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Equivalent dose to the fetus from occupational exposure of pregnant staff in diagnostic radiology

¹Princess Margaret Hospital, Department of Clinical Physics, 610 University Avenue, Toronto, Ontario M5G 2M9, Canada and ²Regional Medical Physics Department, Newcastle General Hospital, Westgate Road, Newcastle Upon Tyne NE4 6BE, UK

	Abstract

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The protection of the unborn children of pregnant women from ionizing radiations is very important because the fetus is particularly vulnerable to the effects of ionizing radiation. From the radiation protection perspective, the International Commission on Radiological Protection regards the unborn child as a member of the public when considering the occupational exposure of pregnant workers. The determination of the equivalent dose to the unborn child in diagnostic radiology is of interest as a basis for risk estimates from occupational exposures of the pregnant worker. In this paper, coefficients for converting dosemeter readings to equivalent dose to the fetus have been calculated using Monte Carlo simulation. X-ray transport was simulated by tracing individual photons through soft tissue phantoms. Equivalent dose to the uterus was used to simulate the equivalent dose to the fetus during the first 2 months of pregnancy. The Monte Carlo model was validated experimentally by direct measurements made in an Alderson female Rando phantom for a range of irradiation conditions. The two sets of data indicated good agreement with the Monte Carlo results, being relatively greater than the experimental results to a maximum of about 15%.

	Introduction

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There have been a number of approaches to limiting exposures to ionizing radiation during pregnancy. The International Commission on Radiological Protection (ICRP) has recommended that, for the protection at work of women who may be pregnant, additional controls have to be implemented to protect the unborn child as the radiation risk factors are higher than those for the mother. The ICRP considers that the protection accorded to the fetus of an occupationally exposed worker should be comparable with that for members of the general public [1]. A further recommendation of the ICRP is that the fetus should be protected by the application of a supplementary equivalent dose limit of 2 mSv to the surface of the woman's abdomen (lower trunk) for the remainder of the pregnancy, once it has been declared. Implicit is an assumption that the dose limit to the fetus is 1 mSv. The ICRP also suggests that the employee's duties should not carry a significant probability of high accidental doses. Concerning medical exposures of pregnant women, the ICRP recommends that procedures causing exposures of the lower abdomen of women likely to be pregnant should be avoided unless there are strong clinical indications.

Radiation doses to occupationally exposed staff working with radiological equipment are generally low [2] and it is unlikely that the equivalent dose limit recommended by the ICRP [1] and as adopted in the UK Ionising Radiations Regulations 1999 [3] will be approached. However, for some fluoroscopic examinations there is a potential for higher radiation doses to staff [4–7]. During interventional radiology procedures, particular radiation protection problems arise from the extended fluoroscopy times and from the use of dedicated radiological equipment which usually does not have lead rubber protective curtains [4]. Consequently, the implications of the ICRP recommendations on the radiation exposure of the fetus of staff performing fluoroscopy procedures should be assessed.

Radiation dose to a number of organs, including the uterus, for a range of X-ray energies has been discussed in the literature [8–10]. Whilst these papers contain useful data for estimating fetal dose, they are usually for calculating dose from the medical exposure of a pregnant patient and may not give a good estimation of the dose to a fetus from the occupational exposure of a pregnant worker who is only exposed to scattered radiation. Faulkner and Marshall [4] have estimated the ratio of equivalent dose to the uterus to film badge reading. When a lead apron was worn, the doses to the uterus and to film badge dosemeters were deduced from the unshielded results by applying a transmission factor for the attenuation of scattered radiation through the lead apron. Data were presented for scattered spectra generated from only three tube potentials and when the dosemeter was placed under the lead apron. One shortcoming, which they stated, was that their approach did not take into account the effect of beam hardening through the lead apron. In view of the lack of data in the scientific literature for the estimation of equivalent dose to the fetus from scattered radiation, it was felt that this subject merited further study.

The main objectives of this investigation were, for simulations of fluoroscopy procedures (for both overcouch and undercouch X-ray tube orientations): (1) to determine the relationship between the equivalent dose to the fetus of a pregnant worker relative to dosemeter readings on the abdominal surface for scatter and primary irradiations (using the Monte Carlo approach)—data will be established for situations when the entrance dose (absorbed dose to thermoluminescent dosemeter (TLD)) is calculated both under or over a lead apron if one is worn; (2) to validate the Monte Carlo method with experimental measurements made in an Alderson female Rando phantom (Alderson Research Laboratories Inc., Stamford, CT); and (3) to use the data presented here and measured entrance doses from occupational exposures in the literature to quantify some fetal equivalent doses.

The work presented here differs from previously published data [4] in that we have determined normalized equivalent dose data from scattered energy spectra generated from a wide range of tube potentials. Normalized doses take into account beam hardening when a lead apron is worn and we have also provided normalized equivalent dose data for situations when the dosemeter reading is taken over and under the lead apron. The optimal position of the dosemeter (TLD or film badge) used in whole body radiation dose to staff in a diagnostic X-ray department has been a subject of recommendation and debate. Whereas in some countries the recommended placement of the dosemeter is outside the lead apron if worn, it is common practice in the UK to monitor staff doses under the apron. An advantage of the work presented here is that data are presented for dosemeters worn both above and below the apron.

The Monte Carlo code calculates the absorbed dose to the uterus but, in radiological protection, it is the absorbed dose averaged over a tissue or organ and weighted for the radiation quality that is of interest [1]. Hence, in terms of dose to the uterus from occupational exposure, equivalent dose will be used instead of absorbed dose. Numerically, for X-rays, the two quantities are the same since the radiation weighting factor for X-rays is 1. In this paper, equivalent dose to the uterus was used as an approximation of the equivalent dose to the fetus during the first 2 months of pregnancy. As pregnancy advances, the geometrical configuration of the fetus (combination of weight, length, width and anteroposterior (AP) thickness) changes and the mean uterus dose may no longer be a good approximation of fetal dose [10–12]. It has been estimated from Monte Carlo calculations for pregnant patients [11, 12] that the mean uterus (mass of 66.3 g) dose would overestimate fetal (mass of 2.5 kg during the last trimester) dose by about 40% depending on beam size on the patient's abdomen.

	Materials and methods

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Monte Carlo approach
The Electron–Gamma–Shower (EGS4) code was used to carry out Monte Carlo simulations of radiation transport through the block-shaped phantoms representing the member of staff (i.e. the trunk) and the uterus. This code calculates the amount of energy deposited in the uterus and follows the photons until they are absorbed or escape from the phantom. In the computer program, a simplified model for the exposure of the pregnant worker to scattered radiation from fluoroscopy procedures is used (Figure 1a). A pregnant worker and uterus are represented by two simple geometrical block shapes. The dimensions of these blocks approximate the anatomical dimensions of an average adult's trunk (length 100 cm, width 35 cm, AP thickness 20 cm) and uterus (length 7.5 cm, AP thickness 2.5 cm, width 3.54 cm). The uterus dimensions (length l and AP thickness t) were taken from Verralls [13] and a mass m of 66.3 g suggested by Rosenstein [14] was used. The width w of the uterus was estimated from w=m/(lt), where is the density and was taken to be 1 g cm^-3. Block-shaped phantoms (especially to represent the adult trunk) have been used by several investigators [15, 16] for different studies in diagnostic radiology. The entire body of the phantom was considered to be homogeneous in composition and density. Elemental compositions of soft tissues for both the adult phantom and the uterus (the elemental composition of soft tissue for the newborn was used) were taken from a report by Eckerman et al [17]. A mean fetal depth of 8 cm from the phantom (representing the pregnant worker) surface to the mid-plane of the inner phantom (representing the uterus) was used. The mean depth was based on measurements of fetal depth [11] with the patient in a supine position and may not be a true representation for a standing position. However, in the absence of such measured data, the chosen value may be a good approximation.

View larger version (21K): [in this window] [in a new window]   Figure 1. (a) Schematic presentation of a simplified model of irradiation of staff (particularly the staff geometry) used in the Monte Carlo simulation. The outer block represents the trunk of an adult member of staff and the inner block represents the uterus. (b) Schematic presentation of the geometry used in the experimental validation of the Monte Carlo code.

In the Monte Carlo simulation, the energy released is assumed to be absorbed at the same point (i.e. the kerma is equal to the absorbed dose). The incident photon spectrum was converted from plane energy fluence to absorbed dose to TLD. Normalized equivalent dose C_eq in millisieverts (mSv) per milliGray (mGy) between the equivalent dose to the uterus in mSv and the entrance surface dose to TLD in mGy (including backscatter) is given by:

where (E) is the number of photons with energy E in MeV, and (µ_en/)_E,tld is the mass energy absorption coefficient for TLD in m² kg^-1. D_eq is the equivalent dose to the uterus, which is the uterus absorbed dose averaged over the entire organ and weighted for the radiation quality of X-rays [1].

Experimental verification
The experimental set-up used for the estimation of equivalent dose to the uterus from occupational exposure of the pregnant member of staff is shown in Figure 1b. An Alderson female Rando anthropomorphic phantom was used to represent the member of staff. The phantom consists of a human skeleton encased in tissue-equivalent material, with 33 transverse sections. Each transverse section contains a matrix of holes of 5 mm diameter on a 3 cm x 3 cm grid. It has an AP thickness of 20 cm, a width (shoulder-to-shoulder) of about 34 cm and a height of 90 cm. The phantom was placed on a support at a distance of 45 cm [19] from the centre of the X-ray couch and corresponded to an individual staff member 165 cm tall working at the X-ray couch side. All holes not containing TLDs were filled with plugs of the same material as the phantom. Thermoluminescent dosemeters of the type TLD-100 (lithium fluoride (LiF) chips; Harshaw, USA) were used for all dosimetry work. As in the Monte Carlo simulations, the equivalent dose to the uterus was used to simulate the equivalent dose to the fetus.

An anthropomorphic pelvis phantom (3M, USA) representing the patient was used to provide a source of scattered radiation realistic of clinical practice. The pelvis phantom was laid in a supine position on the couch with the X-ray beam centred on it. The X-ray field size measured on the patient phantom abdominal surface was 20 x 20 cm² and the focus-to-skin distance was 1.0 m. Scattered radiation was generated at tube potentials of 74 kVp, 90 kVp and 101 kVp. The female phantom representing the member of staff was loaded with 180 TLD chips to measure the absorbed dose to the uterus as a function of depth for each scattered energy spectrum generated. 30 TLD chips contained in polyethylene bags in groups of five and loaded in slices 30 and 31 of the phantom were used for each dose level measurement. Scattered radiation doses in the vicinity of the couch are relatively low for both undercouch and overcouch X-ray tube geometries, so prolonged irradiation times were used in order to have dosemeter readings well in excess of their detection limit (0.02 mGy).

The entrance surface dose with backscatter was measured at the abdomen and chest of the phantom representing the staff member for each irradiation. Groups of five TLD chips contained in polyethylene bags were attached at waist and chest level on the right and left sides of the phantom to represent personal dosimetry badges worn at these positions. Left and right results were averaged together. Data from the waist level dosemeters only were used in the verification and are the ones presented.

Further experimental verification work was done with the staff member (female phantom) wearing a 0.25 mm lead equivalent apron for the verification of the Monte Carlo method when a lead apron layer was included. In practice, it is very difficult to investigate this experimentally using scattered radiation, as the exposure time required to obtain doses above the background on the TLDs beneath the apron is extremely long. Therefore, direct irradiation was used for this verification. The female phantom was irradiated with primary X-rays while wearing a 0.25 mm lead equivalent apron. Field size on the phantom surface was 28 x 26 cm². In this case, the entrance doses were measured by placing 20 TLDs each both over and under the lead apron and positioned along the central axis of the X-ray beam. The uterus doses were normalized to both entrance doses (i.e. TLD absorbed dose with TLDs placed over and under the lead apron).

During fluoroscopy examinations there are occasions when the radiologist or scrub nurse will turn to face the TV monitor and be irradiated from the side. The change in the ratio of equivalent dose to the uterus to dosemeter reading with orientation of the member of staff was also experimentally investigated. In this case, the female phantom representing the member of staff (see Figure 1b) was orientated at an angle of 45° to the couch while being irradiated with scattered radiation.

	Results

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
The ratio of the calculated equivalent dose to the uterus to entrance dose (absorbed dose to TLD), using Monte Carlo simulations, for entrance doses estimated both over and under the lead apron and for simulations of overcouch X-ray tube/undercouch image intensifier irradiation are given in Table 1. Table 2 shows the corresponding data for undercouch X-ray tube/overcouch image intensifier conditions. Equivalent dose to the uterus has been normalized to the absorbed dose to TLD. Equivalent dose to the fetus calculated using data presented here and entrance doses from the literature [20, 21] are presented in Table 3. A comparison of the Monte Carlo model with direct experimental measurements in a female Rando phantom of the ratio of equivalent dose to the uterus normalized to absorbed dose to TLD for scattered radiation generated at 74 kVp, 90 kVp and 101 kVp tube potentials is presented in Figure 2. A similar comparison for primary irradiation using 70, 80 and 90 kVp tube potentials is given in Figure 3. The indicated error bars in Figures 2 and 3 are the 95% confidence level (i.e. 2 x standard error) and correspond to only the random uncertainties from the measurements. For the TLD system used, the non-random uncertainty has been estimated to be less than 10% [22].

View larger version (19K): [in this window] [in a new window]   Figure 2. Comparison of normalized equivalent dose to the uterus for scattered irradiation generated at different tube potentials (kVp). , experimental data; , Monte Carlo data.

View larger version (31K): [in this window] [in a new window]   Figure 3. Comparison of experimentally measured data () with Monte Carlo calculated data () for primary irradiation and when the phantom is covered with 0.25 mm lead, for 90 kVp (top), 80 kVp (middle) and 70 kVp (bottom). Equivalent dose to the uterus nomalized to thermoluminescent dosemeter readings when placed (A) under and (B) over the lead apron.

where _occ is the normalized equivalent dose data averaged over the range of energies (Tables 1 and 2). It should be noted that different values of _occ may be used depending on the position (i.e. over (Tables 1a and 2a) or under (Tables 1b and 2b) the lead apron if it is worn) of the dosemeter or if no lead apron is worn (Tables 1a and 2a). For accidental primary beam irradiation when a 0.25 mm lead apron is worn, normalized data in Figure 3 may be applicable.

Kicken et al [20] have performed dosimetry of occupationally exposed persons in diagnostic and interventional arteriography. The entrance doses to occupationally exposed workers were measured at different parts of the body including the abdomen using TLD dosimetry. All the workers wore 0.5 mm lead equivalent protective aprons and entrance doses were measured with the TLDs outside the apron. The measured mean and maximum entrance dose per procedure for the operators for some procedures are shown in Table 3. The X-ray tube orientation was mostly undercouch. Using normalized data for a 0.5 mm lead equivalent apron, the equivalent dose to the fetus ranges from 0.4 µSv per procedure for cerebral arteriography to 1.5 µSv per procedure for percutaneous transluminal angioplasty (PTA) (the maximum equivalent dose to the fetus for this procedure is 4.8 µSv per procedure). The data imply that a pregnant operator performing PTA and wearing a 0.5 mm lead equivalent apron would only exceed the equivalent dose limit to the fetus after 208 procedures (based on the maximum dose of 240 µGy per procedure).

A survey of radiation exposures received by staff at two cardiac catherization units has been carried out by McParland et al [21] using TLD dosimetry. Both units used an undercouch tube and overcouch image intensifier arrangement and operated under automatic exposure control with tube potentials during fluoroscopy ranging from 63 kVp to 105 kVp and tube currents from 2.8 mA to 4.5 mA. All staff wore torso shielding, with a reported thickness of 0.25 mm lead equivalent. The upper estimates of the dose per procedure to the abdominal surface of the cardiologist and the nurse (measured under the lead apron) were 78 µGy and 31 µGy, respectively. Using the data in Table 2b, the equivalent dose limit to the fetus of a pregnant cardiologist could be exceeded at a workload of 34 cardiac catherizations and to a pregnant nurse at a workload of 87 examinations. These figures are based on an upper estimate of doses during cardiac catherizations.

	Discussion

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
The data presented here enable the equivalent dose to the fetus to be deduced from a dosemeter reading used for routine monitoring of personnel working in the diagnostic radiology department. The data in Tables 1 and 2 are only applicable for dosemeter readings when they are placed at waist level, and either placed under or above the lead apron if one is worn. In all instances, the ratio of equivalent dose to the uterus normalized to dosemeter reading is less than 1, indicating that the uncorrected dosemeter reading will usually overestimate the equivalent dose to the fetus.

As may be deduced from Tables 1 and 2 and Figures 2 and 3 (at a depth of 8 cm), the ratio of equivalent dose to the uterus to dosemeter reading is always less than 0.5. This implies that the assumption made by the ICRP of the equivalent dose to the fetus being half of that at the surface of the abdomen, implicit in the determination of the dose limit to the surface of the abdomen during the term of pregnancy, not unreasonably errs on the side of safety for scattered radiation distributions in fluoroscopy. If a personal monitoring service therefore measured a dose of 2 mSv on the surface of the abdomen of a pregnant worker who worked in an environment in which fluoroscopy was performed, then the results presented here imply that the equivalent dose to the fetus would actually be less than 1 mSv. The data in Tables 1 and 2 were generated using a mean uterus depth of 8 cm. Variations in the depth of the uterus among individuals and at different gestational ages even for the same individual may contribute to either an overestimation or an underestimation of the fetal equivalent dose if the actual depth is less than or greater than 8 cm, respectively.

During fluoroscopy examinations there are occasions when the radiologist or scrub nurse will turn to face the TV monitor and be irradiated from the side. The ratio of the equivalent dose to the uterus normalized to the entrance surface dose for AP irradiation to 45° orientation irradiation (i.e. NUD_AP/NUD_{45° oblique}) was about 1.3 for 90 kVp and 100 kVp tube potentials used. Kicken et al [23] quoted a range of 1.3–1.5 for this ratio. Such factors should be taken into account when estimating fetal doses from occupational exposures. The effect of beam hardening through the lead apron can be observed in the data in Tables 1b and 2b, which were generated using the Monte Carlo method.

The Monte Carlo model was experimentally verified by direct measurements of uterus dose as a function of depth in an Alderson female Rando phantom for scattered irradiation (Figure 2). Another comparison was also made for primary irradiation and with the staff member (female phantom) wearing a 0.25 mm lead apron (Figure 3). The indicated error bars in Figures 2 and 3 correspond to the 95% confidence level (i.e. 2 x standard error). In general, the two sets of results agree reasonably well with the Monte Carlo data, being relatively higher than the experimental values up to a maximum of 15%. There are several sources of error that might contribute to the difference between the two results. For example, the scattered energy spectra produced for the experimental work may differ from that used in the Monte Carlo. The half valuelayers of the scattered spectra used for the experimental work were up to about 6% lower than those used in the Monte Carlo simulation. The covering of the lead apron, which may contain carbon, may also contribute some backscatter to the TLDs and this was not taken into consideration in the Monte Carlo code. The simplified model of the irradiation geometry (and the phantom) of the staff worker and the uterus may also contribute some uncertainty in the results.

From the data presented in Figure 3, it may be deduced that if a lead apron is worn (and assuming the uterus depth is 8 cm from the abdominal surface of the phantom), the ratio of equivalent dose to the uterus relative to waist level dosemeter reading is approximately 0.5 for primary radiation. Similarly, the ratio of equivalent dose to the uterus relative to waist level dosemeter reading is between 0.24 and less than 0.40 for scattered irradiation (Tables 1b and 2b) in diagnostic radiology. Thus, in the unlikely event of an individual wearing a lead apron (up to 0.25 mm lead) being exposed to primary radiation from diagnostic X-rays, the application of a 2 mSv dose limit to the surface of the abdomen will result in about 1 mSv equivalent dose to the fetus based on the data presented here. Also, for scattered radiation, a 2 mSv equivalent dose measured on the surface of the abdomen will result in less than 1 mSv equivalent dose to the fetus.

	Conclusion

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Normalized data between equivalent dose to the uterus and entrance surface absorbed dose to TLDs have been presented for various scattered and primary energy spectra and for different placements of the TLD. These normalized equivalent doses have been provided for the estimation of the equivalent dose to the fetus (during the first 2 months of pregnancy when the physician or the mother may not be aware of pregnancy) from dosemeter (worn by the pregnant worker) readings, which in turn can be used for risk estimates in diagnostic radiology. The data indicated that, assuming a mean fetal depth of 8 cm from the abdominal surface of the pregnant worker and provided the equivalent dose to the surface of the abdomen is 2 mSv, the equivalent dose to the fetus will be about 1 mSv for both scattered and accidental primary irradiations.

Although radiation doses to occupationally exposed persons in radiology are, in general, low, it is important for pregnant staff to be aware of the need to minimize their exposure to ionizing radiation and hence the risk to the fetus. Staff should always be positively encouraged to follow good radiation protection operational procedures, including reporting pregnancy to their Approved Dosimetry Service (normally through their Radiation Protection Supervisor) as soon as pregnancy is confirmed.

	Acknowledgments

 
This work has been supported by a Commonwealth studentship (E K Osei). The authors wish to express their appreciation to Dr N W Marshall for providing the scattered energy spectra data used in the Monte Carlo code and to Dr K Faulkner for his contributions and advice.

Received for publication May 5, 2000. Accepted for publication April 4, 2001.

	References

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 

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This Article Abstract Figures Only Full Text (PDF) Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Osei, E K Articles by Kotre, C J Search for Related Content PubMed PubMed Citation Articles by Osei, E K Articles by Kotre, C J