Estimation of foetal radiation dose to occupationally exposed staff in diagnostic radiology and nuclear medicine.

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1 INTRODUCTION

The use of ionizing radiation in medicine is of special concern when applied to women in fertile age, due to the possible irradiation of a foetus. In diagnostic medicine this applies especially to female patients undergoing examinations or interventions using X- rays or radionuclides, as well as to pregnant staff involved in such procedures. Patient examinations are governed by the ALARA principle. It states that all examinations that involve ionizing radiation should be optimized. Naturally, only examinations that are considered justified for obtaining relevant and necessary diagnostic information of the patient should be performed. In addition, special regulations apply to women in fertile age who undergo examinations involving irradiation of the lower trunk and/or pelvis region. Provided these guidelines for patients are properly taken into account, there are very few situations in diagnostic medicine in which the absorbed dose to the foetus will exceed 100 mGy, considered a relevant abortion indication level (ICRP Publication 84 (2000)). With female medical staff, the problem of foetus irradiation becomes more complex, as in this case the unborn child is treated like a member of the general public, with a dose limit of 1 mSv (effective dose) during the gestational period (ICRP Publication 84 (2000)).

The protection of the unborn child from radiation is indeed very important because the foetus is particularly sensitive to its harmful effects (ICRP Publication 84 (2000)). The type of effect will depend on the gestational age of the embryo at the time of irradiation.

The post conception period can be divided into three major phases. These are ¹⁾the preimplantation phase, ²⁾ phase of major organogenesis (3rd to 8th week post- conception), and ³⁾foetal development (9th week until birth) (ICRP Publication 84 (2000)). The last phase includes the development of the central nervous system (CNS).

Radiation risks are most significant during organogenesis and early foetal period, somewhat less in the second trimester and least in the third trimester (ICRP Publication 84 (2000)). The most severe deterministic effects of prenatal exposure to ionizing radiation are intrauterine mortality (IU), organ malformations, and mental impairment.

Stochastic effects (leukaemia, solid tumours, and genetic anomalies) are also included

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in the list of possible radiation induced effects from foetal exposure. The radiation risks at different gestational age of the foetus have been summarized by Fattibiene et a1 (1999), see table 1.

Table 1

Major effects of prenatal irradiation during different gestational periods [3].

Embryonic or

foetal age (wk) Development stage Effects 0 -2 Pre- implantation;

implantation Abortion

3 - 7 Early organogenesis IU mortality

Organ malformation 8 - 15 Brain development

stage I

IU mortality SMR Epilepsy attack

Decreased IQ 16-25 Brain development

stage II IU mortality

SMR

>25 IU mortality

Whole pregnancy Child. fatal cancers

IU - intrauterine mortality SMR – sever mental retardation IQ - Intelligence Quotient

Based on a national survey carried out by ISPESL (National Institute for Occupational Safety and Prevention, Italy), Parisi et al (1994), reported that 30% of hospital female staff working with nuclear medicine and X-rays were exposed to effective doses higher than 1 mSv during one year¹. For comparison, in 2004 and 2005 approximately 60% of nuclear medicine staff (nurses) at Karolinska University Hospital/Site Solna was exposed to yearly effective doses of at least 1mSv.

Although radiation doses to occupationally exposed staff working with radiological equipment are generally low, the results presented by the Italian research group indicate that the situation with a pregnant female staff requires special attention, especially as there is no threshold dose under which stochastic effects are not believed to be operative

1 In case of homogeneous, whole body, gamma and X-ray exposures, the effective dose (mSv) can be considered equal to the absorbed dose to any organ in the body.

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under the concept of the LNT-model (LNT=Linear No Threshold; ICRP Publication 84 (2000)).

The aim of this work was to define guidelines on how to interpret the dosimeter readings for staff involved in high-dose procedures in X-ray and nuclear medicine applications, such that the foetal dose for pregnant staff will not exceed the limit of 1 mSv. The absorbed dose to the foetus depends on several factors. In X-rays the main factors that influence (staff) foetal dose are type of equipment, exposure parameters (incident beam quality, exposure time), and irradiation geometry, size of irradiated volume, and material and thickness of protective apron. In nuclear medicine, the (staff) foetal dose due to internal and external contamination of the mother depends on the energy of the radionuclide, activity (Bq), decay modes (γ, β, α), if is inhaled or ingested, acute or prolonged intake, and the foetal stage and foetus size. Data on these parameters can be input to physio-biological models in order to estimate foetal dose (ICRP Publication 88 (2001)). It can be concluded that for radionuclides that do not cross the placenta, the foetal dose is derived from the radioactivity in maternal tissues. With radionuclide that potentially can cross the placenta and concentrate in a specific organ, such as ¹³¹I in the thyroid gland, foetal irradiation can cause conditions like permanent hypothyroidism (ICRP Publication 84 (2000)). According to ICRP 84, ^99m Tc does not cross the maternal placenta. However, it has been shown that ^99mTc (NaTCO4) (sodium pertechnenate) has the potential to cross the placental barrier (Isopharma AS (IFETEC code: MO.3S)).

In order to evaluate how these factors impact on foetal dose, a survey of published data on foetal dose to staff from a variety of diagnostic X-ray and nuclear medicine examinations was carried out. As data on staff foetal dose from positron emission tomography (PET) examinations were limited, PET studies were excluded from this survey. However, it should be noted that the contribution to staff foetal dose from PET examinations is not negligible considering that the PET nuclides (¹⁸F, ¹¹C, ¹⁵O) yield annihilation photons of high energy. The contribution from PET nuclides to the staff dose is expected to increase due to that the amount of PET examinations is rapidly increasing. The survey was complemented by additional measurements on X-ray and

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nuclear medicine staff working at the Karolinska University Hospital/Site Solna, in Stockholm.

Based on the results of this study, the recommended threshold dose of 2 mSv (entrance dose) measured on the abdomen of a female worker, and suggested by the ICRP in Publication 84 (2000), was assessed.

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2 MATERIAL AND METHODS

Initially, procedures in X-ray and nuclear medicine applications that can deliver high doses to staff were defined. These include angiographic and computed tomography (CT;

especially with trauma-patients) procedures in X-ray. In nuclear medicine, staff exposure relates to the handling of radionuclides with high activity and/or high energies, and from patients whom have had nuclides injected or orally administered for examinations or therapy. Direct handling of nuclides (extraction, injection and oral administration) can result in external and/or internal contamination of nuclear medicine staff. Such exposure can be augmented due to possible contamination also of laboratories/examination rooms.

2.1 X-RAY

2.1.1 Angiographic procedures

Publications on foetal dose in X-ray focus on different kinds of angiographic procedures. This work summarizes the techniques used and the major results reported by four groups: Kicken et al (1999), Mcparland et al (1990), Osei and Kotre (2001), and Faulkner and Marshal (1993). In order to compare the results from the different groups, special attention was given to the type of examination, beam quality, irradiation geometry, and to the technique used for dose estimation.

Irradiation geometry

Most angiographic procedures are performed using undercoach X-ray tube and overcoach image intensifier (II). This geometry is illustrated in figure 1. All groups report results using this geometry. Osei and Kotre (2001) and Faulkner and Marshall (1993) additionally report results using overcoach tube/undercoach II.

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Figure 1. Schematic presentation of the most common irradiation geometry used in angiographic X-ray procedures; reproduced from [6].

Foetal equivalent dose versus entrance dose

In the article by Osei and Kotre (2001), Monte Carlo simulations of foetal dose to staff in angiographic procedures were performed assuming a mean foetal depth of 8 cm (i.e.

distance from a block-shaped phantom surface to the mid-plane of an inner phantom representing the uterus (see figure 2)). To validate the Monte Carlo results, TLD measurements were made using an Alderson female anthropomorphic phantom.

Experimental verification was performed both in scattered (geometry as in figure 1) and in primary radiation beam.

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Figure 2. Schematic presentation of the irradiation geometry used to estimate staff dose using the Monte Carlo simulation method. The outer block represents the trunk of an adult staff member, and the inner block represents the uterus. Reproduced from [6].

Results presented by other groups Kicken et al (1999), Mcparland et al (1990) and Faulkner and Marshall (1993) are based on similar irradiation geometry (see table 2).

Consequently, a mean foetal depth of 8 cm was assumed to be representative for the results of foetal equivalent dose versus entrance dose also in these studies.

2.1.2 Computed tomography

Under normal conditions, computed tomography (CT) examinations are performed such that staff leaves the examination room during acquisition of image data. In this way, radiographers and other medical staff stay in radiation protected areas during the CT examination, and there is no need for additional protective barriers, such as lead aprons, to reduce staff doses.

However, in some situations staff needs to stay close to the patient during CT-scanning.

A typical example is patients suffering from some kind of trauma that requires a close-

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to-patient assistance from medical staff during the CT-examination. In order to evaluate staff doses from such procedures, measurements using a diode dosimeter (EDD-30;

Unfors Instruments) were performed on a total of eight trauma patients undergoing CT- examinations of the trunk at the emergency ward at the Karolinska University Hospital/Site Solna. In CT, the patient dose can be estimated from the dose-length- product (DLP; [mGycm]). The DLP-value for each patient was then used together with the measured staff dose to calculate a staff dose burden coefficient [μSv/mGycm]. All measurements on staff were performed with the dosimeter positioned on the waist level below a protective apron (0.25-0.35 mm Pb equivalence).

2.2 NUCLEAR MEDICINE

Publications on foetal dose (staff) from nuclear medicine procedures focus on the one hand on external irradiation originating in the patient and from contamination of clothes and the body surface, and on the other hand on internal irradiation due to ingestion or inhalation of radionuclides. This work summarizes the results in publications focused on estimating foetal dose to nuclear medicine staff (Stather et al (2003), Barber et al (2003), Mountford and Steele (1995), Alvem (2002) [private communication]). In order to compare the results from the different groups, special attention was given to the irradiation source, work task, and to the techniques and/or models used for dose estimation.

2.2.1 External irradiation

Mountford and Steele (1995) used ^99mTc and ¹³¹I to irradiate an Alderson anthropomorphic phantom equipped with TL-dosimeters at different depths, thus simulating different foetal stages. The foetal dose was estimated at a reference depth of 7,1 cm, corresponding to the mean midline foetal abdominal depth along an anteroposterior (AP) projection, averaged over the full period of pregnancy (range 6.0- 8.5 cm). Based on measured attenuation factors together with published data on exposure time and dose rate, they report maximum effective dose for the mother (staff) and foetus from one examination.

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Based on their results, the corresponding ratio (foetal/maternal effective dose) was then calculated in this work.

Alvem (2002) reports on the dependence of the dosimeter reading on its position on the trunk. Double dosimeter measurements (one at breast level plus one at abdominal level) were performed on 11 non-pregnant staff members during a period of 4 weeks. All types of work tasks (preparation and measurements of activity of radionuclides, injection, cleaning and patient handling) typical of a nuclear medicine department were being monitored during this period. The results were used to calculate a conversion coefficient between the measured doses at breast level (normal dosimeter position) to the dose at abdominal level (CFbreast/abdomen).

Using data from Mountford and Steele (1995) the ratio of effective dose foetus/staff doseabdomen (see table 3), together with data of CFbreast/abdomen from Alvem (2002), the ratio of effective dose foetus/staff dosebreast was estimated by:

breast foetus abdomen

breast

abdomen foetus

tivedose Staffeffec

ose Effectived CF

tivedose Staffeffec

ose Effectived

=

/

/ (1)

2.2.2 Internal irradiation

Stather et al (2003) report different intake scenarios in order to estimate foetal doses due to inhalation and ingestion of radionuclides. They applied the biokinetic and dosimetric models suggested in ICRP Publication 88 (2001) in order to calculate foetal dose following radionuclide intake by the mother both before and during pregnancy.

Barber et al (2003) performed whole-body counter measurements on staff in charge of cleaning following ¹³¹I-treatment of patients. Measurements were performed before the on-set of cleaning (pre-cleaning internal+external contamination), directly after the cleaning was completed (post-cleaning internal+external contamination), and after changing into clean clothes (post-cleaning internal contamination). They did not report any results from purely external contamination in their series of measurements on cleaning staff, and assumed that the contribution from external contamination could be

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disregarded. They calculated the cumulative effective dose to the foetus from data on the dose coefficients for the foetus at different foetal stages following acute maternal intake by ingestion of ¹³¹I. They used dose coefficients reported by the National Radiological Protection Board (NRPB) (2001), which are based on the model for iodine by Berkovski (1999).

In order to estimate the ratio of effective dose to the foetus and to the mother in this work, data on the effective dose to a non-pregnant woman following injection of ¹³¹I (non-saturated thyroid gland) was taken from ICRP Publications 53 (1998) and ICRP Publications 80 (1998) and was combined with the results by Barber et al (2003).

According to the ICRP data, an administered activity of 1 kBq ¹³¹I to an adult female yields an effective dose of 24 µSv/kBq. The ratio of the foetal dose coefficients by NRPB (2001) and the ICRP estimated adult female effective dose, was calculated according to equation 2, and used as an estimate of the foetal/maternal effective dose ratio:

Mother Foetus kBq

Sv ose

Effectived

kBq Sv oefficient

Do

adult

foetus =

) / (

) / ( sec

μ

μ (2)

Alvem (2002) reports staff total body internal contamination detected by whole-body counter measurements on eight nuclear medicine staff members at the Karolinska University Hospital/Site Solna, following administration of ^99mTc with a mean administrated activity of 2.5 GBq.

The cumulative effective dose to the foetus from daily internal contamination of ^99mTc (nuclear medicine nurses) and ¹³¹I (cleaning staff) was then estimated in this work by using data from Alvem (2002) and Barber et al (2003) on administered versus detected activities together with foetal dose coefficients ICRP 88, according to equation 3.

Cumulative effective dose

∑

= − − + −

= ⁿ

i

i i

i

i x f x f x

x

1 ( 1)( ( ) ( 1))

2

1 (3)

xi = week index no; i=[0,5]; i = 0 (week no 0), i = 1 (week no 5), i = 2 (week no 10), i = 3 (week no 15), i = 4 (week no 25), i = 5 (week no 35) f(xi) = dose coefficient data (ICRP 88)

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3 RESULTS AND DISCUSSION

3.1 X-ray

3.1.1 Angiographic procedures

The results reported by four groups on staff foetal dose from X-ray angiographic procedures are summarized in table 2.

Table2

Staff foetal dose from X-ray procedures. Summary of methods and results reported by four groups.

Reference Method Staff/phantom Irradiation

geometry Procedure Mean staff entrance dose per procedure (µGy)

Foetal equivalent dose per procedure (µSv)

Ratio equivalent dose to the uterus and entrance dose Kicken et

al (1999)

TLD at abdominal level, outer surface of a 0.5 mm lead apron

Operator Undercouch tube and overcouch image intensifier

Cerebral arteriography

Abdominal arteriography

Peripheral arteriography Percutaneous translumina angioplasty

22 58 51 75

240 (m.d.p.p)

0.4 1.2 1.0 1.5

4.8 (m.d.p.p)

0.018 0.021 0.019 0.02

0.02 (m.d.p.p) McParland

et al (1990)

TLD at waist level under the 0.25 mm lead apron

Cardiologist

Nurse

Undercouch tube and overcouch image intensifier

Cardiac

catherization 78 (max) 21 (min)

31 (max) 14 (min)

29 (max) 7.8 (min)

11 (max) 5.2 (min)

0.37 (max) 0.37 (min)

0.35 (max) 0.37 (min) Osei and

Kotre (2001)

1) Monte Carlo (EGS4)

Block- shaped phantom

Both of overcouch and undercouch tube

Fluoroscopy 0.038

(a, oc,o.l.a, 0.25mm Pb,MC) 0.37 ( a, oc, u.l.a, 0.25mm Pb,MC) 0.047 (a, uc,o.l.a,

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2a) TLD at waist;

secondary radiation

2b) TLD at waist both over and under 0.25mm lead apron;

primary radiation

Alderson female Rando anthropomorphic phan-tom

0.25mm Pb,MC) 0.37 ( a, uc, u.l.a, 0.25mm Pb,MC)

0.24-0.40 (u.l.a, 0.05- 0.35mmPb)

0.5 (u.l.a,0.25m mPb)

Faulkner and

Marshall (1993)

Film at chest and waist level under a lead apron;

secondary radiation

Alderson Rando phantom

Fluoroscopy 0.09 (oc,at

90kVp, u.l.a, c.l ) 0.14 (oc,at 90kVp,u.l.a w.l ) 0.35 (uc,at 90kVp, u.l.a, c.l ) 0.29 (uc,at 90kVp,u.l.a w.l ) m.d.p.p (the maximum dose per procedure), [4]

a, oc, o.l.a,0.25mm Pb; MC (average at tube potentials 61,90 and 112kVp, overcouch tube , over lead apron, 0.25 mm equivalent lead, Monte Carlo simulation), [6]

a, oc, u.l.a,0.25mm Pb; MC (average at tube potentials 61,90 and 112kVp, overcouch tube , under lead apron, 0.25 mm equivalent lead, Monte Carlo simulation), [6]

a, uc, o.l.a,0.25mm Pb; MC (average at tube potentials 61,90 and 112kVp, undercouch tube , over lead apron, 0.25 mm equivalent lead, Monte Carlo simulation), [6]

a, uc, u.l.a,0.25mm Pb; MC (average at tube potentials 61,90 and 112kVp, undercouch tube , under lead apron, 0.25 mm equivalent lead, Monte Carlo simulation), [6]

oc.at 90 kVp, u.l.a, c.l ; (overcouch tube, at 90 kVp tube potential, under lead apron and independent of lead apron thickness, at chest level), [7]

oc.at 90 kVp, u.l.a, c.l ; (overcouch tube, at 90 kVp tube potential, under lead apron and independent of lead apron thickness, at waist level), [7]

oc.at 90 kVp, u.l.a, c.l ; (undercouch tube, at 90 kVp tube potential, under lead apron and independent of lead apron thickness, at chest level), [7]

oc.at 90 kVp, u.l.a, c.l ; (undercouch tube, at 90 kVp tube potential, under lead apron and independent of lead apron thickness, at waist level), [7]

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Influence of irradiation geometry on foetal dose

The effect of irradiation geometry on foetal dose has been studied by Osei and Kotre (2001), and by Faulkner and Marshall (1993). Their results are illustrated in figures 3 a- b and 4, respectively.

a) Entrance dose measured in front of lead apron

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,00

0,05 0,10 0,15 0,20 0,25 0,30

Overcoach X-ray tube

Equivalent dose to uterus / entrance dose

Lead thickness [mm]

61 [kVp]

90 [kVp]

112 [kVp]

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,00

0,05 0,10 0,15 0,20 0,25 0,30

Undercoach X-ray tube

Equvalent dose to uterus / entrance dose

61 [kVp]

90 [kVp]

112 [kVp]

b) Entrance dose measured behind lead apron

0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,20

0,25 0,30 0,35 0,40

Overcoach X-ray tube

61 [kVp]

90 [kVp]

112 [kVp]

0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,20

0,25 0,30 0,35 0,40

Undercoach X-ray tube

61 [kVp]

90 [kVp]

112 [kVp]

Figure 3. Ratio between the equivalent dose to the uterus and entrance dose at waist level of a staff member standing beside the patient during a fluoroscopy procedure. Data were generated using Monte Carlo simulations [6]. Results using a 0.25 mm lead apron at 90 kVp (representative of the clinical situation) are indicated by the dotted lines.

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70 80 90 100 110 0,00

0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40

Equivalent dose to uterus / entrance dose

Tube potential [kVp]

c.l,u.l.a,oc w.l,u.l.a,oc c.l,u.l.a,uc w.l,u.l.a,uc

c.l,u.l.a,oc - chest level; under lead apron; overcouch x-ray tube w.l,u.l.a,oc - waist level; under lead apron; overcouch x-ray tube c.l,u.l.a,uc - chest level; under lead apron; undercouch x-ray tube w.l,u.l.a,uc - waist level; under lead apron; undercouch x-ray tube

Figure 4. Ratio between the equivalent dose to the uterus and entrance dose estimated from film badge readings at chest and waist level under a lead apron of a staff member standing beside the patient during a fluoroscopy procedure [7]. Results at 90 kVp are indicated by dotted lines.

The measurements by Osei and Kotre (2001) on an Alderson female Rando Phantom using TL-dosimeters yielded similar results to the Monte Carlo simulations. With the set-up depicted in figure 1 and including both over- and undercoach tube geometry, the ratio between equivalent dose to the uterus and entrance dose measured under a lead apron (0.05-0.35 mm Pb) was in the range 0.24-0.40.

From table 2 it can be concluded that all four groups report similar conversion factors between uterus dose and entrance dose with undercoach X-ray tube geometry. In case of entrance dose measurements performed with a detector positioned in front of a lead apron, the uterus dose is below 10% of the detector dose for typical beam qualities and with an apron thickness >0.25 mm Pb equivalence. With a detector positioned behind the protective lead apron, the uterus dose is always below 40% of the detector dose for apron thickness up to 0.35 mm Pb equivalence.

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3.1.2 Computed tomography procedures

In this series of measurements on trauma patients, the mean DLP value was 2705 ± 934 (1 STD) mGycm, and the staff dose burden coefficient was 1.6 ± 5.6*10^-4 (1 STD) nSv/mGycm. Assuming a DLP of 5000 mGycm per examination, and that the uterus dose amounts to 40% of the staff abdominal dose, such an examination would imply a foetal dose of approximately 3.2 µGy.

3.2 NUCLEAR MEDICINE

The results reported by four groups (including one private communication) on staff foetal dose from in nuclear medicine procedures are summarized in table 3.

Table 3.

Staff foetal dose from nuclear medicine procedures. Summary of methods and results reported by four groups (including one private communication).

Reference Method Staff/Phantom Irradiation source

Procedure Maximum effective doseto the mother from one patient (µSv)

Maximum effective dose to the foetus from one patient (µSv)

Ratio of effective dose to the foetus versus maternal dose¹ Mounford

and Steele (1995)

TLD - at abdominal level for different source to phantom distances, and at different depths.

Imaging technologist

Nurse (clinical ward)

99mTc

131I

99mTc

Bone Lever Dynamic renal

Thyroid ablation (w-b.c) Thyroid therapy (w-b.s)

1.53 0.23 0.35

8.9 11.8

155 63 29 4.7 2.3

1.12 0.17 0.26

6.7 9.0

86 37 18 3.3 1.6

0.73 0.74 0.74

0.75 0.76

0.55(pc.t.h) 0.58(pc.p.h) 0.62(pc.cf/bf) 0.70 (pc.s.a) 0.69 (pc.t.a) Barber et

al(2003)

Whole-body counter.

The iodine model of Berkovski

Cleaning staff Intake by ingestion

131I 0.003(f.s.0w)

0.003(f.s.5w)

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(1999) and NRPB(2001) used to calculate the foetal effective dose.

0.008(f.s.10w) 0.5 (f,s.15w) 1.42(f.s.25w) 2.30(f.s.35w)

Stather et al(2003)

Biokinetic and dosimetric models given in ICRP Publication 88 (2001) used to calculate the foetal dose coefficient.

Intake by inhalation

90Sr

131I

99m Tc

Intake by ingestion

90Sr

131I

99mTc

0.04 (b.c.-26) 0.10 (f.s.0 w) 0.31(f.s.25w) 0.26(f.s.35w)

0.01 (b.c.-26) 0.01 (f.s.0 w) 0.01(f.s10.w) 1.62(f.s.25w) 2.84(f.s.35w) 0.01 (b.c.-26) 0.01 (f.s.0 w) 0.02(f.s10.w) 0.02(f.s.25w) 0.02(f.s.35w)

0.03 (b.c.-26) 0.09 (c.0 w) 2.25(f.s.25w) 2.50(f.s.35w)

0.01 (b.c.-26) 0.01 (f.s.0 w) 0.01(f.s.10w) 1.55(f.s.25w) 2.73(f.s.35w)

0.01 (b.c.-26) 0.11 (f.s.0 w) 0.70(f.s.10w) 0.91(f.s.25w) 0.81(f.s.35w) Alvem

(2002) TLD- at the breast and abdominal level

Nuclear medicine nurse

99mTc Ratio breast

to

abdominal dose:

1.15±0.46 ( 1 STD)

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1 For external irradiation, the maternal dose corresponds to the abdominal surface dose; for internal irradiation it is given by the dose inhalated or ingested by the mother.

Data related to references [9],[10] and [11] were calculated from their corresponding published results.

w-b.s – ( whole- body scan) [ref]

pc.t.h – (patient category totally helpless) [9]

pc.p.h – (patient category partially helpless) [9]

pc.cf/bf – (patient category chair/bedfast) [9]

pc.s.a – (patient category semi -ambulant) [9]

ps.t.a – (patient category totally ambulant) [9]

intake at indicated time; negative times are prior to pregnancy [10]

b.c.-26 –acute intake 6 months before conception [10]

f.s.0 w – acute intake at time of conception [10]

f.s.25w - intake of 25 weeks of foetal stage [10],[11]

f.s.35w - intake of 35 weeks of foetal stage [10, [11]

STE – standart deviation of mean (standard error) [15]

3.2.1 External irradiation

Based on the results presented by Mountford and Steele (1995), the ratio of the effective dose to the foetus and the dose registered on the abdominal surface of the pregnant staff was calculated. The results for an imaging technologist performing patient studies were almost identical for both ^99mTc and ¹³¹I, with a foetal/maternal dose ratio close to 0.73- 0.75. For a nurse at a clinical ward, the corresponding ratio was in the range 0.55-0.70 depending on the caring needs of the patient (data only for ^99mTc). These results indicate that the foetal dose from external irradiation of the mother is almost independent on the type of nuclide and type of work task, and yield a mean ratio foetal/maternal effective dose close to 0.7 (0.69 ± 0.08 (1STD)) for a dosimeter positioned at waist level.

As regards the dependence of the dosimeter reading on its position on the trunk for nuclear medicine staff, Alvem (2002) reports a dose reading breast/abdomen ratio of 1.15± 0.46 (1 STD). This results in a mean ratio foetal/maternal effective dose for a dosimeter positioned at breast level (external irradiation) of about 0.6 (0.59 ± 0.12 (1STD)).

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3.2.2 Internal irradiation

Dependence of gestational age on foetal dose

The dependence of gestational age on the foetal versus maternal dose from inhalation and ingestion for different nuclides as reported by Stather et al (2003), (2002) is presented in figure 5a and 5b. The dose ratio for both ¹³¹I and ⁹⁰Sr display a significant dependence of gestational age. The sharp increase in the relative dose to the foetus for

131I and ¹²⁵I after the first trimester is related to the increased uptake once the thyroid gland is fully developed (Stather et al (2003), Stather et al (2002)). ⁹⁰Sr is chemically similar to calcium, and can cause a significant uptake in the skeleton towards the end of the gestational period. This is pronounced in case of ingestion of ⁹⁰Sr, in which case the placenta easily transmits this nuclide (Stather et al (2003), Stather et al (2002), Phipps et al (2003)). ^99mTc is a short lived radionuclide that does not cause large fetal doses, especially in case of inhalation, as the nuclide does not cross the placenta (ICRP publication (2000)). The foetal dose for ^99mTc is then derived from the radioactivity in maternal tissue. Comparing the two most commonly used radionuclides, ¹³¹I and^99mTc, it can be seen that for an acute maternal inhalation after the first trimester, ¹³¹I results in a higher effective dose ratio than ^99mTc. This reflects the different type of decay and half-life of these nuclides, with ^99mTc being a short-lived nuclide (half-life=6.02 h) and has a γ- dacay, while¹³¹I has a beta minus (β^-) decay and a significantly longer half-life (8 days).

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-30 -20 -10 0 10 20 30 40 0,0

0,5 1,0 1,5 2,0 2,5 3,0

Ratio of effectiv dose foetus/mother

Foetal stages [Week]

90Sr

99mTc

125I

131I

137Cs

Inhalation

Figure 5a. Ratio of the effective dose to the foetus and to a female staff member following an acute maternal inhalation of different nuclides [10], [19]. Foetal stage “0” indicates the moment of conception.

-30 -20 -10 0 10 20 30 40

0,0 0,5 1,0 1,5 2,0 2,5 3,0

Ratio of effective dose foetus/mother

90Sr

99mTc

125I

131I

137Cs Ingestion

Figure 5b. Ratio of the effective dose to the foetus and to a female staff memberfollowingto an acute maternal ingestion of different nuclides [10], [19]. Foetal stage “0” indicates the moment of conception.

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Dependence of intake scenario on foetal dose

The results of the calculation of the ratio of effective dose foetus/mother in case of ingestion of ¹³¹I, using the same dose coefficient data as in Barber et al (2003), are displayed in figure 6. Corresponding results reported by Stather et al (2003), using dose coefficient data from ICRP Publication 88, are included for comparison, together with their data on inhalation. As can be seen in figure 6, there is a discrepancy between the two data sets on ingestion. This is despite the fact that the dose coefficient data used by Barber et al (i.e., NRPB (2001)) are identical to the ICRP data (Publication 88). It is assumed that this reflects different techniques in maternal dose estimation. However, as Stather et al (2003) do not give details on how they estimate maternal dose, it was not possible to verify this assumption.

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40

0,0 0,5 1,0 1,5 2,0 2,5 3,0

Ratio of effective dose foetus/mother

Ingestion [Barber],[Calc by Narine G.]

Inhalation [Stather]

Ingestion [Stather]

Intake scenarios of ¹³¹I by inhalation and ingestion

Figure 6. Ratio between the effective dose to the foetus and to the mother at different gestational age due to internal contamination with ¹³¹I [10, 11]. Foetal stage “0” indicates the moment of conception.

Whole-body counter measurements of internal contamination as reported by Alvem (2002) for nuclear medicine nurses (^99mTc), and by Barber et al (2003) for cleaning staff (¹³¹I ) are summarized in figure 7.

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0 2 4 6 8 10 12 0,00

0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70

Detected activity [kBq]

Administreted activity [GBq]

99mTc (Nuclear medicine nurses)

131I (Cleaning staff) Internal contamination

Figure 7. Whole body counter measurements of internal contamination of cleaning staff [11], and nuclear medicine nurses [15], respectively, after changing clothes.

In the case of ^99mTc, the mean activity administered (2.5 GBq) was considered typical for nuclear medicine nurses during one day. The corresponding cumulative effective dose to the uterus during a pregnancy was estimated according to the method by Barber et al (2003), and using dose ingestion coefficients from ICRP Publication 88 (2001); [se table 4], together with maximum (0.32 kBq) and mean (0.11 kBq) internal contamination activity (Alvem (2002) [se table 5]).

Table 4

Dose ingestion coefficient in utero for ^99mTc and ¹³¹I (ICRP 88 [13]).

Dose ingestion coefficient [µSv/kBq]

Foetal stage (Week no)

99mTc ¹³¹I

0 0.0097 0.078

5 0.0094 0.081

10 0.015 0.21

15 0.017 12.0

25 0.017 34.0

35 0.016 55.0

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Table 5

The cumulative effective dose to the foetus from regular intake of ^99mTc by nuclear medicine nurses, based on dose ingestion coefficients and maximum and mean contamination activities during one day.

Effective dose from regular intake [µSv]

Foetus at

Week max - 0.32 [kBq] mean - 0.11[kBq]

5 0.018 0.005

10 0.038 0.012

15 0.064 0.021

25 0.091 0.030

35 0.117 0.039

In the case of ¹³¹I, corresponding dose estimations were performed for the maximum detected activity, that is, 0.68 kBq Barber et al (2003). According to Barber et al, their results should be considered “worst case” as regards internal contamination of cleaning staff working close to patients undergoing ¹³¹I therapy due to very strict cleaning routines in the UK. The result is shown in table 6.

Table 6

The cumulative effective dose to the foetus from regular intake of ¹³¹I by cleaning staff, based on the dose ingestion coefficient (see table 4, ICRP 88) and maximum contamination activities during one day.

Effective dose from regular intake [µSv]

Foetus at Week

max-0.68 [kBq]

5 0.27

10 0.76

15 21.5

25 177

35 480

The results show that the foetal dose due to internal contamination from ^99mTc for nuclear medicine staff is insignificant, and can be neglected at the time of estimating staff foetal doses. In case of internal contamination of ¹³¹I, the foetal dose can be considerably higher, and routines to estimate such dose burden to pregnant staff should be considered. A reasonable approach seems to be to exclude pregnant staff in the third trimester from working with ¹³¹I.

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4 CONCLUSION

X-ray

Radiation doses to staff in X-ray diagnostic radiology are in general low. Procedures that, under normal work conditions, potentially could yield doses in excess of the current foetal dose limit of 1 mSv during the full length of the pregnancy, are angiographic examinations (including interventions), and CT procedures on patients in trauma conditions.

Based on the publications included in this study, and including a “safety margin”, a dose ratio factor of 2 between the dose registered by a dosimeter positioned below a lead apron (at least 0.25 mmPb) and the dose to a foetus is suggested. This means that an accumulated dose during pregnancy (i.e. sum of dosimeter readings) of 2 mSv, measured behind a lead apron at the surface of the trunk (breast or waist level), is the threshold dose to a pregnant staff member working in X-ray diagnostic radiology in order to maintain the foetal dose below 1 mSv.

Computed tomography examinations on trauma patients often involve the assistance by staff not wearing dosimeters. The dose measurements performed during CT-scanning of the thorax-abdomen region of adult trauma patients in this work indicate a staff dose burden coefficient of approximately 1.6 nSv/mGycm (measured below lead apron at waist level). Assuming a maximum DLP of 5000 mGycm during which staff is standing beside the trauma patient, and applying a dose ratio factor of 2 for the dose to the foetus, would restrict the number of such examinations for a pregnant staff member to 250 during the pregnancy. This maximum DLP is almost twice the DLP estimated for such examinations in this study, and therefore yields a very conservative estimate of the maximum number of patients for which a pregnant member of staff can assist.

This recommendation naturally implies that the dose limits applied for other body parts, such as hands, are not exceeded, and that the pregnant staff member is not exposed to ionizing radiation while performing other work tasks.

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Nuclear medicine

In nuclear medicine, the foetal dose depends on if the irradiation of the mother is external or internal.

The foetal dose from external irradiation of the mother is almost independent on the type of nuclide (^99mTc, ¹³¹I) and type of work task (nuclear medicine nurse, cleaning staff), with a mean ratio foetal/maternal effective dose close to 0.7 (0.69 ± 0.08 (1STD)) for a dosimeter positioned at waist level. Corresponding data for a dosimeter positioned at breast level was about 0.6 (0.59 ± 0.12 (1STD)).

Foetal doses from internal contamination of the mother depend on the type of intake (inhalation/ingestion), type of radionuclide, and gestational stage of foetus. For ¹³¹I there is a very steep increase in the effective dose ratio foetus/mother in the period 10- 35 weeks post conception, starting with a dose ratio of about 1%, increasing to almost a factor of 3 at the end of the pregnancy. The results are almost identical for both inhalation and ingestion. For ^99mTc there is a marked difference between the foetal dose due to maternal inhalation compared to ingestion, with a very insignificant foetal dose from inhalation during the whole gestational period (about 1-2% of maternal dose), while ingestion yields substantial foetal dose starting from 10 weeks post conception (maximum about 90% of maternal dose). This difference is most likely related to that intake by ingestion of ^99mTc has the potential to cross the placenta, and thereby contributing substantially to foetal dose.

In general, staff foetal dose from internal contamination of ^99mTc can be neglected, and only external irradiation needs to be considered. In this case, an accumulated dose during pregnancy (i.e. sum of dosimeter readings) of 1.5 mSv, measured behind a lead apron at the surface of the trunk (breast or waist level), can be used as the threshold dose to a pregnant nurse in nuclear medicine, in order to maintain the foetal dose below 1 mSv. In case of ¹³¹I, internal contamination can no longer be neglected, especially during the third trimester of pregnancy. As it is difficult to properly monitor internal contamination, it is suggested that pregnant staff do not participate in work with ¹³¹I starting from the third trimester until the end of the pregnancy. This is especially important in case of cleaning staff caring for patients undergoing ¹³¹I therapy. Following such a recommendation, the reported threshold dose of 1.5 mSv), should ensure that the foetal dose is within the 1 mSv limit.

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5 ACNOWLEDGEMENTS

I am very grateful to my main supervisor, Dr. Annette Fransson, for her guidance and encouragement, and to my co-supervisor Dr.Linda Persson for introducing and helping me in understanding work and measurements in nuclear medicine and for interesting conversations.

This diploma work would never have been accomplished without the help from these people. It has been interesting and enjoyable working with them.

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6 REFERENCES

1. Alvem C (private communication) Occupational effective dose estimates between two different posions of TLD-chips, on the abdominal and in the breast level. 2002.

2. Barber R W, Parkin A and Goldstone K E Is it safe to work with iodine-131 when you are pregnant? A risk assessment for nuclear medicine staff involved with cleaning and decontamination. Nuclear Medicine Communications 2003 (24: 571- 574).

3. Berkovski V Radionuclide biokinetics in the mother and fetus: part 1: pregnant women and part 2: fetus. In: Radiation and thyroid cancer. Publication No. EUR EN 18552 EN of the European Commission. 1999 (319-332).

4. Fattibene P, Mazzei F, Nuccetelli C and Risica S Prenatal exposure to ionising radiation: sources, effects and regulatory. Acta Peadiatr.1999 (88:693-702).

5. Faulkner K and Marshal N M Pesonal monitoring of pregnant staff in diagnostic radiology. Journal of Radiology Protection 1993 (13: 259-265)

6. ICRP Publication 53 Radiation dose to the patients from radiopharmaceuticals.

Ann.ICRP 28(3) 1998. Pergamon Press, Oxford.

7. ICRP Publication 80 Radiation dose to the patients from radiopharmaceuticals.

Ann.ICRP 28(3) 1998. Pergamon press, Oxford.

8. ICRP Publication 84 Pregnancy and Medical Radiation. Ann.ICRP 30(1) 2000.

9. ICRP Publication 88 Dose to the embryo and fetus from intakes of radionuclides by the mother. Ann.ICRP 31(103) 2001.

10. Isopharma AS (IFETEC code: MO.3S.) Instituttveien 18 P.B 65, N-2027 Kjeller, Norway.

11. Kicken H, Kemerink G J and van Engelshoven J M A Dosimetry of occupationally exposed persons in diagnostic and interventional arteriography. Part I: Assessment of Entrance Doses. Radiation Protection Dosimetry 1999(82:93-103).

12. Mcparland B J, Nosil J and Burry B A survey of the radiation exposures received by the staff at two cardiac catheterization laboratories. The British Journal of

Radiology 1990(63:885-8).

13. Mountford P J and Steele H R Fetal dose estimates and the ICRP abdominal dose limit for occupational exposure of pregnant staff to technetium –99m and iodine- 131 patients. European Journal of Nuclear Medicine 1995 (22: 1173-1179).

14. Osei E K and Kotre C J Equivalent dose to the foetus from occupational exposure of pregnant staff in diagnostic radiology. The British Journal of Radiology 2001(74:

629-637).

15. Osei E K and Faulkner K Fetal position and size data for dose estimation. The British Journal of Radiology 1999 (72:363-370).

16. Osei E, Darko J, Faulkner K and Kotre J Software for the estimation of foetal radiation dose to patients and staff in diagnostic radiology. Journal of Radiology Protection 2003 (23: 183-194).

17. Parisi A, Bonanni A and Loppa A Rapporto delle dosi da radiazioni ionizzanti ricevute dai lavoratori degli istituti pubblici di ricovero e cura. Prevenzione oggi 1994 (83-123).

18. Phipps A W, Harrison, J D, Fell T P, Eckerman K F and Nosske D Some aspects of the fetal doses given in ICRP Publication 88. Radiation Protection Dosimetry 2003 (105, Nos 1-4, 279-284).

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19. Phipps A W, Smith T J, Fell T P and Harisson J D Dose to the embryo/ fetus and neonate from intake of radionuclides by mother. NRPB contrasct research report 397/2001. Didcot, Oxon: National Radiological protestion board (NRPB), 2001.

Available on website http://www.hse.gov.uk/research/crr_pdf/2001/crr 01397.pdf.

20. Swedish Radiation Protection Authority Regulations SSI FS 1998:4.

21. Stather J W, Phipps A W, Harrison J D, Eckerman K F, T J Smith Dose coefficient for the embryi and foetus following intakes of radionuclides by the mother. Journal of Radiology Protection 2002 (7-24).

22. Stather J W, Phipps A W, Smith T J and Fell T P Dose coefficients for the embryo and fetus following intakes of radionuclides by the mother. Radiation Protection Dosimetry 2003 (105: 257-264).

Estimation of foetal radiation dose to occupationally exposed staff in diagnostic radiology and nuclear medicine.

Contents

1 INTRODUCTION

2 MATERIAL AND METHODS

∑

3 RESULTS AND DISCUSSION

4 CONCLUSION

5 ACNOWLEDGEMENTS

6 REFERENCES