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 Table of Contents 
ORIGINAL ARTICLE
Year : 2020  |  Volume : 43  |  Issue : 2  |  Page : 108-114  

Calibration of MTS-N (LiF: Mg, Ti) chips using cesium-137 source at low doses for personnel dosimetry in diagnostic radiology


1 Department of Radiology, Medical Physics Unit, Federal Medical Centre Asaba, Asaba, Delta, Nigeria
2 Department of Radiation Biology, Radiotherapy and Radiodiagnosis, College of Medicine, University of Lagos, Lagos, Nigeria

Date of Submission05-Feb-2020
Date of Decision18-Mar-2020
Date of Acceptance28-Jun-2020
Date of Web Publication27-Aug-2020

Correspondence Address:
Mr. Akintayo Daniel Omojola
Department of Radiology, Medical Physics Unit, Federal Medical Centre Asaba, Asaba, Delta
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_6_20

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  Abstract 


Thermoluminescent dosimeter (TLD) is still in use for many applications such as radiation protection, medical dosimetry, environmental research, and personnel dosimetry, with the overall aim of estimating radiation dose within a given medium or material. The aim of this study was to determine the coefficient of variation (CV) for thermoluminescent (TL) element within the same bar-coded slide and to establish calibration factors (CFs) at dose equivalent of 0.07 mm depth in tissue (Hp [0.07]) and dose equivalent of 10 mm depth in tissue (Hp [10]) for newly purchased TL elements alongside a new RadPro TLD manual reader and annealing oven. Annealed TL elements were taken to a Secondary Standard Dosimetry Laboratory (SSDL) for irradiation using a cesium-137 source at known doses (0.2–2 mGy). A RadPro Cube 400 manual TLD Reader was used to determine corresponding TL signal. The CV between two identical TL element within a bar-coded slide for (Hp [10]) and (Hp [0.07]) was determined and a graph of dose (mGy) against TL signal (Coulomb) was plotted to determine the elements CF. CVs from the raw data for 40 TL elements for Hp (10) and Hp (0.07) were 14.6% and 15.02%, respectively. Further selection of sensitive TL elements reduced the CVs of Hp (10) and Hp (0.07) to 3.73% and 3.21%, respectively, which was seen to be within ±10% accepted limit. The maximum percentage deviation for the calculated and actual dose for Hp (10) and Hp (0.07) was 16.7% and 14.3%, respectively. The CFs were power of 10 − 6 and the Coefficient of determination (R2) for Hp (10) and Hp (0.07) was 0.9998 and 0.9981, respectively, after adjustments were made on the initial graphs. Although large deviations were observed at low doses from the results of the raw data. Re-selected “golden Chips” had R2 close to unity and CV was within recommended standards.

Keywords: Annealing oven, bar-coded slide, calibration factor, cesium-137, coefficient of variation, thermoluminescent dosimeter


How to cite this article:
Omojola AD, Akpochafor MO, Adeneye SO, Aweda MA. Calibration of MTS-N (LiF: Mg, Ti) chips using cesium-137 source at low doses for personnel dosimetry in diagnostic radiology. Radiat Prot Environ 2020;43:108-14

How to cite this URL:
Omojola AD, Akpochafor MO, Adeneye SO, Aweda MA. Calibration of MTS-N (LiF: Mg, Ti) chips using cesium-137 source at low doses for personnel dosimetry in diagnostic radiology. Radiat Prot Environ [serial online] 2020 [cited 2020 Sep 26];43:108-14. Available from: http://www.rpe.org.in/text.asp?2020/43/2/108/293628




  Introduction Top


The phenomenon of thermoluminescence (TL) has been known and it has been studied for years by early Scientists. The use of TL dosimeter (TLD) is still regarded as “gold standard” in radiation protection, medical dosimetry, experimental physics, environmental research, personnel dosimetry, and testing food irradiation levels.[1],[2],[3],[4] Most commonly used TLDs in medical applications are LiF: Mg, Ti, LiF: Mg, Cu, P, and Li2B4O7: Mn, because their property is tissue equivalent. Other TLDs, which are being used because of their high sensitivity, are, Al2O3: C, CaSO4: Dy, and CaF2: Mn among many others.[5],[6],[7],[8],[9],[10] TLDs possess advantageous characteristics that allow for easy use and reproducibility in both clinical and research settings, such as their small size, reusability, and few correction factors to be applied among others.[11],[12] The lithium fluoride doped with magnesium and titanium LiF: Mg, Ti (TLD-100) is the most widely used TLD in routine personal dosimetry, environmental monitoring, space dosimetry, and clinical dosimetry.[13] Sensitivity of TL materials has been studied; for instance, CaSO4: Dy have been shown to be 80% more sensitive than TLD-100 but studies have shown that CaSO4: Dy is not tissue equivalent, unlike Lithium Borate doped with Magnesium (Li2B4O7: Mn) which is regarded as a material that is tissue equivalent but known to be light sensitive, which makes it has background signal. Similarly, LiF: Mg, Cu, P have been shown to be 60% more sensitive than TLD-100.[14],[15] TLDs are available for use in different and various forms (e.g., powder, chips, rods, and ribbons). Before they are used for clinical or research purposes, the nature of their performance characteristics needs to be verified to rule out possible errors. The general use of TLDs requires that they are first annealed to erase the residual TL signal using an annealing oven at known temperatures after which they are placed in a TLD reader. The measurement chamber contains a photo multiplier tube (PMT) module, heating unit, exchangeable filter unit, and nitrogen gas supply unit. Once element is heated through the heating unit, trapped energy is released in the form of light, from which a PMT does the light amplification, before they are converted into electrical signal which is linearly proportional to the detected photon fluence and an electrometer for recording the PMT signal as a charge or current.[16],[17],[18],[19]

This study used a newly acquired RadPro TLD Manual Reader and a TLD Furnace Type LAB-10/400 for annealing the TL elements. The type of TL element used was MTS-N (LiF: Mg, Ti) also known as TLD-100. Essentially, this study would determine coefficient of variation (CV) from raw and re-selected TL elements. Comparison would be made with other studies. Similarly, it would determine calibration factor (CF) from re-selected elements (i.e., Golden chips) and would compare them with relevant studies. The choice of the dose range (0.2–2 mGy) was to investigate the behavior of the TL elements for personnel in diagnostic radiology.


  Materials and Methods Top


A total of 80 MTS-N (LiF: Mg, Ti) chips were selected with similar property. The chips were arranged on an annealing tray having a cover screwed at diagonal ends to avoid direct heat contact [Figure 1]. They were placed in a TLD Furnace Type LAB-01/400 at a temperature of 400°C for 1 h and allowed to cool to room temperature. The image and specification of the TLD furnace are described [Figure 2] and [Table 1]. In other to remove lower peaks, they were subjected to another temperature of 100°C for 2 h. The chips were later inserted using a Vacuum tweezer into the bar-coded slide, respectively. This was meant to avoid the use of bare hands in handling the TL elements. The first two bar-coded round holes were used to determine the CF for the deep dose (Hp10) close to the numbering on it and last two were for the shallow dose (Hp0.07). The bar-coded slides were inserted into a holder, which was covered with a dosimeter cover [Figure 3]a-c]. Irradiation was done using a Cesium-137 source in a Secondary Standard Dosimetry Laboratory (SSDL) to known doses. Each bar-coded slide was exposed to 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 mGy, respectively. A RadPro Cube 400 manual TLD Reader (Freiberg Instruments GmbH, Germany) was used to determine corresponding photon count or signal for the irradiated chips as described [Figure 4] and [Table 2]. Glow curves were generated for individual elements as described [Figure 5].
Figure 1: Storage dish for 120 thermoluminescent dosimeter elements

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Figure 2: Annealing Oven (TLD Furnace Type LAB-01/400)

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Table 1: Technical specification for the thermoluminescent dosimeter Cube 400 manual reader

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Figure 3: (a) Bar-coded slides with the first two hosing deep dose Hp (10) and shallow dose Hp (0.07). (b) Bar-coded slide in a Holder with Al-filters 1 mm thick at both ends (c) a complete thermoluminescent dosimeter badge

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Figure 4: RadPro Cube 400 Manual thermoluminescent dosimeter Reader

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Table 2: Technical data for the annealing oven

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Figure 5: Workstation of the Cube 400 thermoluminescent dosimeter manual reader, showing a glow curve of a particular thermoluminescent element

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A total of 40 TL elements were later analyzed after the removal of TL elements with high deviation for (Hp10) and (Hp 0.07), respectively. These measurements were taken as raw values from which further re-selection was made. The equation for the percentage CV between two similar chips for (Hp10) and (Hp0.07) at a known dose is given by:



In the same vein, the CF was determined by plotting a linear graph of dose (mGy) against TL signal (C) from which a line equation of y = mx + c was obtained. Where m is the slope of the line graph and is regarded as the “CF.” Only CVs below 10% were selected for this purpose to compute the CF.

Similarly, an evaluation was carried out to determine the percentage deviation of the calculated doses in relation to the actual dose values that was delivered (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2 mGy). The equation for the % deviation was given as:



Where,

Dcalculated= is the dose obtained using line equation of y = mx + c

Dactual= is the dose to which the chips were exposed to


  Results Top


The CV for the raw values after the selection of 40 TL elements for the deep dose (Hp [10]) for batches irradiated with 0.2–2 mGy, which was denoted as TLD1–TLD10, have maximum and minimum values of 0.15% and 14.6% respectively. The results showed that only 50% of the TL chips had CV < ±10%, while the remainder had CV > ±10%. The maximum CV was noticed in the 5th element with 14.6% [Figure 6]. Similarly, a One Sample Statistics show that there was statistically significant difference in CV among the chips (P = 0.004). Further selection of TL element reduced the CV value to a range of 0.15%–3.73%.
Figure 6: Coefficient of variation for a single readout between two chips exposed to the same dose for Hp 10 (this result was based on raw data from 20 thermoluminescent elements)

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Similarly, a line graph of dose against TL Signal (C) was plotted for deep dose (Hp[10]). The obtained line graph from eight selected points was: y = 3.789 × 10 − 6 x–1.760, with Coefficient of determination (R2) = 0.9866. Furthermore, re-adjustment was made to improve the CF of Hp(10) with (R2) = 0.9866 [Figure 7]a. After this was done, the adjusted (R2) for Hp(10) became 0.9998 [Figure 7]b, with a relative difference (RD) of 1.3% between both graphs. Where the slope practically is regarded as the CF for the TLD elements. The percentage deviation between the calculated and the actual dose ranged between 0% and 16.7%, this was achieved by removing TL element with CV above ± 10%. In addition, an Independent Sample t-test shows that there was no statistically significant difference between the calculated and actual dose values (P = 0.752).
Figure 7: (a) Dose (mGy) against thermoluminescent signal (C) for Hp (10) (b) Adjusted dose (mGy) against thermoluminescent signal (C) for Hp (10)

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The CV for the raw values after the selection of 40 TL elements for the shallow dose (Hp [0.07]) for batches irradiated with 0.2–2 mGy, which was denoted as TLD1–TLD10, have maximum and minimum values of 0.94% and 15.02% respectively. The result shows that only 90% of the TL chips had CV < ±10%, while the remainder 10% had CV > ±10%. The maximum CV was noticed in the 4th element with 15.02% [Figure 8]. Similarly, a One Sample Statistics show that there was statistically significant difference in CV (P = 0.001). Further selection of TL element reduced the CV value to a range of 0.94%–3.21%.
Figure 8: Coefficient of variation for a single readout between two chips exposed to the same dose Hp 0.07 (this result was based on raw data from 20 thermoluminescent elements)

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A line graph of dose (0.2–2 mGy) against TL Signal (C) was plotted for deep dose [Hp(0.07)]. The obtained line graph was: y = 4.804 × 10 − 6 x–2.622, with Coefficient of determination (R2) = 0.9817. Furthermore, re-adjustment was made to improve the CF of Hp(0.07) with (R2) = 0.9817 [Figure 9]a. After this was done, the adjusted (R2) for Hp(0.07) became 0.9981 [Figure 9]b, with a relative difference (RD) of 1.7% between both graphs. Where the slope practically is regarded as the CF for the TLD elements. Percentage deviation between the calculated and the actual dose ranged between 0% and 14.3%, this was achieved by removing TL element/chips with CV above ± 10%. In addition, an Independent Sample t-test show that there was no statistically significant difference between the calculated and actual dose values (P = 0.862).
Figure 9: (a) Dose (mGy) against thermoluminescent signal (C) for Hp (0.07) (b) Adjusted dose (mGy) against thermoluminescent signal (C) for Hp (0.07)

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  Discussion Top


The CV or variability index between two identical chips within the same bar-coded slide was seen to be below 15.1% both for Hp (10) and Hp (0.07), respectively, based on our raw data. Only 50% of Hp (10) TL were within ± 10% recommended standard. Similarly, Hp (0.07) had a TL response of 90%, indicating that the TL elements were more sensitive. The above raw data were in line with Bhuiyan et al., whose response to raw data shows a wide-ranging sensitivity of the chips varying up to 20% using90 Sr and137 Cs as sources.[20] What we did in this study was to remove TL elements above ± 10%, so we got our maximum CV for Hp (10) and Hp (0.07) as 3.75% and 3.21%, respectively.

A study by Sadeghi et al., who investigated the reproducibility of LiF: Mg, Ti (TLD-100) using (81, 162, and 40.5 mGy) with 662 keV photons of Cs-137, showed that the CV for 40 TL elements were below 10%. The result was in line with our study which had a maximum CV of 3.75%.[21]

In a related study by Abd El-Hafez et al. who quantified various factors influencing the repeatability and reproducibility of TLD-600 Detector; his finding show that at lower doses, CV becomes very large while at higher doses, it appears to be more constant.[22] This may have been why our CV was above ± 10% since we used doses in the range of 0.2–2 mGy which is regarded as low dose. A study by Bauk et al., who investigated precision of low-dose response of LiF: Mg, Ti dosimeters exposed to 80 kVp X-Rays. The study observed that at 0.3 mGy standard deviation of the TL response was <10%. Similarly, the standard deviation was very high (above 40%) at doses below 0.3 mGy. This finding indicates that LiF: Mg, Ti TL measurements below 0.3 mGy were no longer precise and reproducible.[23]

TL response to dose in a study by Bhuiyan et al. showed that after selection of 10 golden chips from 90 chips which were to be used for dose conversion factor with close sensitivity, a variation within ± 3.5% was obtained, corresponding to standard deviation below 2.5%.[20] These values were comparable to our study where the variation for Hp10 and Hp0.07 was 3.73% and 3.21%, respectively. The corresponding standard deviations were 1.59% and 1.04%, respectively.

A study by Akpochafor et al. shows that out of 20 TLDs that were irradiated to known absorbed doses in the 180 kVp and 300 kVp therapy beams. Only 40% of the TLDs were within ± 10% of the delivered absorbed dose in the 180 kVp beam and only 25% in the 300 kVp beam.[24] The above findings show that the behavior of TL elements vary and may go above ± 10% recommended standard. An investigation to the effect of TLD-700 energy response to low energy X-ray encountered in diagnostic radiology using ceasium-137 source by Herrati et al. show that maximum deviation could reach 60%. This was similar to what we encountered using the 80 TL element at the initial stage of selection, before they were cut down to forty TL element. The maximum deviation that was calculated went as high as 50%.[25]

The coefficient of determination (R2) for this study for Hp10 and Hp0.07 was 0.9998 and 0.9981, respectively, after slightly non-linear point was removed. Although changes were not well pronounced between the first and second R2 for Hp (10) (0.9866 and 0.9998) and Hp (0.07) (0.9817 and 0.9981). The standard deviation between first and second R2 was 0.93%, while that of Hp (0.07) was 1.2%, respectively. From the above results, the CF for the deep equivalent dose was more stable and accurate than the surface/skin dose. Hence, it is necessary to consider both parameters for personnel monitoring. There was good relationship between the TL signal and dose generally. Similarly, R2 in a study by Rahman et al., was 0.98., which had good agreement with what we obtained.[3] Similarly, a study by Liuzzi et al., show that for 6MV, 5 meV, 7 meV, and 9 meV the coefficient of determination was 0.9995, 0.97, 0.9937, and 0.9962, respectively.[26] This was in line with this study.

The Coefficient of determination (R2) in a study by Yusof et al. with low energy X-ray using TLD-100, OSLD, and Ionization Chamber was 0.9999, 0.9936, and 0.995. The above result was comparable to our study which was 0.9998 and 0.9981, respectively. Deviation between this study R2 and Yusof's was 1.2%.[27]

Statistical analysis

The data analysis was performed using SPSS for Windows, Version 20.0 (SSPSS Inc., Chicago, IL, USA). Descriptive statistics, the one-sample t-test, and the independent sample t-test were used at a 95% level of significance. P < 0.05 was considered statistically significant.


  Conclusions Top


A study to determine CF and variation in TL elements has been carried out. This study findings show that at 0.2–2 mGy there was significant variation among the chips during the first sorting, which was similar to what most authors encountered. However, in other to achieve a coefficient of determination close to unity for lower dose, TL element with closer count or TL signal must be selected; this will help eliminate/reduce CF error.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Lawless MJ, Junell S, Hammer C, DeWerd LA. Response of TLD-100 in mixed fields of photons and electrons. Med Phys 2013;40:012103.  Back to cited text no. 12
    
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Wesolowska PE, Cole A, Santos T, Bokulic T, Kazantsev K, Izewska J. Characterization of three solid state dosimetry systems for use in high energy photon dosimetry audits in radiotherapy. Radiat Meas 2017;106:556-62.  Back to cited text no. 13
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
 
 
    Tables

  [Table 1], [Table 2]



 

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