|Year : 2019 | Volume
| Issue : 8 | Page : 123-126
Comparison of thermoluminescent dosimeter calibration irradiated in gamma knife and 60Co instruments
Mohadese Moafi, Ghazale Geraily, Ali Reza Shirazi
Department of Medical Physics and Medical Engineering, Tehran University of Medical Sciences, Tehran, Iran
|Date of Web Publication||22-Mar-2019|
Dr. Ghazale Geraily
Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran
Source of Support: None, Conflict of Interest: None
Aim: By necessity of dosimeters calibration for evaluating delivered dose accuracy to organs out of the radiation field in patients undergoing gamma knife radiosurgery, we calibrated thermoluminescence dosimeters in gamma knife and 60Co instruments, and then, compared both results to investigate when one of these devices was out of reach, can we use one of this instruments instead of the ther.
Materials and Methods: To individual calibration by 60Co, thermoluminescent dosimeters (TLDs) were placed in a Perspex sheet with conditions of source-skin distance = 80 cm, field size = 10 cm × 10 cm, and dose = 100 cGy. For individual calibration by Gamma knife, TLDs placed in flat Perspex were located in a special sphere and were exposed with conditions of source to axis distance = 40 cm, field size = 18 mm, and dose = 100 cGy, and for group calibration, TLDs were divided into six groups and were exposed with doses of 0–1000 cGy in both devices.
Results: According to Fisher's exact test, calculated P = 0.27, so the difference is not significant.
Conclusions: The result showed despite differences in calibration conditions, 60Co unit can be used to calibrate TLD dosimeter for estimating the accuracy of measurement of delivered dose to organs of patients undergoing Gamma Knife 4C radiosurgery treatment.
Keywords: 60Co, calibration, gamma knife 4c, thermoluminescent dosimeter
|How to cite this article:|
Moafi M, Geraily G, Shirazi AR. Comparison of thermoluminescent dosimeter calibration irradiated in gamma knife and 60Co instruments. J Can Res Ther 2019;15, Suppl S1:123-6
|How to cite this URL:|
Moafi M, Geraily G, Shirazi AR. Comparison of thermoluminescent dosimeter calibration irradiated in gamma knife and 60Co instruments. J Can Res Ther [serial online] 2019 [cited 2020 Nov 28];15:123-6. Available from: https://www.cancerjournal.net/text.asp?2019/15/8/123/243474
| > Introduction|| |
Calibration is the procedure of finding a relationship between two quantities. Definition of Calibration is determined by the International Bureau of Weights and Measures which is the function that under identified conditions. That is connection between the quantities with uncertainties in measurement which is supplied by measurement standards.
The goal of calibration is an effective establishment to control over the accuracy of the metrological criterions of the measurement devices and all the equipment functions influencing the quality of this process to ensure the conformity of the measurement undertaken with international standards used.
Measuring devices should be calibrated periodically. The passage of time, burnout, unpredictable events, cause their capabilities to track results to standards under question which results in confirmation again. Hence, the main purposes of calibration are ensuring the device takes readings, determining the accuracy of readings from the device and establishing a traceability system to reference standards.,
The calibration process is used to perform dosimetric measurements with dosimeters such as thermoluminescent dosimeter (TLD) and film that their responses do not give use absorbed dose directly. Therefore, dosimeters need calibration. The most important target of radiotherapy is delivering prescribed dose to the tumor lesion and decreasing the minimum dose to surrounding normal tissues. Hence, we used thermoluminescence dosimeter to measure delivered dose to normal tissues while the tumor is being treated by radiotherapy.,,
Gamma ray is one of the most usable radiations in radiotherapy. Instruments that produce gamma ray by 60Co source divide into two groups, 60Co unit and gamma knife machine. First one is used for conventional radiotherapy and the second one is used for radiosurgery goals as stereotactic radiosurgery. 60Co is a radioactive isotope of cobalt with a half-life of 5.2714 years. It decays by beta particle to the stable isotope 60 Ni. The activated nickel nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV which result in average energy of 1.25 MeV.,, Stereotactic radiosurgery was presented by Leksell in 1951, and Gamma Knife system was manufactured by Elekta Company which developed for treating brain tumors such as blood vessel defects, Parkinson's disease, epilepsy, and trigeminal neuralgia.,, The technique is originated in using multiple radiation beams, focusing and localizing on a small volume. In this way, a high dose will be delivered to the target, while the dose in the surrounding tissue is low. The gamma ray is emitted from 201 sources of 60Co, distributed along parallel circles. The photons, emitted from each source, are collimated into beams by four different helmets with collimator sizes of 4, 8, 14, 18 mm and focused to a common volume, with a source to focus distance of 400 mm.
60Co machine which emits gamma ray is already used for calibration of TLDs, and we want to figure out if calibration of TLDs is performed by gamma knife system, it (response) is different from its calibration by 60Co machine or not.
| > Materials and Methods|| |
In this study, irradiation in conventional radiotherapy was performed by Theratron 780C with 60Co source. An ionization chamber-PTW was used to estimate dose rate by AAPM's TG-51 protocol. It is common that we called this system as 60Co machine. Irradiation in stereotactic radiosurgery was performed by Leksell Gamma knife 4C.
In this study, we used cubic chips (GR100M) which manufactured by FIMEL that have the following characteristics of lithium fluoride (LiF), crystals doped with magnesium (Mg), and titanium (Ti), sizes of 3.2 mm × 3.2 mm × 0.9 mm, approximate weight of 24 mgr and linear energy range of 50 μGy to 500 Gy.
The chips were first annealed at 285°C for 30 min, followed by fast cooling and for typical annealing heating at 400°C for 1 h. After irradiation and before reading, the TLDs were stored for 24 h at room temperature (20°C) to clear the low energy traps.,, To calibrate TLDs for 60Co machine to individual calibration, TLDs were placed in a Perspex slab with conditions of source-skin distance = 80 cm, field size = 10 cm × 10 cm, build up layer = 5 mm, and exposed 100 cGy uniformly in the isocenter of the cobalt's source [Figure 1].,,, To calibrate TLDs for gamma knife during individual calibration, TLDs placed in flat Perspex were located in a special sphere of the Gamma knife and were exposed uniformly in its isocenter with conditions of source to axis distance (SAD) = 40 cm and collimator field size = 18 mm [Figure 2].
|Figure 1: (a) Theratron 780 C or 60Co machine, (b) perspex sheets in the field of radiation|
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|Figure 2: (a) Leksell Gamma Knife system with collimator 18 mm, (b) special cassette in the middle of spherical phantom for calibration of thermoluminescent dosimeters|
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Analysis of results
Efficient correction coefficients (ECCs) were obtained by the equation to determine the sensitivity of each irradiated individual TLD.
Where ECCj is the ECC of each TLD, TLDj, and <TLD> are individual reading and average reading of the total TLDs, respectively.,,
TLDs were read by TLD reader (Fimel LTM) with considering the particular number of each TLD. In the next step, we measured standard deviation and average afterward of TLDs with readings higher or lower than the subtraction of mean and standard deviation was omitted.
To convert the read number to delivered dose, group calibration should be done. In the interest of group calibration of remaining TLDs, they were divided into six groups and exposed respectively with doses of 0, 200, 400, 600, 800, and 1000 cGy in both devices. Time of every group radiation has been calculated according to the output of every machine.,,,,,,,,,,,,,
| > Results|| |
Initially using ECCs, two calibration curves for 60Co unit and gamma knife machine were obtained by the best-fitted equation with Matlab and SPSS software which y axis is determined absorbed dose and x axis is TLDs reading or response of TLDs. For drawing calibration curve according to formula, value of x axis will be obtained.
Where Rj is TLD reading and B is background TLD reading.
According to [Figure 3], regression coefficients of both curves are more than 0.99 with 5% error bar. Both curves equation is linear. X coefficient of 60Co and Gamma knife curves are respectively 15,636 and 14,403 which are indicated in formula.
|Figure 3: (a) Calibration curve with line equation for 60Co unit, (b) calibration curve with line equation for Gamma knife 4C|
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| > Discussion|| |
Eventually, we analyzed our data with Excel and Matlab. The curve fitting for both systems has been plotted. For two systems response of thermoluminescence, dosimeters are growing by increasing dose. According to the results despite linearity equations, line slope of each equation has become different therefore curves and equations slop of every device is different from each other, but serial numbers of remaining TLDs are the same. Corresponding to the null hypothesis test that the correlation between X and Y in one population is the same as the correlation between X and Y in another population we can use the procedure developed by Fisher in 1921. P value was obtained 0.27 by this test which was more than 0.05 and the variables relationship or difference is not significant and meaningful. Therefore, two calibration curves do not show any considerable distinctness between Theratron 780C as a conventional radiotherapy machine and Leksell Gamma Knife as a stereotactic radiosurgery system.
Although many studies have been done about calibration, nobody has compared the thermoluminescence dosimeters calibration of two devices, because sometimes, many researchers have used other systems and other devices to calibrate dosimeters instead of calibration of dosimeters by the machine which used for treatment. Our performance is related to absolute dosimetry thus it is better to consider the type of study and research which is evaluated. Relative dosimetry is practical for assessment of the radiation dose, but absolute dosimetry is for accurate measurement; so we cannot consider measurements by estimates and possibilities; therefore, accurate measurements should be considered in per instruments and every system should be calibrated individually. Despite this fact and differences of two machines and also different calibration conditions, as already mentioned P value showed no significant relationship or difference between two calibration curves for both systems which discussed above, hence, we can calibrate the alternative machine instead of other with the same source which emits identical radiation. Hence, machines with 60Co source can be calibrated instead of each other in both absolute and relative dosimetry.
Comparing our data with Banaee and Nedaie, Camargo and Ribeiro, and also Farid Ahmed showed that TLD calibration curve is linear too. The thermoluminescent which Camargo and Ribeiro have used was GR-100 and also Farid Ahmed have used LiF-100, but Banaee and Nedaie have used GR-207A with different radiation energies, but in our study, thermoluminescent which is used was GR-100 and energy was the same. In Banaee's study, Calibration source was 60Co, and it was the same as our calibration source but in Camargo's study, TLDs was calibrated by 137 Cs source and second device which estimates absorbed dose, was linear accelerator. In Farid Ahmed's study, calibration source was 60Co and for estimating dose distribution in distances up to 40 cm from the field edge of the field axis, different teletherapy units had been used such as 60Co gamma ray, 120 kVp, and 250 kVp X-ray.,,,,,
In Camargo and also da Costa studies, the sources for calibration of TLD dosimeters and treatment systems were different. Although the calibration curves are linear and regression of their obtained curves is up to 0.99 which is the same as our study. Dosimeters which Camargo and da Costa have used were GR-100 equal to our study.
In Hassanzadeh's study, calibration of TLD dosimeters has been done by 60Co with TLD-100 dosimeter, but assessment of absorbed dose to organs out of radiation field have been done by Gamma knife and for evaluating delivered dose to every site has been estimated by 60Co machine calibration curve, but in our study, we have calibrated dosimeters with 60Co unit and Gamma knife system.
In Najafi's study, EBT3 film was the dosimeter which was used, but in our study, TLD-100 is the dosimeter which is used. His study showed that calibration curves of EBT3 film dosimeter have had the same response for 60Co and Gamma knife 4C machines and calibration curves have not had any meaningful difference which is the same as our result.
| > Conclusions|| |
The results show that using the best fitting curve from measured data the calibration curve for TLDs of the two systems are the same despite the different SAD, number of sources, calibration depth, size, and shape of the radiation field and dose rate between 60Co machine and Gamma knife unit. Hence, 60Co machine can be used to calibrate TLD dosimeter for measuring and estimating delivered radiation dose to tissues and organs of patients undergoing Gamma knife 4C radiosurgery treatment.
Financial support and sponsorship
This study has been supported by grant No.29995 of Tehran University of Medical Sciences.
Conflicts of interest
There are no conflicts of interest.
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