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ORIGINAL ARTICLE
Year : 2018  |  Volume : 14  |  Issue : 6  |  Page : 1341-1349

Evaluation of optically stimulated luminescence dosimeter for exit dose in vivo dosimetry in radiation therapy


1 Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Bengaluru, Karnataka; Department of Radiotherapy, Christian Medical College, Vellore, Tamil Nadu, India
2 Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Bengaluru, Karnataka, India

Date of Web Publication28-Nov-2018

Correspondence Address:
Ravikumar Manickam
Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Dr. M.H. Marigowda Road, Bengaluru - 560 029, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.191066

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 > Abstract 


Aim: The aim of this study was to assess and analyze the exit dose in radiotherapy using optically stimulated luminescence dosimeter (OSLD) with therapeutic photon beams.
Materials and Methods: Measurements were carried out with OSLD to estimate the exit dose in phantom for different field sizes, various phantom thicknesses, and with added backscatter material. The data obtained were validated with ionization chamber data where applicable. A correction factor was found to determine the actual dose delivered at the exit surface using measured and theoretical dose.
Results: The exit dose factor with Co-60, 6 MV, and 18 MV beams for 10 cm phantom thickness was found to be 0.752 ± 0.38%, 0.808 ± 0.34%, and 0.882 ± 0.42%. The dose enhancement factor with field size was ranging from 3% to 7.7% for Co-60 beam, from 2.6% to 6.6% for 6 MV, and from 2.5% to 4.7% for 18 MV beams at 10 cm depth of the phantom with 20 cm backscatter. The percentage reduction in exit dose with no backscatter material at 25 cm depth with field size of 10 cm × 10 cm was 5.6%, 4.4%, and 4.0%, less than the dose with full backscatter thickness of 20 cm for Co-60 beam, 6 MV, and 18 MV beam.
Conclusions: The promising results confirm that accurate in vivo exit dose measurements are possible with this potential dosimeter. This technique could be implemented as a part of quality assurance to achieve quality treatment in radiotherapy.

Keywords: Backscatter, exit dose, ion chamber, optically stimulated luminescence dosimeter, radiotherapy


How to cite this article:
Ponmalar R, Manickam R, Saminathan S, Ganesh K M, Raman A, Godson HF. Evaluation of optically stimulated luminescence dosimeter for exit dose in vivo dosimetry in radiation therapy. J Can Res Ther 2018;14:1341-9

How to cite this URL:
Ponmalar R, Manickam R, Saminathan S, Ganesh K M, Raman A, Godson HF. Evaluation of optically stimulated luminescence dosimeter for exit dose in vivo dosimetry in radiation therapy. J Can Res Ther [serial online] 2018 [cited 2020 Jul 2];14:1341-9. Available from: http://www.cancerjournal.net/text.asp?2018/14/6/1341/191066




 > Introduction Top


The prime aim of radiotherapy in cancer treatment is to provide a lethal dose to tumor volume while keeping minimum dose to surrounding tissues and nearby sensitive parts of the body. The prognosis of tumor depends on how accurate the dose is being delivered to target and normal tissue tolerance. One of the approaches to ensure the actual dose delivery to the patient is the quality assurance at each step of the treatment chain,[1],[2] including in vivo measurements of entrance or exit dose. The practical information at the exit surface is extremely important to provide sufficient data on dose to exit skin surface if the contribution of exit dose is clinically significant to cover the tumor volume or to save critical structures. The survey of practice on postmastectomy radiotherapy indicated that 80% of the respondents suggested considering the skin as a part of tumor volume in such clinical cases is essential.[3] The exit dose depends on the energy of the incident beam, treatment technique, tumor dose, and the presence or absence of backscatter material.[4] A technique that can be translated to real patients' situation to achieve sufficient dose delivery to the skin or scar at the exit surface by restoring the build-down effect and an appropriate dosimeter to quantify the exit dose in clinical situation are essential; hence, the problem addressing this issue has become the scope of this study. Optically stimulated luminescence dosimeters (OSLDs) have been introduced recently into medical dosimetry and are quickly gaining popularity for use in clinical dose measurements in radiotherapy.[5] To date, no studies have been reported that showing the results of exit dose measurement using OSLDs. Hence, an attempt has been made to investigate the feasibility and usefulness of OSLD in in vivo dosimetry as a prelude to clinical measurements on patients.


 > Materials and Methods Top


Optically stimulated luminescence dosimetry system

The nanoDot OSLDs were procured from Landauer, Inc., (Glenwood, USA) that consist of 5 mm diameter and 0.2 mm thick discs of Al2O3:C samples encased in a 1 cm × 1 cm × 0.2 cm light-tight plastic case [Figure 1]a to prevent signal depletion due to light exposure. The bar code information enables to record the data with ease and to track the history of each OSLD. The disk can slide out of the plastic case during readout and optical bleaching process. The nanoDots are read using a MicroStar reader (Landauer, Inc., Glenwood, USA) with a reader warm up time of 10 min, and the reader has been shown in [Figure 1]b. OSLDs were stimulated during readout with a light of wavelength 540 nm, and the wavelength of the emitted luminescence was 420 nm. The special dosimetry software installed in the computer in conjunction with reader allows, the results to be displayed and can be exported to an Excel spreadsheet for the data analysis.
Figure 1: (a) NanoDot optically stimulated luminescence dosimeter with an adaptor (b) MicroStar reader

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An Optical Annealer was used to bleach the OSL signal by resetting the dosimeter for reuse by exposing it to light. Bleaching time of 6 h was found to be sufficient enough to remove the residual signal almost equal to the background signal. All OSLDs were read before irradiation, and the resultant OSL signal was the difference between post- and pre-irradiation signals.

Parallel plate ionization chamber

Ionometric measurements were made using a calibrated Nordic Association of Clinical Physicists-02 parallel plate ionization (Scanditronix Wellhofer AB, Sweden) chamber along with dose 1 (Scanditronix Wellhofer AB, Sweden) electrometer and shown in [Figure 2]. The chamber was operated at the polarizing voltage of 200 V and has a sensitive volume of 0.16 cc. The chamber was fitted into a polystyrene phantom with the measuring entrance window facing farther from the source. Where applicable, the data acquired with OSLD have been validated with ionization chamber.
Figure 2: Parallel plate ionization chamber with dose 1 electrometer

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Irradiation

Irradiations were done with Varian DHX linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) capable of delivering 6 and 18 MV photon beams and with Co-60 gamma rays from Theratron-780E telecobalt unit (Theratronics, Canada). Dosimeters were irradiated with a dose of 200 cGy, and the source to detector distance was maintained at 80 cm in Co-60 beam and 100 cm in linear accelerator with the field size of 10 cm × 10 cm unless otherwise mentioned. Measurements were carried out in solid water phantoms of size 30 cm × 30 cm made of white polystyrene (ρ =1.04 g/cm3). A perspex slab was machined to have small slots into which the OSL detectors would fit tightly with a minimized air gap.

Exit dose calibration

The calibration of exit dose using OSLD was carried out in Theratron (Co-60 beam) and in Varian DHX linear accelerator for 6 MV and 18 MV photon beams at the measurement depth of 10 cm in a phantom of size 30 cm × 30 cm with 20 cm height to account for the changes in actual radiation conditions compared to calibration situation. The dosimeters were irradiated with different doses ranging from 50 to 300 cGy. The calibration curve has been plotted with the response of dosimeter versus delivered dose. Using calibration curve, the measured dose in cGy at the exit surface was calculated and compared with the theoretical dose derived from tissue-maximum ratio (TMR) data. A correction factor has to be known to obtain the actual dose at the exit surface as the dose estimation by theoretical method includes full backscatter; whereas in actual situation, this is not so. Hence, a correction factor was determined by taking the ratio of measured exit dose without backscatter medium to that of derived exit dose with backscatter. Hence, the actual dose delivered at the exit surface could be calculated by applying this correction to the theoretical exit dose.

Exit dose with phantom thickness variation

The exit dose measurements were carried out by varying the thickness of the phantom from entry to exit surface without any backscatter medium. Phantom thicknesses were varied from 4 to 28 cm insteps of 2 cm, and the dosimeters were kept at the exit surface of the phantom farther from source. [Figure 3] shows the schematic representation of the experimental setup for exit dose measurement with phantom thickness using OSLDs. Exit dose measurements were done with beams in gantry 180° position (under couch) to reduce the positional inaccuracy of phantom and dosimeter. Nevertheless, in few clinical situations, the exit dose has to be measured with gantry positioned at 0°. Therefore, measurements were carried out with beams both at anterior and posterior positions, and the results were compared to find the contribution of couch during exit dose measurements.
Figure 3: Experimental setup for exit dose measurement with phantom thickness

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Exit dose with field size variation

Measurements with varying field sizes were performed at exit depths with three different phantom thicknesses (10, 20, and 25 cm) without any backscatter material. The dosimeters were placed at the exit surface of the phantom, and the field size was varied from 5 cm × 5 cm to 30 cm × 30 cm. In addition to the measurements without backscatter material, measurements were also performed at the exit side of the beam with full backscattering thickness of 20 cm where the contribution due to scatter attains saturation. The dose enhancement factor (DEF) was determined as the ratio of the response of dosimeter with full backscattering condition to that of the response without any backscattering medium.

Exit dose with added backscatter

Exit dose measurements with or without added backscatter medium were carried at the exit depths of 10, 20, and 25 cm. The added backscatter thickness beyond the measurement depth was varied from 1 mm upto 20 cm, which is sufficient for scatter saturation. The relative exit dose (RED) with varying phantom thicknesses (10, 20, and 25 cm) was calculated by measuring the ratio of the response of detector with backscatter material to the response without backscatter material. The setup with added backscatter phantom beyond the dosimeter during measurement is shown in [Figure 4].
Figure 4: Experimental setup with added backscatter material beyond the dosimeter

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 > Results Top


Exit dose calibration

A calibration curve has been plotted for exit dose measurement with the response of dosimeter versus delivered dose [Figure 5]a and [Figure 5]b by taking the response of dosimeter along the Y axis and the dose delivered in cGy along the X axis. Using linear fit equation from the curve, the measured dose from dosimeters was obtained. The R2 value obtained from the linear fit equations was 0.999, 0.9974, and 0.996 for OSLD with Co-60, 6 MV, and 18 MV beams, respectively; whereas for ion chamber, R2 = 1 for all three-photon beams, indicating the trend line perfectly fits the data. The measured dose in cGy at the exit side with 6 MV, 18 MV, and Co-60 beam for ion chamber, and OSLD was obtained by solving the following equations of the form Y = mX + C.
Figure 5: Calibration curve (a) ion chamber (b) optically stimulated luminescence

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6 MV,

RIon = 0.0057 DExit + 0.002 (1)

ROSL = 680.7 DExit − 8122 (2)

18 MV,

RIon = 0.0058 DExit + 0.0005 (3)

ROSL = 672.77 DExit − 8376.3 (4)

Co-60,

RIon = 0.0065 DExit + 0.0163 (5)

ROSL = 738.19 DExit − 10477 (6)

where RIon, and ROSL are the responses of ion chamber and OSLD. DExit is the exit dose measured in cGy from ion chamber and OSLD.

A correction factor from the measured dose and the theoretical dose derived using TMR data was determined using the following expression:



[Table 1] provides the correction factor for Co-60, 6 MV, and 18 MV photon beams with various phantom thicknesses. The actual dose delivered at the exit surface in real situation with patients by giving correction to the basic data obtained from theoretical method for the lack of back scatter condition was calculated using the following expression:
Table 1: Correction factor to find the actual exit dose

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Exit dose with phantom thickness variation

The results of exit dose measurements with OSLDs by varying phantom thicknesses of 4–28 cm from entry to exit surface without any backscatter material are presented in [Figure 6]. Exit dose factor with phantom thickness variation was determined by normalizing the response of the detectors to 4 cm thick phantom and it was observed that as the phantom thickness increases, the exit dose decreases. The exit dose factor for 10 cm phantom thickness was 0.752 ± 0.38%, 0.808 ± 0.34%, and 0.882 ± 0.42% with Co-60, 6 MV, and 18 MV photon beams, respectively. For 20 cm phantom thickness, the factor was 0.416 ± 0.62, 0.516 ± 0.57, and 0.658 ± 0.54 with Co-60, 6 MV, and 18 MV photon beams; whereas with 28 cm phantom thickness, the factor was 0.246 ± 0.81 in Co-60 beam, 0.345 ± 0.94 in 6 MV beam, and 0.539 ± 0.98 in 18 MV beam.
Figure 6: Response of the dosimeters as a function of phantom thickness

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The measurements done in linac with beams vertically up position was compared with beams in a vertically down position to assess the contribution of couch which is essential in some clinical situations. A graph has been plotted with the ratio of under couch to vertically down (G180°/G0°) as a function of phantom thickness and shown in [Figure 7]. The variation observed between G180° and G0° measurement was <1% indicating that the difference between these two techniques is insignificant.[6] However, the presence of support bar attached underneath the couch leads to a variation of 2% when it is inside the field during irradiation; hence, it should be ensured that the bar has been moved to the couch end away from the field.
Figure 7: Comparison between under couch (G180°) and vertical beam (G0°) technique

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Exit dose with field size variation

The results obtained with field size variation of three different phantom thicknesses (10, 20, and 25 cm) on exit dose are depicted in [Figure 8] for (a) Co-60, (b) 6 MV, and (c) 18 MV photon beams. Exit dose factor with field size was determined by normalizing the response of the detectors to 10 cm × 10 cm values. The exit dose factor for the smallest (5 cm × 5 cm) and the largest field (30 cm × 30 cm) in Co-60 beam was 0.882 ± 0.11% and 1.143 ± 0.1% with 10 cm phantom thickness, 0.830 ± 0.05% and 1.295 ± 0.09% with 20 cm phantom thickness, and 0.818 ± 0.07% and 1.365 ± 0.21% with 25 cm phantom thickness. In addition, the exit dose factor in 6 MV for the smallest (5 cm × 5 cm) and the largest field (30 cm × 30 cm) was 0.90 ± 0.15% and 1.117 ± 0.3% with 10 cm phantom thickness, 0.860 ± 0.29% and 1.225 ± 0.30% with 20 cm phantom thickness, and 0.86 ± 0.2% and 1.266 ± 0.56% with 25 cm phantom thickness. Similarly, the exit dose factor in 18 MV for the smallest (5 cm × 5 cm) and the largest field (30 cm × 30 cm) was 0.925 ± 0.01% and 1.072 ± 0.01% with 10 cm phantom thickness, 0.901 ± 0.01% and 1.118 ± 0.11% with 20 cm phantom thickness, and 0.895 ± 0.01% and 1.143 ± 0.04% with 25 cm phantom thickness.
Figure 8: Response of the dosimeters as a function of field size for (a) Co-60 (b) 6 MV (c) 18 MV photon beams

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DEF was plotted as a function of field sizes ranging from 5 cm × 5 cm to 30 cm × 30 cm with different phantom thicknesses of 10, 20, and 25 cm and shown in

[Figure 9] for (a) Co-60, (b) 6 MV, (c) 18 MV photon beams. It was observed that the DEF values at 10 cm depth of the phantom with 20 cm backscatter vary from 3% to 7.7% with field size ranging from 5 cm × 5 cm to 30 cm × 30 cm for Co-60 gamma rays. For 6 MV photon beams, the DEF values vary from 2.6% to 6.6%; whereas at 10 cm depth of the phantom with 20 cm backscatter, DEF varies from 2.5% to 4.7% with 18 MV beams. The percentage increase in the exit dose with the backscatter saturation thickness of 20 cm for the field sizes of 10 cm × 10 cm and 20 cm × 20 cm with Co-60, 6 MV, and 18 MV photon beams are summarized in [Table 2].
Figure 9: Dose enhancement factor as a function of field size for (a) Co-60 (b) 6 MV (c) 18 MV photon beams

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Table 2: Percentage increase in exit dose with full scatter

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Exit dose with added backscatter

The RED for 10 cm × 10 cm field was found from the responses of the dosimeters with or without added backscatter material beyond the measurement depths of 10, 20, and 25 cm for Co-60, 6 MV, and 18 MV photon beams. Graphs have been plotted with RED versus added backscatter thickness and are shown in [Figure 10] for (a) Co-60, (b) 6 MV, and (c) 18 MV photon beams. The results show that the measured dose at the exit surface of the phantom with no backscatter material is 4.7%, 5.4%, and 5.6% less than the dose with full backscatter medium of 20 cm thickness for Co-60 beam with 10, 20, and 25 cm phantom thicknesses. The percentage increase in RED for 6 MV photon beam with phantom thicknesses of 10, 20, and 25 cm are 3.9%, 4.2%, and 4.4%, respectively. For 18 MV beam, the RED is more by 3.7%, 3.8%, and 4.0% with enough backscattering medium present beyond the measurement depths of 10, 20, and 25 cm.
Figure 10: Relative exit dose versus added backscatter thickness for (a) Co-60 (b) 6 MV (c) 18 MV photon beams with various phantom thicknesses

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 > Discussion Top


The knowledge of exit dose is crucial, and the dose contribution from multiple external beams during treatment should be assessed properly as it exits the patient. Theoretically, the estimation of exit dose done in semi-infinite phantoms using Percentage depth dose (PDD)/TMR data always includes full backscattering condition. However, in clinical situations, the measurement points in exit surface receive lower dose than the predicted dose due to insufficient backscattering material beyond exit surface. Hence, it is important to investigate the exit dose as it contributes considerably to the skin dose in head and neck or breast treatment, or in other cases where the critical structures are located close to the skin (e.g., scrotum, lens). The proposed work was to estimate the exit dose in phantom for various phantom thicknesses, different field sizes, and with added backscatter material. The appropriateness of exit dose measurements using OSLD in radiotherapy was validated with parallel plate ionization chamber as they are high in accuracy and precision. Nevertheless, this dosimeter is not used frequently for the patient measurement due to high applied voltage, size of the detector, and the cables attached to the chamber.

The variation in exit dose as a function of phantom thickness and field size is the result of variation in dose per pulse and the photon energy spectrum. It was observed that as the phantom thickness increases, the dose per pulse decreases for a particular field size.[7] This is due to the change in energy spectrum as the original photon beam and low-energy scattered photon from the phantom were getting attenuated. At all phantom thicknesses, the exit dose was found to be high for higher energy beam. This would enable to choose the appropriate energy to provide sufficient dose while including skin as a tumor volume or to deliver minimum dose to save critical structures. For instance, while using tangential fields in postmastectomy breast irradiation, it is important to cover the scar as well as skin at the exit surface by 100% isodose line.[8] Sathiyanand Ravikumar[9] have reported that the reduction in exit dose of 5.8% for Co-60, 4.4% for 6 MV and 3.7% for 18 MV photon beams, and 3–7% for phantom thickness of 1–20 cm for a Co-60 beam was reported by Goede et al.[10]

The exit dose as a function of field size without any backscatter medium showed an increase in the response of dosimeters. The increase in field size causes an increase in dose per pulse and number of low-energy photons due to phantom scatter. The DEF values do not have any significant variation with the thickness of phantom through which the radiation beam had traversed.[9] DEF values as a function of field size in full backscattering condition compared with no backscattering medium showed a slightly higher response for Co-60 than 6 MV, and 6 MV little higher than 18 MV. The reason is that the ionization component due to backscattered electrons and photons is less at higher energies as the Compton electrons and scattered photons are projected more in the forward direction as energy increases. As energy reduces, the amount of backscattering increases thus leads to higher response. If there is no medium beyond the exit surface; then, the contribution in dose due to backscattered photons and electrons are absent, resulting in less exit dose. The short ranged back scattered electrons due to the presence of backscatter medium beyond the exit surface results in increasing exit dose. The increase in exit dose saturates when the thickness of the phantom exceeds the range of backscattered photons.

The evaluation of exit dose measurement with added backscatter thickness is useful to determine the required thickness of the bolus to provide sufficient dose at the exit surface. The International Commission on Radiation Unit report 24[11] emphasizes that it is important to ensure the presence of full backscattering medium during exit dose measurements. The build-down effect at the exit portal due to the reduction in scatter contribution complicates the actual estimation of exit dose. The build-down effect due to the absence of backscatter medium was gained rapidly up to 1 cm and thereafter, a gradual increase was noticed. It was observed that the exit dose is restored to within 3% of full backscatter condition for 1 cm added backscatter slab.[12] This shows that a minimum 1 cm thickness bolus is necessary to restore the build-down effect. It was reported in other literature[8] that the use of >:1 cm bolus is advisable to restore the build-down effect completely if the exit dose is considered as a part of tumor volume. The build-down effect with no backscatter medium was attributed due to the absence of backscattered electrons and photons. The addition of appropriate bolus thickness would enable to restore the dose and the reduction in dose under this circumstance is only due to the absence of backscattered photons that would be negligibly small. This added backscatter technique ensures the validity of exit dose evaluation so that this described technique could be translated to clinical situation with the patient. The investigation on exit dose by Banjade et al.[8] has shown that 1 cm added thickness is required to restore the reduction in exit dose to 3% of full backscatter medium. Gagnon and Horton[4] and Legare[13] have reported that the lack of full backscatter material at the exit surface leads to a variation of 2–3% during tumor dose delivery and significant dose reduction in the skin surface.

The measurement of exit dose is normally more complicated than entrance dose measurement as the beam has to pass through the phantom and dosimeter before the response of dosimeter is being registered in sensitive volume.[6],[12],[14] Although there are many studies on entrance dose, exit dose has not received similar attention and only very few studies have been reported in literature.[15],[16],[17],[18] Ferguson et al.[19] suggested that dose measurements in exit surface can be made during every fraction of the external beam therapy due to no perturbation in tumor dose. In vivo dosimetry is an essential tool to assess dose delivery at the exit surface during treatment and the uncertainty between the estimated dose and the actual dose delivered to patients.[14],[20],[21],[22] The OSLDs have been widely used for the past couple of decades to determine dose in the areas of personal dosimetry, space dosimetry, environmental dosimetry, and retrospective dosimetry;[23],[24],[25] the results obtained from this in vivo dosimetric measurements are also promising. Hence, this dosimeter could be used in radiotherapy clinical settings as it overcomes the drawbacks of other in vivo dosimeters such as consuming read-out method and annealing process before irradiation with thermoluminescent dosimeter, temperature dependency, nonuniform angular response with diodes, limited lifetime with metal oxide semiconductor field effect transistor (MOSFET)[1] and also cumbersome to use the cables in clinical settings with these dosimeters when many dosimeters are used simultaneously.[1],[16],[17],[26],[27],[28],[29],[30]


 > Conclusions Top


OSLD can be used widely in radiation oncology applications for various purposes because of their small size, ruggedness, ease with which they can be attached to the body of the patient without disturbing the hospital routine and their capability of accumulating absorbed dose. The promising results obtained on phantom indicate that this practice could be translated to the patient situation with nanoDot OSLDs to evaluate the exit dose during irradiation. In addition, it reveals that this technique can be implemented as a part of quality assurance to achieve quality treatment in radiotherapy and suitable to use in various other clinical radiotherapy settings. The overall results confirm that OSLD could be a potential dosimeter in providing accurate in vivo dosimetry measurements.

Acknowledgment

The authors would like to thank Mr. Vijaya Reddy, M/s Kavya Medical Equipments for designing dosimeter accessories.

Financial support and sponsorship

This study was supported by a grant from Atomic Energy Regulatory Board (AERB), India.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
 
 
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