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 Table of Contents 
ORIGINAL ARTICLE
Year : 2017  |  Volume : 40  |  Issue : 3  |  Page : 142-148  

Measurement of dose reduction factors for X-rays and its relevance in eye lens monitoring applications


1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
3 Ex-Scientific Officer, RP&AD, BARC and Operating Plant Safety Division, Atomic Energy Regulatory Board, Mumbai, Maharashtra, India

Date of Submission11-Oct-2017
Date of Decision25-Nov-2017
Date of Acceptance08-Dec-2017
Date of Web Publication16-Feb-2018

Correspondence Address:
Munish Kumar
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai - 400 094, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_27_17

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  Abstract 


Lead (Pb) goggles used by radiation workers while performing fluoroscopy-guided diagnostic and interventional procedures reduce the doses to eye lenses substantially. In majority of the cases, the dosimeter intended to measure eye lens doses is worn close to the eyes but outside the lead (Pb) goggles. In such situations, additional reduction factors are required to approximate the actual doses which eye lens might have received. However, the value of the dose reduction factor (DRF) for a given energy may vary due to the dependence on the methodology of measurement, that is, whether the dose is estimated free-in-air or on a phantom. Due to this reason, the DRFs quoted by suppliers or manufactures cannot be implemented directly for dose estimation. Free-in-air as well as on phantom studies for the measurement of DRFs for lead (Pb) goggles having equivalent thickness of 0.75 mm were performed using optically stimulated luminescence (OSL)-based dosimeters. Measurements were carried out for photon energies having relevance in interventional cardiology (IC) and radiology (IR), and other facilities where eye lens monitoring might be important. It has been found that the DRFs differ substantially for irradiations done free-in-air and on the phantom. The results showed that for X-ray energies relevant to IC/IR procedures, the DRF values were found in the range of ~60–90 and 7–15 for free-in-air and on phantom measurements, respectively. Similarly, the DRF values for N-80 as well as 241Am photons were found to be ~15 and ~3.50 for free-in-air and on phantom measurements, respectively. The difference in the DRF values for free-in-air and on phantom measurements would arise due to the presence of backscattering associated with the phantom studies.

Keywords: Backscattering, cylindrical phantom, dose reduction factor, eye lens dose, lead goggles, medical X-ray machine and optically stimulated luminescence (OSL) dosimetry


How to cite this article:
Kumar M, Singh SK, Gaonkar UP, Sharma S D, Ratna P, Koul D K, Kulkarni M S, Datta D. Measurement of dose reduction factors for X-rays and its relevance in eye lens monitoring applications. Radiat Prot Environ 2017;40:142-8

How to cite this URL:
Kumar M, Singh SK, Gaonkar UP, Sharma S D, Ratna P, Koul D K, Kulkarni M S, Datta D. Measurement of dose reduction factors for X-rays and its relevance in eye lens monitoring applications. Radiat Prot Environ [serial online] 2017 [cited 2020 Sep 25];40:142-8. Available from: http://www.rpe.org.in/text.asp?2017/40/3/142/225585




  Introduction Top


In view of the decrease of the annual dose limit for the eye lenses from 150 mSv to 20 mSv per year (not exceeding 50 mSv in a single year) by the International Commission on Radiological Protection, the eye lens dosimetry has received worldwide attention.[1],[2],[3],[4] The revised dose limit has already been adopted by the International Atomic Energy Agency.[5],[6] More recently, the International Standards Organization (ISO) has published a report about calibration, monitoring, and operational aspects related to eye lens monitoring.[7] As per ISO, the positioning of the dosimeter for the lens dosimetry is very important. The dosimeter should be worn as close as possible to the eye, if possible in contact with the skin, and facing toward the radiation source.[7] ISO has also suggested that in case of interventional cardiology (IC)/interventional radiology (IR), the side closest to the X-ray tube shall be chosen for wearing the dosimeter.[7]

It is important to mention here that eye lens dosimetry would be important for facilities performing IC/IR procedures. In these installations, the X-ray machines are commonly operated in the energy range of 70–110 kV, and the radiation workers receive doses mainly from the scattered radiation fields, which arise due to the interaction of the primary beam with patient's body, machine components, floor, ceiling, and other surrounding objects. It may be noted that the scattered fields are highly directional, so the dose rates at chest, eye lens, and head levels may be different and generally depend on the field size. In many IC/IR facilities, Pb aprons are commonly used to reduce the whole body (effective) as well as skin doses at the chest level and the dosimeter is commonly worn under the Pb apron.[8] In view of this, the chest dose cannot be correlated to the eye lens doses even for most simple situations of uniform and homogeneous exposures. Therefore, a dedicated eye lens dosimeter must be worn at the center of head level or even close to one of the eyes for reliable eye lens dosimetry. In addition, Pb goggles are also used by radiation workers while performing fluoroscopy-guided diagnostic and interventional procedures, which reduces the doses to eye lenses substantially and the dosimeters worn near eyes/head level intended for estimating eye lens doses may overestimate the doses hugely. In view of these findings, ISO has recommended that while using protective lead glasses, the eye lens dosimeter shall be worn preferably behind them.[7] However, this is often not very practical and the dosimeter is generally worn outside the lead goggles/glasses.[7] Under such situations, dose correction/reduction factors should be applied so that the actual dose to eye lenses could be reasonably estimated.[7]

In recent years, large number of researchers have performed studies to test the efficacy of various protective gadgets, especially lead goggles and ceiling suspended shields. The dose reduction factor (DRF) is defined as the ratio of the dose with and without eyewear like Pb glasses. Magee et al.[9] have derived DRFs for X-rays incident on the front of lead glasses and found that DRFs vary from 5.2–7.6, while DRF values for orientations similar to those used in the majority of clinical practices were between 1.4 and 5.2. Hu et al.[10] performed simulations as well as measurements for the eye lens doses of the radiologists wearing different kinds of lead glasses and found that the eye lens doses were reduced by a factor of 3–9 for lead glasses having Pb equivalent thickness up to 1 mm. McVey et al.[11] has performed the assessment of lead eyewear in interventional radiology and found that the mean DRF to be about 5 (percentage reduction of 80%) for goggles with a lead equivalence of 0.75 mm. The authors further hypothesized that a DRF of 5 can conservatively be applied to all lead eyewear having Pb equivalent thickness >0.5 mm.[11] Domienik and Brodecki [12],[13] studied the effectiveness of lead glasses in reducing the doses to eye lenses during cardiac implantation procedures. Sanchez et al.[14] used OSL nanodots and electronic dosemeters for eye lens dose measurements and compared the results with those obtained with thermoluminescent (TL)-based eye lens dosemeters. For phantom measurements, OSL dose values on the average varied by −21.5% as compared to measurements done with the TL dosimeters (TLD). Furthermore, the tests carried out with other electronic dosemeters revealed differences of up to +20% with respect to TLD.[14] In addition, the influence of dosemeter position for the assessment of eye lens dose during IC procedures was investigated by Principi et al.[15]

More recently, Yokoyama et al.[16] evaluated the effect of use of lead goggles on eye lens doses in IC procedures using optically stimulated luminescence dosimeters. The ratio of the doses measured inside the eyewear to that outside the eyewear ranged from 0.31 to 0.48 for medical X-rays when the operating voltages were 73.4 to 97.7 kV. Teles et al.[17] reviewed radiation protection requirements as well as technical possibilities of dose reduction to staff and patients in computed tomographic (CT) fluoroscopy and reported that patient absorbed dose per procedure with CT fluoroscopy was much less when compared with the doses associated with conventional CT scanning. Rivett et al.[18] performed comprehensive studies on the assessment of the dose reduction of commercially available lead protective glasses and found that the direct transmission results were very similar for all models of glasses. The typical transmission was very low ~1.0%-1.2% at 102 kV and ~0.3%–0.4% at 81 kV and the DRFs for all glasses were observed to be extremely high. However, the mean DRF was about 7 for head phantom measurements. Further details about the effect of irradiation angles, lens sizes, etc., have been reported by Rivett et al.[18] In a recent study by Martin,[19] it has been suggested that ceiling suspended shields and disposable radiation absorbing pads can reduce eye lens doses by a factor of 2–7, whereas Pb glasses that shield against exposure from the side can lower the doses by 2.5–4. 5 times. Comprehensive details about the methodology and testing requirements of various radiation protection gadgets including eyewear are available from the reports of International Electrotechnical Commission.[20],[21],[22]

It may also be noted that the dose reduction/correction factors for X-ray photon energies relevant to IC/IR procedures may be influenced not only by the geometry and thickness of Pb goggles but also by the energy of the photons. In addition, the generated DRFs may also vary for free-in-air or on phantom irradiations. This indicates that the DRFs as mentioned by the suppliers might differ from actual reduction factors. The reason behind the discrepancy would be the fact that scattering of photons due to Compton effect is more prevalent in lower energy region (<100 keV) as compared to higher energy photons associated with 137 Cs or 60 Co sources. Moreover, the backscatter phenomenon generally increases the absorbed dose, thereby reducing the efficacy of the Pb goggles. At energies around 50 to70 keV, the backscattering from typical water-filled phantom is maximum and increases the dose at least by ~50% compared to one measured for free-in-air irradiation conditions (in the absence of phantom). This would also lead to differences in the values of DRFs as backscattering plays an important role in dosimetry. In view of this, a study was performed for X-rays/photons in the energy range which is commonly used in IC/IR procedures. The values of the DRFs for lead goggles were measured using OSL-based eye lens dosimeters for free-in-air as well as on phantom irradiations for various X-ray beams.


  Materials and Methods Top


α-Al2O3:C OSL-based eye lens dosimetry badge designed exclusively for the measurement of eye lens doses in our country was used. It has two regions, namely, metal (Cu + Al) filters and the second region has plastic buildup of 300 mg/cm 2.[23] The OSL discs having diameter and thickness of 7 mm and ~0.14 mm, respectively, are prepared by sandwiching α-Al2O3:C powder of grain size (75–105) μm between two thin plastic sheets. The minimum measurable dose for the eye lens dosimetry badge is~50 μSv. Further details about the eye lens dosimetry badge can be obtained from the publications of Kulkarni et al.[23] and Munish et al.[24],[25],[26]

For the readout of the OSL cards, the OSL badge reader developed indigenously for four elements-based badge was used with an adopter card. The reader was operated in the continuous wave OSL (CW-OSL) mode at 20 mW/cm 2 blue (~470 nm) light stimulation intensity. Two high-power light (blue)-emitting diodes (~470 nm) placed at an angle of 45° with respect to the sample holder are used as stimulation light source in the reader system.[27],[28] A GG-435 color glass filter was fixed in front of the blue LEDs to cut off the stimulation wavelengths below 435 nm. An UG-1 color glass filter is placed in front of the photomultiplier tube (PMT-Electron Tube, 9125B) to prevent the scattered stimulating light from reaching the PMT.[26] In the studies, CW-OSL curves were recorded for 10 s with acquisition interval of 0.1 s, and total area under the CW OSL curve was considered while performing analysis.[27],[28] CW OSL for the control (unirradiated) cards stored in radiation free area was also recorded under above mentioned conditions, and the control/background reading was subtracted from the readout of the exposed cards.[26]

In the present studies, lead goggles (Ray protection glasses make) having Pb equivalent thickness of 0.75 mm with side shields was used. Although typical average energies in IC/IR facilities are ~30–40 keV, for the sake of completeness; the studies were also performed for photons having energy up to 100 keV. It is worth mentioning here that photons having energy ~60 keV emitted from 241 Am also need attention as dose gradients might prevail between the chest and head levels while performing certain jobs such as glove box operations. In view of this, measurements were also performed using 241 Am (59.5 keV) photon source. For other energies, the studies were performed using X-ray machine (model YXLON MG325). Dosimeters were irradiated at 2 m distance from the focal spot of X-rays and were kept at 1.3 m height from the ground. Various narrow (N) and wide (W) X-ray beams having designations of N-40, N-60, N-80, W-80, W-110, and W-150 were used.[21] These beams have effective energy of 33 keV (N-40), 48 keV (N-60), 63keV (N-80), 57 keV (W-80), 79 keV (W-110), and 104 keV (W-150).[29],[30],[31] The measurements stated above were also performed at operating voltage of 80 kV as such X-rays are typically used in various medical procedures.

In addition to the studies performed using YXLON MG325 X-rays machine, measurements of DRFs were performed using medical X-ray machine (Siemens POLYDOROS-LX with high-frequency generator). The machine can be operated up to 150 kV and maximum current of 800 mA. There is inbuilt filtration in the X-ray machine achieved using 4.5-mm thick Al sheet which cuts low energy X-rays, and thereby reduces the skin dose. The present studies were performed in the 50–100 kV range. The machine is routinely used for testing the lead equivalent thicknesses of lead aprons and Pb goggles as well as quality control of the radiographic films used in our country.[32] In this study, doses in the range 4–10 mGy were delivered for above mentioned X-ray beams/photon sources.

For free in air irradiation, a wooden frame having dimensions of 30 cm × 30 cm was used and dosimeters were pasted in its central region with and without using Pb goggles [Figure 1]a and [Figure 1]b. For on phantom studies, cylindrical phantom shown in [Figure 2]a having both diameter and height of 20 cm with wall thickness of 5 mm as recommended by Optimization of RAdiation protection for MEDical staff project was used.[33] The phantom has a volume of 6280 cm 3 and is a reasonable approximation of the human head. On phantom studies were performed with and without Pb goggles [Figure 2]a and [Figure 2]b. While using Pb goggles for free-in-air or on phantom irradiations, the dosimeters were kept under both the left as well as the right lens of the goggle and mean of these readings is presented. All the results reported are for normal/perpendicular irradiations. The experiments were repeated and the overall reproducibility of the measurements was ≤10%.
Figure 1: Free-in-air irradiations of eye lens dosimeters without (a) and with (b) lead goggles

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Figure 2: On phantom irradiations of eye lens dosimeters without (a) and with (b) lead goggles

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  Results and Discussion Top


The values of the DRFs for Pb goggle having equivalent thickness of 0.75 mm measured for free in air and on phantom measurements for various photon energies are given in [Table 1]. Typical DRFs for free-in-air study varied from 92 to 6.78 for X-rays beams corresponding to lowest ~25 keV (80 kV) and highest energies ~104 keV (W-150) utilized in the measurements. In case of N-80 or 241 Am beam/energy, the value of DRF was found to be ~17. Similarly, the DRFs for on phantom measurements varied from 17 to 3.13 for X-ray beams corresponding to 80 kV (~25 keV) and W-150 (~104 keV), respectively, whereas for N-80 or 241 Am beam/energy, the value of DRFs was ~3.75. The graphical variation of DRFs for free-in-air and on phantom measurements is shown in [Figure 3]a. Further in [Figure 3]b, the variation of DRFAir/DRFPhantom for various photon energies is shown. The shape of the curve essentially represents the transmission characteristics of Pb goggles and the backscattering effect from water-filled phantom. It needs to be mentioned here that the backscattering of photons is highest at photon energy range of ~50–70 keV, depending on the size and dimensions of phantom.
Table 1: Dose reduction factors and transmission percentage for free-in-air and on phantom irradiations for various photon energies

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Figure 3: (a) Variation of DRFs for irradiations done free-in-air and on phantom for various photon energies. (b) Variation of DRFAir/DRFPhantomfor various photon energies. DRF: Dose reduction factor

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This study demonstrated that for X-ray energies relevant to IC/IR procedures, the typical DRF values for on phantom measurements were in the range of 5–15 for energies up to 50 keV. Furthermore, the DRF values for N-80 X-ray beam and 241 Am for free-in-air and on phantom measurements were ~17 and ~3.5, respectively. The values were found to be same for both the beams as they have nearly same average energy, although the spread in energy associated with the N-80 beam is much higher than the monoenergetic photons emitted from 241 Am. The study has relevance for various operations, such as glove boxes handing of Pu/Am sources, where there are huge dose gradients between eye lens and chest levels.

The results of the similar study performed for medical X-ray machine showed that the DRFs in air varied from ~100–60 for 50–100 kV range, respectively, and the results are shown in [Figure 4]. Further details are given in [Table 2]. The values corresponding to on phantom measurements vary from ~12 to 7 for 50 to 100 kV operating voltages, respectively. The mean DRFAir/DRFPhantom was found to be ~8.64 ± 1.00 for medical X-ray machine operated up to ~100 kV. It needs to be mentioned here that in this study, the DRFs investigated for the medical X-ray machine also includes backscattering not only from water-filled phantom but also from the patient couch which enhances the scattering of X-rays. In addition, the DRFs were measured for primary (direct) wide beams under full scatter conditions. The difference in the values of DRFs for free-in-air and on phantom measurements for different photon energies may be due to the Compton scattering popularly called backscattering effect in dosimetry. It also needs to be noted that the backscatter from the head is the limiting factor for dose reduction potential of lead eye goggles, as mentioned by McVey et al. in their study.[11] This implies that the actual values of the DRFs in the field are much lower than generally quoted by suppliers. In general, it is presumed that the lead glasses/goggles absorb ~99% of the incident X-rays. However, the fact is that the typical transmission with Pb goggles can be as high as ~15% for X-ray energies relevant to IC/IR facilities. It may be noted that the DRFs are influenced by a large number of factors such as oblique irradiation, the impact of the body backscattering as well as irradiation beam (narrow or wide) geometries and same may be taken care while applying DRFs.
Figure 4: Variation of dose reduction factors for irradiations carried out free-in-air and on phantom for various photon energies using conventional medical X-ray machine

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Table 2: Dose reduction factors and transmission percentage for free-in-air and on phantom irradiations for various photon energies

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  Conclusions Top


  1. The DRFs differ substantially for irradiations done free-in-air and on phantom and are highly influenced by the backscattering from phantom
  2. A DRF of ~5 may be appropriate for X-rays installations where typical average energy is ≤45 keV, whereas for facilities handling sources having higher photon energy ~60 keV, as prevalent in glove boxes, a DRF of ~2 may be suitable for Pb goggles having equivalent thickness of 0.75 mm
  3. It is recommended that DRFs for protective (lead) goggles must be established by performing measurements on phantom for various photon/X-ray energies so as to avoid undue underestimation of the eye lens doses. The information may be useful for those situations in which eye lens dosimeters are worn outside protective goggles.


Acknowledgment

The authors are thankful to Dr. K S Pradeepkumar, Associate Director, HS and E Group, BARC for constant support, guidance, and encouragement. Assistance from Smt. P. S. Thatte, Smt. Shramika H. Pawar, and Shri. Vivek Kadam from RP and AD, BARC is gratefully acknowledged.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2]



 

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