadvantage of ICs is that they require a relatively high voltage to work (this is usually in the range of 10s of volts or more) and thus cannot be used in-vivo, and only for QA purposes. A further disadvantage of ICs is the requirement for dose ionization conversion factors, which can result in a strong dependence on electron beam dose rate.
To overcome these shortcomings and meet actual clinical demand, a method using scintillators has been presented (Beddar et al 1992) [16]. This optical fiber based dosimeter has a simple structure: a small quantity of scintillating material is located at the end of a normal plastic optical fiber (POF) and acts as a signal source and the tip is further coated with carbon which acts as a light shield to external inward light entrainment. As the scintillators are exposed to ionizing radiation, such as high-energy photons and/or electron beams (typically in the single MV and MeV range respectively), a visible wavelength optical signal is generated and guided by the Poly Methyl Methacrylate (PMMA) fiber toward distal photomultiplier tubes (PMTs) or other photon sensitive detectors placed far away from the irradiation zone (Usually 20 to 30 m). This kind of dosimeter arrangement can also be sub-divided into plastic scintillation dosimeters and inorganic scintillation dosimeters. Plastic scintillator dosimeters have many desirable dosimetric characteristics compared with traditional detector systems: these small-volume detectors can offer reproducibility, continuous sensitivity, linearity of response, a high degree of water-equivalence (through their effective atomic number, Zeff), resistance to radiation damage, and real-time operation [17–24]. However, plastic scintillator dosimeters also have a shortcomings, namely the low signal-to-noise ratio (SNR) issue generated by the low fluorescence response and the associated Cerenkov radiation emission as a potential noise source. Unlike the plastic scintillator dosimeters, the inorganic scintillator dosimeter has very high efficiency and exhibits all the advantages shared by the plastic scintillator dosimeter, except the water-equivalence which is currently a major obstacle preventing inhibiting widespread use of inorganic scintillating material based dosimeters in clinical applications.
In this paper, a parallel-paired optical fiber light guide structure dosimeter with two different inorganic materials embedded separately in each fiber is presented. In addition to all the advantages of other inorganic dosimeter listed above, it is shown that this dosimeter also exhibits a similar depth-dose characteristic to organic scintillator dosimeters [25].
2. Principle and method
Due their relatively high effective atomic number (Zeff), inorganic scintillating materials have little or no water-equivalence. The density and atomic composition of the scintillator materials are quite different from those of the water and organic materials. For relatively low energy X-Ray beams (30keV-25MeV), the predominant mode of interaction in low-Z materials, such as water and other organic materials is the Compton Effect. As for some inorganic materials, such as Gd2O2S: Tb, a high-Z material, there exists a higher probability of secondary electron generation through the photoelectric effect. Hence, during a standard depth-dose experiment, a different depth-dose curve from the curve measured by the IC during QA is obtained. For example, when the beam energy of the accelerator is 6 MV (photon Energy), the depth of the maximum measured dose (Dmax) as measured by the IC is 1.5 cm below the irradiated surface. However, the Gd2O2S: Tb dosimeter produces a corresponding value at around 3cm. Despite this situation, for a fixed water depth, the inorganic dosimeter has near perfect dose linearity: Linear regression analysis has revealed a coefficient (R2) of over 0.9999 for this type of Sensor [26]. From the standard depth-dose experiments, the ratio of real absorbed dose as measured by IC (DIC) and the values measured by the optical fiber based inorganic dosimeter (DDosi) changing with the water depth can be obtained. That is to say, as long as the water depth and the ratio are known, the absorbed dose can potentially be accurately calculated. However, for several treatments of human patients it is necessary to rotate the gantry of the linac. This produces a large uncertainty as to the real value of depth below the surface of a targeted tumour and hence would introduce a corresponding uncertainty and unacceptable error to the dose calculation.
To solve this problem, a method involving embedding two different scintillation materials in the dosimeter is presented. First, the two kinds of inorganic scintillation materials should have different and distinct dose-depth characteristics. In the case of obtaining the resulting dose responses from these two materials, the depth can be calculated by utilizing an equation based on fitting to data arising from results of the individual dose-depth experiments corresponding to the two materials. The second step is to associate the newly calculated depth to the depth DIC/DDosi ratio curve, so as to determine this ratio as a calibration coefficient. Finally, through multiplying the value measured by the dosimeter with the correction coefficient, the true absorbed dose is thus calculated.
3. Dosimeter design and fabrication
The experimental setup for measuring the absorbed dose is presented schematically in Fig. 1(a). The inorganic scintillation dosimeter was submerged under the water within a clinical standard water-equivalent tank in the radiotherapy bunker room. The photograph in Fig. 1(b) shows the immersed dosimeter and IC placed in water tank and are effectively submerged in the water. However, the Optical Fibre Sensor is located directly in the water at the required depth unlike the IC which needs to be protected and hence occupies a purpose made dry compartment.
Fig. 1 The experimental test facility at the Harbin Hospital Oncology Clinic: (a) Schematic layout.(b) The dosimeter immersed in the test water tank.
The fiber-optic dosimeter is located in the center of the rectangular 10 cm by 10 cm radiation beam field provided by an IX 3937 Varian Linear accelerator (linac). The inorganic scintillation dosimeter shown in Fig. 1 comprises two different inorganic scintillator embedded in the core of two PMMA optical fibers, a parallel-paired fiber light guide, and two identical photodetectors, Hamamatsu type C11208-350 MPPCs (multi-pixel photon counters). These two inorganic scintillators are terbium-doped gadolinium oxysulfide (Gd2O2S: Tb) and thallium activated cesium iodide (CsI:Tl) (Supplied by Phosphor Hoddesdon & Herts, UK). They provide different characteristics responses in depth-dose experiments in water as well as high scintillation efficiency. At the tip of the fibers, each core was micro-machined to create a small hole whose diameter is 0.25 to 0.5 mm and depth is 2mm. The two inorganic scintillating materials were embedded in the holes and sealed therein using epoxy adhesive. The materials both fluoresce immediately when exposed to ionizing radiation (X-Ray, electron beam and high-energy photon), and hence their time response can be considered as near instantaneous (actually it is in the sub ns range). The 25-m length parallel-paired optical fiber light guide is shown in Fig. 2 and is manufactured this way for ease of use and installation (Mitsubishi Rayon Co., Ltd, Tokyo, Japan). Two optical fibers placed side by sidecoated with one jacket can be seen in Fig. 2(a). At the receiving end of the light guide, each optical fiber was carefully polished and connected to the Hamamatsu MPPC using a standard SMA Connector. Figure 2(b) is a photograph of the sensor end of the optical fiber with the full protective coating of the fiber as used in the experiments and Fig. 2(c) the sensor with the protective coating removed purely for clarity of the image. Figure 2(d) represents an ‘end on’ view of the sensor end of the fiber showing clearly the core of the fiber and the micro-machined hole into which the scintillation material has been inserted.
Fig. 2 (a) Sectional view of the special optical fiber, (b) the photograph of the fiber, (c) the photograph of the dosimeter with the jacket removed and (d) Sectional view of the tip of the dosimeter.
When the linac beam is on, the incident radiation dose is converted to a measurable visible light signal by fluorescence of the scintillator material located at the tips of each optical fiber [27]. The two visible optical signals from the different scintillators propagate along the parallel-paired fiber light guide to the two distal MPPCs located in the control room which are therefore completely isolated from the radiation field. Using the output signal arising from the two MPPCs, the intensity of the visible fluorescent light signal was measured and transferred to the computer for data analysis and processing, which is discussed below.
4. Results and analysis
The two-fiber dosimeter was tested at the External Radiation Beam Therapy clinic of the First Affiliated Hospital of the Harbin Medical University, Harbin, China using a Varian Linac (IX 3937). The fiber-optic dosimeter and the IC (TW30012-1) were located centrally in the field of the ionizing beam which had a standard field size of 10 × 10 cm2 at a SSD (Source to Surface Distance) of 100 cm. Both the IC and fiber-optic dosimeter were therefore exposed to near identical irradiation conditions for reference measurement and data validation. The dosimeter and the IC were irradiated with a dose rate of 600MU/min and photon energy of 6 MV in a range of sub surface depths from 1cm to 10 cm in steps of 1 cm.
Figure 3 shows the results of depth-dose profile for 6 MV beam as measured for the two types of Scintillator material (Fig. 3(a)) and the IC chamber (Fig. 3(b)). Figure 3(c) represents each of the curves of Fig. 2(a) and Fig. 2(b) normalized to the value of 2 cm and is intended to highlight the difference depth-dose responses of the two scintillator materials in the optical fiber dosimeters as well as the IC.
Fig. 3 The dose versus depth: (a) measured on the dosimeter, (b) measured on IC, and (c) the normalized data.
The two characteristics of the scintillating material sensors represented in Fig. 3(a) have been curve fitted based on a polynomial fit and can be described as:
where yGd2O2S:Tb and yCsI:Tl represent the integral sum of the optical intensity during the beam, corresponding to the radiation dose received by these two materials [26], x is the sub surface depth. The equation obtained for the CsI:Tl material is divided into two parts, because the error would be higher if only one cubic equation was used for fitting. From earlier work presented by the same authors as this article, it has been shown that this kind of optical fiber dosimeter exhibits excellent repeatability in monitoring radiation dose with a maximum percentage error of 0.16%. That result illustrates that the curve fitting is relatively stable in successive uses of the sensor i.e. it can be applied to the sensor in a repeatable manner.From the data of Fig. 3(a) and 3(b), the ratio between the dose absorbed by water (DIC) and the signal response received by the dosimeter (DDosi) for each depth was obtained. This ratio represents the correction coefficient, and when multiplied by the signal response (in this case the response of Gd2O2S: Tb has been used to determine (DDosi)), it is possible to extract the real absorbed-dose for water. Figure 4 shows the resulting correction coefficient obtained from the Gd2O2S:Tb scintillator based sensor versus water depth. The equation governing this curve may be expressed as:
where yratio represents the DIC/DDosi ratio, and x is the sub surface depth.Using the above fitted curves, the dosimeter can be used to measure the real absorbed-dose, as in the case of a plastic scintillator dosimeter. The process for extracting the real dose is shown schematically Fig. 5. Under the same experimental conditions, another experiment was performed at a different time, being two weeks later. The integral sum of the optical intensity of the two materials for different depths is shown in Table 1.
Table 1. The Integral Sum of the Optical Intensity Received by Two Materials in Different Depth
Substituting the data in Table 1 into the Eq. (1), (2), it is possible to obtain a series of results of the water depth which is shown in Table 2. And from the Table 2 it can be observed that the maximum error corresponds to an absolute depth of less than 5 mm over the entire depth range of 1 to 10 cm. The maximum error occurs at a depth of about 3cm. The reason for this is that the curves of Fig. 3 exhibit their minimum slope value in this range and therefore the polynomial fit is more sensitive to any errors that may occur in this range. However, results presented later (in Table 3) of calculated dose show that even with this apparent error in depth, the impact on the calculated dose is much less. Therefore, it can be considered that the dosimeter has sufficient accuracy when compensating the depth (location) of the radiation. Using the calculated depth and Eq. (3), the correction coefficient can also be acquired.
Table 2. The Calculated Depth and the Correction Coefficient
Table 3. The Calculated Absorbed-dose and the Error
The calculated absorbed-dose was calculated by multiplying the integrated intensity (Table 1) by the correction coefficient. The result in Table 3 shows that the novel parallel-paired fiber-optic dosimeter can measure the absorbed-dose in different water depths accurately within 5% error (max error = 4.1%).
5. Conclusion
An investigation has been conducted on the use of two different inorganic scintillating materials in a single two-fiber optical dosimeter for the purpose monitoring ionizing radiation. Using the unique depth dose characteristics of the two embedded scintillators, it has been shown that the two-fiber sensor exhibits a similar depth dose characteristic to the ‘gold standard Ionisation Chamber (IC) detector. Through this study, we have therefore proposed an innovative method of using a parallel-paired fiber light guide method to overcome the difficulty that inorganic scintillating material has little or no water-equivalence. In this paper, the depth-dose characteristics of two inorganic scintillating materials: Gd2O2S:Tb and CsI:Tl, have been curve fitted to polynomial functions, while the relationship of the correction coefficient and the depth has also been obtained. Through forming these curves, it has been shown that the absorbed dose at each depth can be accurately calculated. The veracity of the measurement was confirmed using a co-located IC, and hence it was shown that the real absorbed-dose can be obtained with a relatively small error: being less than 5%. Further work is planned to make the fitting curves more accurate and find two scintillating materials with larger differences in their depth-dose characteristics. A greater difference will improve the accuracy of the solution in the case of identifying the true water depth and hence the overall accuracy of the measurement.
Funding
The International Science & Technology Cooperation Program of China (2014DFE10030), the Joint Research Fund in Astronomy (U1331114) under cooperative agreement between the National Natural Science Foundation of China (NSFC) and Chinese Academy of Sciences (CAS), the 111 project (B13015), the Fundamental Research Funds for the Central Universities to the Harbin Engineering University.
Acknowledgment
We are grateful for the assistance from Professor Zhang Hongquan and Huang Zongjun.
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