Evaluation of Eye Lens Doses
Received by Medical Staff
Working in Interventional
Radiology at Sahlgrenska
University Hospital
M.Sc. Thesis
Viktor Sandblom
gussanvi@student.gu.se
Supervisors:
Charlotta Lundh
Åke Cederblad
Pernilla Jonasson
Department of Radiation Physics
University of Gothenburg
Gothenburg, Sweden
January 2012
ABSTRACT
Background: The International Commission on Radiological Protection (ICRP) recently
lowered their recommended occupational eye lens dose limit from 150 mSv in a year to 20
mSv in a year, averaged over a defined period of 5 years.
Aim: The main aim of the present study was to investigate the eye lens doses received by
the interventional staff at Sahlgrenska University Hospital.
Another aim was to evaluate whether the dose recorded by the PDM, a dosimeter worn at
thorax height, could be used as an indicator of eye lens dose.
Material and Methods: To prepare for the personnel eye lens dose assessments, phantom
measurements were carried out. These preparations included deciding the optimal position
of an eye lens dosimeter, evaluation of different models of lead glasses and estimation of a
ratio between the eye lens dose and the dose recorded by the PDM.
Personnel eye lens doses were assessed using TL-dosimeters held by individual headbands
worn by staff members at the Catheterization Laboratory at the department of Cardiology
and at the division of Peripheral Interventional Radiology at the department of Radiology
at Sahlgrenska University Hospital for one month. Staff members also wore a PDM at
thorax height outside their lead apron.
Results and Discussion: Materials with an equivalent lead thickness of 0.75 mm block
over 95% of incident radiation but the lead glasses evaluated (0.75 mm) reduced the eye
lens dose by only 30-88% due to radiation back scattered in the head and oblique incident
radiation.
Many operators reported annual eye lens doses of over 10 mSv. The eye lens doses of
nurses were generally much lower. The ratios between equivalent doses recorded by TLD
and PDM were as expected higher for nurses than for operators but the ratios varied also
between different operators.
Conclusions: Even though the estimations of annual doses have uncertainties, it can be
seen that the lens doses of operators are high enough for concern. Based on the results of
the present study, operators might be recommended to wear lead glasses in the future.
Estimating eye lens dose from the dose recorded by the PDM is difficult. This method
includes large uncertainties. The PDM should instead be used to indicate whether a more
precise measurement is necessary.
ABBREVIATIONS
dm
The number of days of the measurement period for a given staff member
DTLD
The equivalent dose recorded by the TLD, for a given staff member, during
the measurement period
DAS
DoseAware system
EDD
Educational Direct Dosimeter
ICRP
International Commission on Radiological Protection
ICRU
International Commission on Radiation Units and Measurements
ISO
International Organization for Standardization
nm
The number of procedures, when the PDM (and headband) was worn,
performed by a given staff member during the measurement period
ny
The number of procedures performed by a given staff member during the
last year
ORAMED
Optimization of Radiation Protection for Medical Staff
PDM
Personal Dose Meter
PSC
Posterior subcapsular cataract
SDD
Source-to-detector distance
SSD
Source-to-skin distance
SSM
Strålsäkerhetsmyndigheten (Swedish Radiation Safety Authority)
TLD
Thermoluminescent dosimeter
TABLE OF CONTENTS
1
INTRODUCTION
1
1.1 Aims..........................................................................................................................................1
1.2 New recommendations from the ICRP.................................................................................... 1
1.3 Previous eye lens dose estimations.......................................................................................... 2
1.4 Cataract.....................................................................................................................................2
1.4.1 Mechanisms of radiation cataractogenesis.......................................................................3
1.5 Definition of the personal dose equivalent Hp(d).................................................................... 3
1.6 The interventional radiology procedure................................................................................... 4
1.7 Parameters influencing personnel eye lens doses.....................................................................5
1.7.1 Tube potential and tube current........................................................................................ 6
1.7.2 Patient size and projection angle...................................................................................... 6
1.7.3 Source-to-detector and source-to-skin distance................................................................6
1.7.4 Collimation and magnification......................................................................................... 7
1.7.5 Factors connected to the working method of the operator............................................... 7
1.8 Radiation detection instruments............................................................................................... 7
1.8.1 The EDD-30...................................................................................................................... 7
1.8.2 The DoseAware system......................................................................................................8
1.8.3 Thermoluminescent dosimeters.........................................................................................8
2
MATERIAL AND METHODS
8
2.1 Phantom measurements............................................................................................................ 8
2.1.1 Position of the dosimeter...................................................................................................8
2.1.2 Lead glasses.................................................................................................................... 10
2.1.3 Angular response of the PDM and the EDD-30..............................................................12
2.1.4 Phantom based assessment of the ratio between doses at eye and thorax height...........13
2.2 Personnel eye lens dose assessments......................................................................................15
2.2.1 The Catheterization Laboratory at the department of Cardiology.................................16
2.2.2 The division of Peripheral Interventional Radiology at the department of Radiology...16
2.2.3 Data analysis...................................................................................................................16
3
RESULTS
17
3.1 Phantom measurements.......................................................................................................... 17
3.1.1 Position of the dosimeter.................................................................................................17
3.1.2 Lead glasses.................................................................................................................... 17
3.1.3 Angular response of the PDM and the EDD-30..............................................................20
3.1.4 Phantom based assessment of the ratio between doses at eye and thorax height...........21
3.2 Personnel eye lens dose assessments......................................................................................22
3.2.1 The Catheterization Laboratory at the department of Cardiology.................................23
3.2.2 The division of Peripheral Interventional Radiology at the department of Radiology...23
4
DISCUSSION
24
4.1 The use of Hp(0.07) for measuring lens dose.........................................................................24
4.2 Phantom measurements.......................................................................................................... 25
4.2.1 Position of the dosimeter.................................................................................................25
4.2.2 Lead glasses.................................................................................................................... 26
4.2.3 Angular response of the PDM and the EDD-30..............................................................27
4.2.4 Phantom based assessment of the ratio between doses at eye and thorax height...........28
4.3 Personnel eye lens dose assessments......................................................................................29
4.3.1 Systematic errors and stochastic uncertainties in the DTLD values..............................29
4.3.2 Uncertainties in the annual eye lens dose estimations....................................................29
4.3.3 Recommendation of lead glasses.................................................................................... 30
4.3.4 Using the PDM as an eye lens dose estimator................................................................30
4.3.5 Lens doses in relation to current dose limits...................................................................31
4.3.6 Consistency with other studies........................................................................................ 31
5
CONCLUSIONS
32
6 ACKNOWLEDGEMENTS
33
REFERENCES
34
1 INTRODUCTION
Medical staff working in interventional radiology have relatively high exposure to
radiation compared to other occupational groups working with X-rays [1]. Naturally,
radiation protection is a matter of interest in interventional radiology.
The International Commission on Radiological Protection (ICRP) recently made a
statement about their recommendation of the maximum eye lens dose, due to new insights
about radiation-induced cataract. The recommended equivalent dose limit for the eye lens
is now 20 mSv in a year, averaged over a defined period of 5 years, with no single year
exceeding 50 mSv. The previous recommendation given in ICRP Publication 103 was 150
mSv/year. Also the estimation of a threshold lens dose for radiation-induced cataract have
been lowered. For acute exposures the threshold is now considered to be 0.5 Gy instead of
previously 5 Gy [2].
1.1
Aims
The main aim of the present study was to investigate the eye lens doses received by the
interventional staff at Sahlgrenska University Hospital and from this evaluate whether
additional radiation protection, e.g. lead glasses, could be necessary.
Another aim was to evaluate whether the dose recorded by the PDM, in Philips DoseAware
system, could be used as an indicator of eye lens dose. This would be of great use for
future monitoring of eye lens doses.
1.2
New recommendations from the ICRP
The ICRP has reviewed recently published studies indicating a lower eye lens threshold
dose for cataract than previously used. For example, a study published in 2009 has
reviewed eight epidemiological studies that estimated odds ratios or relative risk for
developing cataract after an absorbed eye lens dose of 1 Gy or 1 Sv [3]. These eight studies
are based on atomic bomb survivors [4, 5], Chernobyl clean-up workers [6] and pilots but
also clinical or occupational exposure. All studies indicate an increased risk of developing
cataract at 1 Gy. The studies that presented threshold doses for developing cataract all
presented values far below the previous ICRP threshold of 5 Gy [7].
These recently published studies have longer follow-up periods than previous studies and
could therefore show that the latency period depends on absorbed lens dose. The lower the
1
lens dose, the longer the latency period (given that cataract will be developed). The latency
period is approximately inversely proportional to the radiation dose [3, 7]. Earlier studies
that influenced the ICRP recommendations were based on too short follow-up periods and
thus indicated a higher threshold dose for developing cataract; the latency period for
individuals developing cataract after receiving low doses were longer than the follow-up
periods [8].
In Sweden, occupational dose limits are set by the Swedish Radiation Safety Authority
(SSM). The regulations of SSM have not yet been affected by the new ICRP
recommendation. The equivalent dose limit for the lens of the eye of medical staff working
with ionizing radiation in Sweden is 150 mSv in a year [9]. However, it is likely that SSM
will lower the dose limit and that the new dose limit will follow the recommendation of the
ICRP.
1.3
Previous eye lens dose estimations
An estimation of eye lens doses of interventional staff at the Catheterization Laboratory at
the department of Cardiology at Sahlgrenska University Hospital, Gothenburg, Sweden,
was made in 2005. From this estimation it was concluded that the doses were far below
150 mSv/year and that no more precise measurement was necessary. However, when the
ICRP lowered their recommended dose limit a more thorough investigation of eye lens
doses at Sahlgrenska became necessary. Furthermore, an Europe-wide project called the
ORAMED project investigated eye lens doses of interventional staff at 34 European
hospitals and, for example, it was concluded that 7 out of 15 operators performing cardiac
angiographies and angioplasties (two commonly performed procedures at the department
of Cardiology) exceeded an annual eye lens dose of 20 mSv [10].
1.4
Cataract
Cataract is a clouding of the lens of the eye causing impaired vision. It is one of the most
common causes of blindness worldwide and is usually classified into three main forms:
nuclear, cortical and posterior subcapsular cataract (PSC) [8].
Symptoms associated with cataract are blurry vision, reduced night vision, sensitivity to
light, seeing halos around objects or lights, reduced contrast sensitivity and double vision.
Possible causes of cataract are diabetes, exposure to ultraviolet light, the use of steroids,
exposure to ionizing radiation, high body mass index, age, tobacco use, alcohol use,
2
previous eye injuries and premature birth. The only possible treatment for cataract is
surgery, where the lens is replaced by an artificial intraocular lens. In the United States the
success rate for such an operation is about 98% [11, 12].
1.4.1 Mechanisms of radiation cataractogenesis
Age related cataract rarely appears in the form of PSC. Instead this form of cataract is
often associated with radiation exposure. In addition, relatively minor PSC can have great
effect on vision impairment [8].
A single layer of epithelial cells is located on the anterior of the lens. These cells divide
and some cells differentiate into mature lens fibre cells. Lens transparency is mainly
dependent on the fact that this layer of epithelial cells is intact and correctly can
differentiate into lens fibre cells. It is likely that disruption of this epithelial cell layer leads
to cataract [7].
It has been shown in animal studies that radiation-induced cataract will not form if the lens
epithelial cells are completely inhibited from cell division. This is one indicator that
cataract is not caused by cell killing but rather that genomic damage results in altered cell
division causing cataract over time. The lens epithelial cells with radiation-induced DNA
damage is believed to be a first step to cataractogenesis. However, more research is
necessary for concluding the true mechanism of radiation induced cataract [7].
The question remains whether radiation-induced cataract is a deterministic or stochastic
effect. Historically it has been considered a deterministic effect [7]. Many recently
published studies have questioned the existence of a threshold dose and instead suggested
that radiation-induced cataract is of a stochastic nature [6, 13 - 16]. There are also studies
14
15
supporting the theory of it being a deterministic effect [17 - 19].
14
18
Radiation-induced lens cataract is currently considered a deterministic effect with a low
dose threshold by the ICRP, who in their latest publication on the subject stated: “Lastly,
although the lower 95% confidence interval in some threshold calculations includes zero
dose, there is no direct evidence that a single damaged progenitor lens epithelial cell can
produce a cataract, and hence radiation-induced lens cataract is still considered a tissue
reaction (deterministic effect) with a dose threshold albeit small” [7].
1.5
Definition of the personal dose equivalent Hp(d)
The personal dose equivalent, Hp(d), is an operational quantity defined by the equivalent
3
dose in ICRU soft tissue material at the depth d (mm) of the human body below the point
of the dosimeter. For assessments of the effective dose, H p(10) is used. To determine
equivalent skin dose, Hp(0.07) is used. For measuring equivalent eye lens dose, H p(3) is
recommended by the ICRP and the ICRU [20, 21].
1.6
The interventional radiology procedure
During an interventional radiology procedure multiple medical conditions can be
diagnosed and/or treated. A small incision is made in which catheters are inserted and
advanced to the source of the condition via blood vessels. An X-ray unit and monitors are
used to visualize the position of the catheter and the anatomy of the patient (other imaging
modalities can also be used). An interventional procedure is minimally invasive and the
disease can be diagnosed and/or treated non-surgically.
There are usually three staff members in the room, one operator and two nurses. The
operator and one of the nurses are dressed in sterile aprons. They are standing close to the
patient table. The operator is standing closest to both the irradiated part of the patient and
the catheter insertion site (the operator is handling the catheters). The second nurse, who is
not dressed in a sterile apron, is assisting both the patient and the other staff members. This
nurse moves around in the room or is stand-by just outside the room during the procedure.
The number and the position of staff members can vary with different procedures, although
the operator handling the catheters is always closer to the irradiated part of the patient than
the nurses. A standard angioplasty procedure (commonly performed at the department of
Cardiology) can be seen in Figure 1.1.
4
Figure 1.1: Image showing a typical angioplasty procedure set-up
and position of the operator (the person behind the ceiling
suspended shield on the right). The nurse dressed in a sterile
apron can not be seen in the image but is usually standing directly
next to the operator. Photography by Curt Warås.
Staff members at the Catheterization Laboratory at the department of Cardiology almost
exclusively perform angiographies and angioplasties; the set-up and operator position
rarely differ from what can be seen in Figure 1.1. Staff members at the division of
Peripheral Interventional Radiology at the department of Radiology on the other hand use
different positions, both in relation to the patient and the X-ray tube, for almost every
procedure.
1.7
Parameters influencing personnel eye lens doses
Most parameters influencing patient doses also influence personnel doses and
consequently also personnel eye lens doses. When considering personnel eye lens dose, the
5
irradiated part of the patient can be seen as the radiation source; photons contributing to
this dose that are not scattered in the patient can be neglected. Both an increased amount of
photons scattered in the patient and the energy of these photons will contribute to a higher
eye lens dose. Minimizing the patient dose is important both in consideration to patient and
personnel.
1.7.1 Tube potential and tube current
Tube potential and tube current are closely related due to automatic output adjustment. The
X-ray unit can automatically adjust the tube current and tube potential setting. The X-ray
unit is set at a fixed “dose to image detector” value. This is important to get sufficient
image quality for the body part examined. Either the tube current, the tube potential or
both can be changed to achieve this. How this is done is dependent on several factors, e.g.
X-ray unit settings. Nevertheless, both increased tube current and increased tube potential
result in increased personnel eye lens dose due to an increased amount of scattered
radiation and an increased energy of these photons, respectively.
1.7.2 Patient size and projection angle
The amount (and the energy) of scattered photons, and thus also personnel eye lens dose,
are strictly related to the radiation penetration distance for a given projection angle. Three
factors that influence this distance are patient size, irradiated body part and projection
angle. The greater the radiation penetration distance, the higher the personnel eye lens dose
due to an increased output. Another aspect of projection angle should also be taken into
account. For X-ray energies, most of the radiation is back scattered. The radiation exposure
will therefore be much higher if the operator is next to the X-ray tube instead of next to the
image detector. For example, an over couch X-ray tube configuration can mean as much as
27 times higher eye lens dose than an under couch X-ray tube configuration [22].
1.7.3 Source-to-detector and source-to-skin distance
The inverse-square law explains why the dose delivered to the image detector will
decrease with increased source-to-detector distance (SDD). This means that for an
increased SDD, due to automatic output adjustment, the tube current and/or the tube
potential will be increased. Another consequence of an increased SDD is collimation of the
X-ray field to still fit the image detector. These two effects will cancel and the personnel
eye lens dose and effective dose of the patient will remain unaffected. The maximum skin
dose of the patient, on the other hand, will be increased.
6
If the source-to-skin distance (SSD) decreases while SDD remains constant the patient will
be moved further away from the image detector. Photons scattered in large angles in the
patient will then not reach the image detector and the dose delivered to the image detector
will decrease. The personnel eye lens dose will then increase, due to automatic output
adjustment.
1.7.4 Collimation and magnification
When a collimated X-ray field hits the image detector it can either be projected onto the
full area of the monitor or onto a reduced area, proportional to the reduced X-ray field
area, of the monitor.
The first of these two options will result in a magnified image. To maintain the same image
quality, the output will automatically be increased. This will result in an increased
maximum patient skin dose while the patient effective dose and the total amount of
scattered photons will be somewhat unchanged, depending on fluoroscopy system.
The second of these two options will result in a collimated image, i.e. a smaller image with
some information simply cut out. The output will then not be increased and the amount of
scattered photons will be reduced proportionally to the reduced X-ray field area.
In summary, collimation of the X-ray field will decrease personnel eye lens dose while
magnification has minor effect.
1.7.5 Factors connected to the working method of the operator
Three factors that are more intuitive to understand and therefore perhaps easier to forget
when considering parameters influencing personnel eye lens dose are fluoroscopy time,
distance from patient and the use of radiation protection, e.g. lead glasses or ceiling
suspended shields. The personnel eye lens dose increases linearly with fluoroscopy time
and decreases with the square of the increased distance from irradiated part of the patient,
according to the inverse-square law.
1.8
Radiation detection instruments
Various kinds of radiation detection instruments were used for the present study. This
section contains a short description of these instruments.
1.8.1 The EDD-30
The Unfors EDD-30 (Educational Direct Dosimeter) consists of a display unit and a sensor
7
connected with a cable. It is calibrated in terms of H p(0.07) and can measure equivalent
dose, equivalent dose rate and exposure time.
1.8.2 The DoseAware system
The DoseAware system (DAS), developed by Philips and Unfors Instruments, is installed
in five interventional radiology rooms at Sahlgrenska University Hospital. The detector
unit of this system is called Personal Dose Meter (PDM) and is worn on the torso outside
the lead apron. The equivalent dose at a depth of 10 mm (i.e. an estimation of the effective
dose if no lead apron is used), H p(10), is measured and the dose rates are available to the
interventional staff live on a monitor. Also the accumulated personal dose is recorded.
1.8.3 Thermoluminescent dosimeters
The TL-dosimeters1 used for personnel eye lens dose assessments consists of a polyimide
film (Kapton) containing a LiF:Mg, Ti TLD-chip (3 mm 2, 0.38 mm thick). When used for
dosimetry, these films are put into a thin protective bag. These TL-dosimeters are normally
used for extremity dosimetry at Sahlgrenska University Hospital. Compared to other
available TL-dosimeters, the design and casing of these dosimeters made them best suited
for the present study. After exposure, the TLDs were read out in the TLD-reader Harshaw
Model 6600 plus, Thermo Scientific.
2 MATERIAL AND METHODS
2.1
Phantom measurements
In order to prepare for the personnel eye lens dose assessments at Sahlgrenska University
Hospital phantom measurements were carried out. These preparations included deciding
the optimal position of the dosimeter used for the eye lens dose assessments, evaluation of
different models of lead glasses and estimation of a ratio between the eye lens dose and the
dose recorded by a dosimeter worn at thorax height.
2.1.1 Position of the dosimeter
The position of the TLD is an important matter when measuring eye lens dose. For
example, a dosimeter positioned on the eyebrow ridge on the side of the X-ray tube can
measure a dose 3-5 times higher than a dosimeter between the eyes [20].
1 Single chipstrate EXT-RAD, Type TLD-100, Thermo Scientific
8
A standard interventional procedure at the department of Cardiology was simulated using a
Philips BV 300 X-ray unit (image detector diameter 31 cm), an anthropomorphic
abdominal phantom2 simulating a patient and a head phantom on a tripod simulating the
operator, see Figure 2.1. The head phantom consisted of a human cranium and tissue
equivalent plastic to simulate a human head. The fluoroscopy time was 60 s, the tube
potential was 82 kV and the tube current was 2.82 mA. A circular field with a diameter of
31 cm (at the image detector) was used. The same X-ray unit settings were used for all
phantom measurements (with exception for the fluoroscopy time in section 2.1.3 and
2.1.4 ). The head phantom was angled straight forward, simulating an operator looking at
the monitor, with an angle of approximately 60º relative to the irradiated part of the
abdominal phantom. No lead glasses were used.
Figure 2.1: The set up used for dosimeter position measurements both seen from the front (a) and
from above (b).
The equivalent doses at 8 different positions across the forehead, see Figure 2.2, were
2 Transparent abdominal phantom, RSD Model RS-113T, Gammex RMI, USA
9
assessed using an Unfors EDD-30.
Figure 2.2: Image showing the 8 different EDD-30 positions used.
The different positions are represented by the dots.
2.1.2 Lead glasses
The dose reduction ability of 12 different models of lead glasses, 11 provided by Scanflex
Medical and one provided by Mediel, was investigated, see Table 2.1 and Figure 2.3. All of
the lead glasses had a specified equivalent lead thickness of 0.75±0.05 mm. Lead glasses
number 11 and 12 can be worn with a pair of regular glasses underneath, although when
worn without they also fit well. The same measuring set up as in section 2.1.1 was used
with exception for the abdominal phantom. An anthropomorphic thorax phantom 3 (of the
same height as the abdominal phantom) was instead used to simulate a patient, the
distances in Figure 2.1 still applies. The EDD-30 was placed at position 1 (in Figure 2.2).
Measurements were made for two different head phantom angles; one when the head
phantom was angled straight forward, simulating an operator looking at the monitor, one
when the head phantom was angled obliquely down and to the left, simulating an operator
looking at the irradiated part of the patient. Equivalent doses were recorded with and
without each of the 12 models of lead glasses. For number 11 and 12, the equivalent dose
was measured both with and without space for regular glasses underneath. The dose
reduction ability (%) was then calculated for each of the 12 lead glasses, using equation
(2.1).
Dose reduction ability=(1−
Dose recorded with lead glasses
)⋅100
Dose recorded without lead glasses
3 RSD Torso Imaging Phantom, Radiology Support Devices, Inc., Long Beach, USA
10
(2.1)
Table 2.1: List of models of lead glasses evaluated. All models
had a specified equivalent lead thickness of 0.75±0.05 mm.
Number as referred to in
Product name of model
this report
1
APX Ray BX04 (Scanflex)
2
APX Ray TX01 (Scanflex)
3
APX Ray TX07 (Scanflex)
4
APX Ray AX06 (Scanflex)
5
APX Ray AX10 (Scanflex)
6
9935 Ultralite Small (Scanflex)
7
9935 Ultralite Large (Scanflex)
8
9941 Ultralite (Scanflex)
9
99 Ultralite (Scanflex)
10
58 Jarrod (Scanflex)
11
89 Fitover (Scanflex)
12
Vista (Mediel)
Figure 2.3: Image showing the designs of the different models of lead glasses. Lead glasses number 11 and
12 can be worn with a pair of regular glasses underneath.
To determine whether 0.75 mm equivalent lead thickness is enough to block incident
radiation the influence on the equivalent dose of photons penetrating the lead glasses was
investigated. An additional 5 mm lead was added for measurements with lead glasses
11
number 1. The 5 mm lead was manufactured to fit the lead glass part of the glasses only.
The head phantom was angled towards the irradiated part of the patient. The 60 s
measurement was repeated 10 times and mean values of equivalent doses with and without
an additional 5 mm lead were calibrated.
The effect on dose reduction ability of changing the tube potential was also investigated.
The head phantom was angled towards the irradiated part of the patient. The EDD-30 was
used to record equivalent doses from the lowest tube potential that gave a measurable
signal to the highest possible tube potential of the X-ray unit. Measurements were made
with and without lead glasses number 12 at 77, 80, 82, 85, 90, 95, 100 and 110 kV. At tube
potentials 82 and 110 kV an additional four measurements were made to statistically test
the difference in dose reduction ability.
An approximation of the actual equivalent lead thickness of each pair of the lead glasses
was measured using a collimated
241
Am radiation source and a Geiger counter (Scaler
ratemeter type 6-90) calibrated for different thicknesses of lead at a fixed SDD of 10 cm.
2.1.3 Angular response of the PDM and the EDD-30
During an interventional procedure, the upper body of a staff member is not always facing
towards the irradiated part of the patient. This means that the incident angle of scattered
radiation hitting the PDM will vary and the angular response of the PDM is a matter of
interest. According to Unfors Instruments the PDM has a specified horizontal angular
response of ±5% within ±5°, ±30% within ±50° and +200%/-100% within ±90°. Since the
EDD-30 was used for phantom measurements containing oblique incident angles it was
also included in the angular response measurements.
A lead apron was wrapped around the thorax phantom to make the amount of back
scattered photons as realistic as possible. Both a PDM and an EDD-30 was placed on the
thorax phantom outside the lead apron at approximately the same height as the abdominal
phantom (which was simulating a patient). A fluoroscopy time of 30 s was used here and in
section 2.1.4 . Equivalent doses were recorded for 11 different incident angles of scattered
radiation hitting the thorax phantom from -90° to 90°. The angles used were -90°, -80°,
-70°, -60°, -30°, 0°, 30°, 60°, 70°, 80° and 90°. To acquire these incident angles, both the
dosimeters and the thorax phantom were rotated around the point of the dosimeters. For
the incident angles -90° and 90°, the radiation was hitting the dosimeters straight from the
right and from the left, respectively. For the incident angle 0°, both the thorax phantom and
12
the dosimeters was facing the abdominal phantom. The distance from the center of the
primary field hitting the abdominal phantom to the dosimeter was 80 cm for all incident
angles. Relative equivalent doses were calculated as the equivalent dose for a given
incident angle relative to the dose recorded at 0°.
Measurements in the 11 incident angles were also made when the PDM and the EDD-30
were turned 90° counterclockwise to investigate the vertical angular response of the
dosimeters rather than the horizontal. Two measurements were made for each incident
angle and dosimeter rotation, the mean value and the standard deviation were calculated.
2.1.4 Phantom based assessment of the ratio between doses at eye and thorax height
As mentioned in section 1.1 one of the aims of the present study was to evaluate whether
the PDM from Philips DoseAware system could be an indicator of personnel eye lens dose.
A ratio between equivalent doses at eye and thorax height was estimated by phantom
measurements.
The abdominal phantom was placed on the table to simulate a patient with the same patient
table and X-ray unit height as in Figure 2.1(a). To simulate a staff member, the thorax
phantom was set up with the head phantom on top, see Figure 2.4. A lead apron was
wrapped tightly around the thorax phantom.
Figure 2.4: Image showing the measurement set up.
13
Dosimeters were placed at eye and thorax height. One EDD-30 was placed on the left eye
of the head phantom and at thorax height on the left side one EDD-30 and one PDM was
placed. The difference in sensitivity of the two EDD-30s were tested and corrected for. As
mentioned under section 1.8 the EDD-30s were calibrated to measure H p(0.07) while the
PDM was calibrated to measure Hp(10).
During an interventional procedure, the operator adjusts the table height to an ergonomic
working height. This means that the distance from the patient to the eyes of the operator is
approximately the same for any operator height. The eyes of a nurse, who is shorter or
taller than the operator, will then either be closer to or further away from the patient. To
evaluate the influence of the nurse height on the ratio, two different nurse heights, in
addition to the operator height, were used during the measurements. A normal operator
working situation was set up with a vertical distance of 30 cm from the abdominal
phantom to the EDD-30 placed on the eye of the head phantom. To simulate a nurse,
shorter or taller than the operator, standing at the operator's position, the height of the
phantoms simulating a staff member was changed. The same patient table and X-ray unit
height was used for all three staff member heights. The three different staff member
phantom heights used resulted in a vertical distance between the abdominal phantom and
the EDD-30 placed on the eye of 20, 30 and 40 cm, respectively. These three staff member
phantom heights will hereafter be referred to as “short nurse”, “operator” and “tall nurse”.
Equivalent doses were recorded at four different distances from the irradiated part of the
patient, see Figure 2.5. In addition to the three different staff member heights two different
head angles were also used for each of these four distances, simulating the staff member
looking at the catheter insertion site (see Figure 2.4) and at the monitor. To simulate the
staff member looking at the monitor, the wedge between the head phantom and the thorax
phantom was removed. For each distance, the staff member was assumed to stand directly
in front of the catheter insertion site.
14
Figure 2.5: Image showing the four different staff member positions used; 0, 25, 50
and 150 cm from the irradiated part of the patient.
2.2
Personnel eye lens dose assessments
At Sahlgrenska University Hospital several departments are using interventional radiology.
The personnel eye lens dose assessments in the present study were carried out at the
Catheterization Laboratory at the department of Cardiology and at the division of
Peripheral Interventional Radiology at the department of Radiology. The staff at these two
departments perform most of their interventional procedures in the thorax and abdominal
region of the patient. Due to the large patient size in these areas the staff are exposed to
high levels of scattered radiation. In addition, it can be seen from the legal dosimetry
measurements that the highest annual accumulated personal doses are reported from these
two departments.
Individual headbands holding TL-dosimeters were fashioned. The TL-dosimeters were
calibrated in terms of Hp(0.07) in a N-80 spectra using a slab phantom. The calibration
spectra was found in ISO 4037-1 [23]. The calibration set-up and the conversion
coefficients were found in ISO 4037-3 [24]. These headbands were worn by interventional
staff for one month during all work containing radiation. The design of the headbands
differed depending on clinical role of staff member and department, see section 2.2.1 and
2.2.2 . No staff member wore lead glasses during the measurement period. One PDM was
also worn, at thorax height outside the lead apron, during the same period of time as the
headbands by all personnel participating in the measurements. Three TL-dosimeters were
placed at each of the two departments to measure the dose contribution from background
radiation. These were placed close to the daily work but fully shielded from the X-ray
units.
15
2.2.1 The Catheterization Laboratory at the department of Cardiology
Seven operators and four nurses, with the highest number of procedures performed per
month, were selected for measurements. The headbands worn by operators held two TLdosimeters to record the maximum potential eye lens dose for all possible incident angles
of scattered radiation. These dosimeters were placed at position 2 and 5 (in Figure 2.2).
The headbands worn by nurses held one TL-dosimeter at position 5.
2.2.2 The division of Peripheral Interventional Radiology at the department of Radiology
The same method of selection, as used for interventional staff at the Catheterization
Laboratory, was used to select seven operators and seven nurses for measurements. The
headbands worn by operators held three, instead of two, TL-dosimeters due to the large
variation of incident angles. These dosimeters were placed at position 2, 5 and 8 (in Figure
2.2). The headbands worn by nurses held one TL-dosimeter at position 5.
2.2.3 Data analysis
From the PDM data, the number of procedures can be acquired. By assuming that the
headband and PDM were always worn together the numbers of procedures performed with
a headband worn by a given staff member could be acquired. For each staff member, the
TLD with the highest dose recorded was selected for data analysis. For operators, the
accumulated doses recorded by the TLD during the measurement period (with the
contribution from the background radiation subtracted), DTLD (mSv), were extended to an
estimation of the annual dose in two ways by using equation (2.2) and equation (2.3).
Annual dose= DTLD⋅
ny
nm
(2.2)
ny is the number of procedures performed by a given staff member during the last year and
nm is the number of procedures, when the PDM (and headband) was worn, performed by a
given staff member during the measurement period.
365−42
Annual dose= DTLD⋅
dm
(2.3)
dm is the number of days of the measurement period for a given staff member (days when
no work was done included). The number of days in a year is 365 and 42 is the number of
days subtracted due to vacations.
Data on the total numbers of procedures a nurse assisted at in a year (ny) were unavailable
16
and thus only equation (2.3) was used for nurses.
Also the average lens dose contribution per procedure (DTLD/nm) and ratios between the
dose recorded by the TLD and the PDM were calculated for each staff member.
3 RESULTS
3.1
Phantom measurements
3.1.1 Position of the dosimeter
The doses recorded at different dosimeter positions across the forehead varied with
approximately a factor 6, see Table 3.1. The results are presented as equivalent dose at a
given position relative to the equivalent dose at position 1, i.e. the position when the EDD30 was placed on the eye of the head phantom.
Table 3.1: Equivalent doses at different positions of the
EDD-30 dosimeter relative to the dose at position 2.
Position of the EDD-30
Relative equivalent dose
1
1.00
2
1.07
3
1.00
4
0.85
5
0.74
6
0.63
7
0.33
8
0.17
3.1.2 Lead glasses
When the head phantom was angled towards the patient, i.e. the scattered radiation source,
minor difference could be seen between different models of lead glasses. A greater
difference in dose reduction ability was seen when the head phantom was angled towards
the monitor and in this angle lead glasses number 11 most efficiently shielded the EDD-30
from scattered radiation, see Table 3.2. For this angle, the same model also reported the
lowest shielding efficiency when space for regular glasses was given underneath.
17
Table 3.2: Dose reduction ability for each of the 12 models of
lead glasses.
Lead glasses
model number
Dose reduction ability (%)
Head phantom angled
Head phantom angled
towards monitor
towards patient
1
81
85
2
79
85
3
67
83
4
75
83
5
79
83
6
48
81
7
66
82
8
71
85
9
67
83
10
38
86
11
82
88
11*
30
87
12
71
87
12*
54
86
* With space for a pair of regular glasses underneath.
With an additional 5 mm of lead added to the lead glasses no statistically significant
difference in dose reduction ability was seen (p=0.48). The mean value of the 10
measurements with and without 5 mm lead added was 85.2% and 85.0%, respectively.
The dose reduction ability was practically independent of tube potential, see Figure 3.1.
18
Dose reduction ability (%)
100
90
80
70
60
50
40
30
20
10
0
75
80
85
90
95
100
105
110
115
Tube potential (kV)
Figure 3.1: Dose reduction ability as a function of tube potential.
The repeated measurements at 82 kV and 110 kV resulted in a small but statistically
significant difference in dose reduction ability with a mean value of 83.9% and 84.6%,
respectively (p<0.006).
The results from the measurement of the equivalent lead thickness varied from 0.68 to 0.82
mm and can be seen in Table 3.3. All glasses had a specified equivalent lead thickness of
0.75±0.05 mm.
Table 3.3: Measured equivalent lead thickness for each of the
12 models of lead glasses.
Lead glasses model number
Equivalent lead thickness (mm)
1
0.73
2
0.68
3
0.71
4
0.78
5
0.74
6
0.71
7
0.73
8
0.75
9
0.77
10
0.80
11
0.82
12
0.77
19
3.1.3 Angular response of the PDM and the EDD-30
The horizontal angular response of the PDM (measured when the PDM was correctly
placed on the thorax phantom) was not symmetrical. For example, the relative equivalent
dose at 60° was 1.12 while the corresponding value at -60° was 0.45. The vertical angular
response (measured when the PDM was turned counterclockwise 90°) had a less
asymmetrical shape, see Figure 3.2. The values here, at 60° and -60°, were instead 0.96
and 1.04, respectively.
1.2
Relative equivalent dose
1
0.8
Horizontal
Vertical
0.6
0.4
0.2
0
-100
-80
-60
-40
-20
0
20
40
60
80
100
Incident angle (degrees)
Figure 3.2: Angular response of the PDM presented as equivalent doses relative to the dose recorded at 0°. The
standard deviation of the two repeated measurements for each angle and PDM rotation is shown as error bars.
For large angles the relative angular response of the EDD-30 was higher than the angular
response of the PDM, see Figure 3.3.
20
1.2
Relative equivalent dose
1
0.8
0.6
Horizontal
Vertical
0.4
0.2
0
-100
-80
-60
-40
-20
0
20
40
60
80
100
Incident angle (degrees)
Figure 3.3: Angular response of the EDD-30 presented as equivalent doses relative to the dose recorded at 0°.
The standard deviation of the two repeated measurements for each angle and EDD-30 rotation is shown as error
bars.
3.1.4 Phantom based assessment of the ratio between doses at eye and thorax height
Equivalent eye lens dose rates decreased with increasing distance and with increasing staff
member height, see Figure 3.4.
Equivalent eye lens dose rate (μSv/h)
2500
2000
1500
0 cm
25 cm
50 cm
150 cm
1000
500
0
Short nurse
Operator
Tall nurse
Figure 3.4: Equivalent eye lens dose rates for the different distances (0, 25, 50, 150 cm shown in Figure
2.5) and staff member heights when the head phantom was angled towards the catheter insertion site.
21
The ratios between equivalent doses recorded by the two EDD-30s were closer to 1 the
further away the staff phantom was moved from the patient phantom, see Table 3.4.
Table 3.4: Ratio between equivalent doses recorded by the two EDD-30s, at eye and thorax
height, for different phantom measurement set ups. The distances (0, 25, 50 and 150 cm)
specified refer to the different distances shown in Figure 2.5.
Head angle
Towards monitor
Towards patient
Staff member height
Ratio
0 cm
25 cm
50 cm
150 cm
Short nurse
0.27
0.24
0.36
1.04
Operator
0.21
0.26
0.39
0.88
Tall nurse
0.20
0.27
0.50
0.85
Short nurse
0.38
0.29
0.43
1.13
Operator
0.28
0.33
0.48
0.99
Tall nurse
0.26
0.33
0.62
0.93
The ratios between equivalent doses recorded by the EDD-30 placed on the eye relative to
equivalent doses recorded by the PDM placed at thorax height were, for small distances,
generally lower than the ratios presented in Table 3.4, see Table 3.5.
Table 3.5: Ratio between equivalent doses recorded by EDD-30 and PDM, at eye and thorax
height, for different phantom measurement set ups. The distances (0, 25, 50 and 150 cm)
specified refer to the different distances shown in Figure 2.5.
Head angle
Towards monitor
Towards patient
3.2
Staff member height
Ratio
0 cm
25 cm
50 cm
150 cm
Short nurse
0.19
0.18
0.31
2.30
Operator
0.15
0.20
0.35
1.35
Tall nurse
0.14
0.26
0.50
1.34
Short nurse
0.26
0.22
0.41
2.49
Operator
0.19
0.28
0.43
1.47
Tall nurse
0.18
0.31
0.65
1.47
Personnel eye lens dose assessments
Staff members who wore headbands and PDMs have been assigned a random number for
22
the presentation of the results. Operators 1-7 and nurses 1-4 are working at the
Catheterization Laboratory at the department of Cardiology. Operators 8-14 and nurses 511 are working at the division of Peripheral International Radiology at the department of
Radiology. Four operators (operator 3, 5, 11 and 14) out of the total 14 operators selected
for measurements did not wear their headbands or PDMs during any procedure and have
been excluded from this section.
3.2.1 The Catheterization Laboratory at the department of Cardiology
The results from the personnel eye lens dose assessments showed that the annual lens dose
of no staff member at the Catheterization Laboratory exceeded the new ICRP
recommendation of 20 mSv, see Table 3.6. However, one operator reported an annual lens
dose of 19.1 mSv. The ratios between accumulated equivalent doses recorded by the TLD
and the PDM during the measurement period were closer to 1 for nurses than for operators.
Table 3.6: Estimated annual lens doses, average lens dose contribution per procedure
during measurement period, number of procedures (both during measurement period when
the PDM was worn, nm, and total number during the last year, ny) and ratio between
equivalent doses recorded by TLD and PDM for staff members at the Catheterization
Laboratory at the department of Cardiology. n/a means not available.
Staff member
Estimated annual
Average lens dose
equivalent eye lens
per procedure
dose (mSv)
(µSv)
nm
ny
Ratio
Equation Equation
(2.2)
(2.3)
Operator 1
19
11
49
20
387
0.63
Operator 2
12
2.3
28
7
410
0.55
Operator 4
7.5
8.7
24
31
305
0.41
Operator 6
7.5
8.3
22
32
335
0.54
Operator 7
12
9.2
35
23
345
0.58
Nurse 1
n/a
1.1
2.8
33
n/a
1.37
Nurse 2
n/a
4.6
10
39
n/a
0.90
Nurse 3
n/a
2.8
5.6
43
n/a
0.85
Nurse 4
n/a
4.6
11
39
n/a
0.73
3.2.2 The division of Peripheral Interventional Radiology at the department of Radiology
One operator at the division of Peripheral Interventional Radiology reported an estimated
annual lens dose that exceeded the new ICRP recommendation, see Table 3.7. Ratios
between accumulated equivalent doses recorded by the TLD and the PDM were generally
23
lower for operators at the division of Peripheral Interventional Radiology than for
operators at the Catheterization Laboratory. The average lens dose contribution per
procedure differed greatly between different operators.
Table 3.7: Estimated annual lens doses, average lens dose contribution per procedure
during measurement period, number of procedures (both during measurement period when
the PDM was worn, nm, and total number during the last year, ny) and ratio between
equivalent doses recorded by TLD and PDM for staff members at the division of Peripheral
Interventional Radiology at the department of Radiology. n/a means not available.
Staff member
Estimated annual
Average lens dose
equivalent eye lens
per procedure
dose (mSv)
(µSv)
nm
ny
Ratio
Equation Equation
(2.2)
(2.3)
Operator 8
12
14
68
17
183
0.44
Operator 9
14
16
83
16
164
0.80
Operator 10
4.4
1.9
32
5
136
0.49
Operator 12
8.5
7.3
36
17
237
0.47
Operator 13
33
13
350
3
93
0.30
Nurse 5
n/a
4.0
12
28
n/a
0.90
Nurse 6
n/a
1.2
3.1
32
n/a
0.89
Nurse 7
n/a
1.7
8.9
16
n/a
0.78
Nurse 8
n/a
2.1
5.0
35
n/a
0.83
Nurse 9
n/a
6.4
36
15
n/a
0.54
Nurse 10
n/a
4.4
11
33
n/a
1.03
Nurse 11
n/a
4.0
19
18
n/a
0.85
4 DISCUSSION
4.1
The use of Hp(0.07) for measuring lens dose
As mentioned in section 1.5 , when measuring eye lens dose, dosimeters calibrated in
terms of Hp(3) is recommended by the ICRP and the ICRU. The dosimeters used here for
measuring personnel eye lens dose were instead calibrated to measure H p(0.07) using a
slab phantom. No conversion coefficients for Hp(3) were available in ISO 4037-3 (from
where the conversion coefficients used were taken).
If the radiation measured consists of photons, which is the case for interventional radiology
staff member lens dose measurements, H p(0.07) adequately assesses eye lens dose.
24
Behrens et al. used Monte Carlo simulations trying to quantify the error of using H p(0.07)
instead of Hp(3) for eye lens dose measurements in interventional radiology and found that
for scattered photons from photon fields emerging from tube potentials above 30 kV, the
eye lens dose was overestimated by only 10% or less (depending on photon energy) when
Hp(0.07) was used. For tube potentials below 30 kV, the eye lens dose was overestimated
by a factor 1.1 to 5.0 [21]. Since tube potentials of below 30 kV are never or extremely
rarely used in interventional radiology, Hp(0.07) is considered to adequately estimate the
eye lens dose.
The use of Hp(0.07) to estimate eye lens dose has even been recommended by the
European Commission [25]. In addition, TLD:s calibrated in terms of Hp(0.07) was used in
the ORAMED project [10] and in a Norwegian study investigating eye lens doses of
interventional staff [26].
4.2
Phantom measurements
4.2.1 Position of the dosimeter
A dosimeter should be positioned where it can give an indication of the maximum potential
eye lens dose. Thus for operators using a standardized method when performing their
procedures, e.g. operators at the Catheterization Laboratory at the department of
Cardiology who almost always have the X-ray tube on their left hand side, position 2 in
Figure 2.2 (on the side of the X-ray tube) should be used. This applies for the conditions
used in the phantom measurements, i.e. that the operator is looking at the monitor. If the
operator instead is looking at the irradiated part of the patient, position 5 should be used.
This is why headbands used by operators at the Catheterization Laboratory at the
department of Cardiology had TL-dosimeters in position 2 and 5. For operators using
different positions in relation to both the patient and the X-ray tube for every procedure,
e.g. operators at the division of Peripheral Interventional Radiology at the department of
Radiology, the matter of dosimeter positioning is more complicated. For the personnel lens
dose assessments here, both position 2 and 8 were used, as well as position 5, to record the
maximum potential eye lens dose for all possible incident angles.
Nurses assisting the operator during procedures stand further away from the irradiated part
of the patient. Therefore the incident angle for scattered radiation does not vary as much
for nurses as it does for operators. Even when nurses look at the monitor, the scattered
radiation strikes them relatively frontal. This is why a sufficient nurse eye lens dose
25
assessment could be made with only one TL-dosimeter, in position 5.
4.2.2 Lead glasses
Materials of an equivalent lead thickness of 0.75 mm block over 95% of X-ray radiation
[27]. The lead glasses however, reduced the dose by only between 30% and 82% of the
incident radiation when the head phantom was angled towards the monitor, due to back
scatter from the head and incident radiation from the side. When the head phantom instead
was angled towards the irradiated part of the patient, the dose reduction ability differed
from 81% to 88%, i.e. less dependent of design of the lead glasses. This means that
contribution from back scattered radiation in the head is almost impossible to avoid by
adjusting the design of the glasses. However, the design is important when considering
incident radiation from the side.
During interventional procedures, the operators look in many different directions, most
commonly at the monitor (depending on procedure). They also look straight at the
scattered radiation source, i.e. the irradiated body part of the patient. When their head is
angled towards the monitor, the scattered radiation has a small gap beside the glasses to
simply pass by and hit the eye lens without penetrating the lead glass. Out of the different
head angles commonly used during interventional procedures, the two angles used for
measurements are the ones in which lead glasses have the highest and lowest dose
reduction ability, respectively. This means that in practice (if the operator has the exact
same head shape as the head phantom) the glasses should have a dose reduction ability
percentage between the two values measured for each model. Regardless of head angle, the
gap between the cheek and the lead glasses needs to be minimized to give as good
protection as possible. Lead glasses should be individually tested to optimally fit around
the shape of the head. Using lead glasses designed to fit outside a pair of regular glasses,
e.g. model 11 and 12, are not recommended due to their low dose reduction ability when
regular glasses are worn underneath.
Dose reduction ability was not affected by adding an additional 5 mm lead to the area of
the glass and thus 0.75 mm equivalent lead thickness is considered enough to block the eye
lens from the radiation trying to pass though the glass. This means that even with
practically 100% attenuation of radiation hitting the glass, the lead glasses still only
reduced the eye lens dose with 30% to 88%, depending on incident angle and design of
lead glasses model rather than equivalent lead thickness. However, the question remains
26
whether 0.75 mm is required to fully block incident radiation. Can 0.50 mm (or even less)
also be enough? Unfortunately none of the glasses provided by Scanflex and Mediel had
and equivalent lead thickness of less than 0.75 mm.
Another factor that may influence the differences in dose reduction ability is the actual
equivalent lead thickness of the different models. Each of the 12 models of lead glasses
had, as earlier mentioned, a specified equivalent lead thickness of 0.75±0.05 mm and the
measured values varied from 0.68 to 0.82 mm. Pair number 11, which had the highest dose
reduction ability, also had the highest measured equivalent lead thickness. However, the
absolute values may not be completely accurate due to the calibration.
The results from the effect on dose reduction ability of changing the tube potential can be
explained by first assuming that the radiation hitting the lead glass was fully attenuated for
all tube potentials used. This would explain the relatively constant dose reduction ability in
Figure 3.1 but can also explain the small increase in dose reduction ability when the tube
potential was changed from 82 to 110 kV. The head phantom was angled in a way that only
photons back scattered in the head contributed to the dose recorded by the EDD-30
(assuming the full attenuation of lead glasses), i.e. there was no gap between the cheek and
the glasses for radiation to slip through and hit the EDD-30. For lower tube potentials the
photons scattered in the head will be scattered in wider angles than photons emerging from
a higher tube potential. This means that the dose reduction ability would be slightly lower
for 82 kV (than for 110 kV) due to the greater proportion of back scattered photons for this
tube potential.
4.2.3 Angular response of the PDM and the EDD-30
An incident angle of 60° (or even more) to the PDM is not unusual during interventional
procedures. The results are therefore not completely satisfying. The estimation of an eye
lens dose based on the dose recorded by the PDM will be even more difficult with a poor
angular response. However, one should keep in mind that the PDM was designed as a
radiation protection instrument rather than a dosimeter and for this purpose the angular
response is sufficient.
The detector part of the PDM consists of four diods which are located in the upper left
corner. This location could explain the non symmetrical shape of the horizontal angular
response. For negative incident angles various internal electronics in the PDM could shield
the diods from incident radiation. It is also possible that for positive incident angles the
27
radiation can more easily reach the diods than for an incident angle of 0° (in which the
calibration was made) which would explain why the relative response is larger than 1 for
some positive angles. The same argumentation can be made for the vertical angular
response which also show a slight tendency of asymmetry.
4.2.4 Phantom based assessment of the ratio between doses at eye and thorax height
The total fluoroscopy time for a standard interventional procedure varies from
approximately 10 minutes to over 60 minutes. The equivalent eye lens dose rates measured
were for some distances over 1 mSv/h, with a maximum value of 2.25 mSv/h (see Figure
3.4) for the simulated scenario when a nurse shorter than the operator is standing directly
in front of the irradiated part of the patient looking at the catheter insertion site hole.
Theoretically this means that in a worst case scenario a nurse can exceed the new ICRP
recommendation of 20 mSv after only 10 procedures with a total fluoroscopy time of about
60 minutes each. A more realistic time, rather than 60 minutes, a nurse stands directly in
front of the irradiated part of the patient would be a few seconds per procedure, which
based on the results would mean an annual dose contribution of less than 1 mSv.
When the staff member height was increased the eye lens dose decreased not only due to
an increased distance from the scattered radiation source but also because the EDD-30
placed on the eye was partially shielded by the image detector of the X-ray unit. When the
distance from the patient was increased, this shielding effect had less influence and the
equivalent dose did not decrease as markedly with increased staff member height, e.g. the
recorded doses at 50 and 150 was almost independent of staff member height.
Photons of the energy used to calibrate the EDD-30 and the PDM has a build up effect on
the depth dose. The equivalent dose at 10 mm is about 9% higher than the equivalent dose
at 0.07 mm [24]. Also the difference in sensitivity of these two dosimeters, which was
assessed during the present study, contributes to the fact that the PDM measures an
equivalent dose 37% higher than the EDD-30 for a given situation. This explains why the
EDD/EDD-ratios were higher than the EDD/PDM-ratios for small distances. However,
when the distance was increased, the EDD/PDM-ratios increased more than the
EDD/EDD-ratios due to the poor angular response of the PDM (see Figure 3.2) compared
to the angular response of the EDD-30 (see Figure 3.3).
28
4.3
Personnel eye lens dose assessments
The operators excluded did not wear their headband or PDM for two different reasons.
Two of these operators did not perform any procedures at all during the measurement
period and the other two were simply not interested in participating. The latter two
performed about as many procedures during the measurement period as the operators who
wore their PDM and headband. However, this does not exclusively mean that their annual
eye lens doses are similar to the doses of the operators participating in the study.
4.3.1 Systematic errors and stochastic uncertainties in the D TLD values
As mentioned in section
4.1 , using dosimeters calibrated in terms of Hp(0.07) for
measuring eye lens dose results in a systematic overestimation of the eye lens dose by 10%
or less.
The fact that the TLDs were not placed on, but instead beside, the eye lens results in a
systematic error. The phantom measurements indicate that the eye lens dose is
overestimated by about 5-10%, but an underestimation is not impossible.
The mean energy of photons scattered in the patients, contributing to the DTLD values, are
lower or the same as the mean energy of the photons used for calibration of the TLDs. This
could possibly lead to a slight overestimation of the eye lens dose, which should be less
than 5%.
The TL-dosimetry system used (including the calibration, the read out, etc.) is assumed to
include a stochastic uncertainty of about ±10% or less.
This means that, theoretically in a worst case scenario, it is possible that a DTLD value could
be overestimated by 35% or underestimated by 10%.
4.3.2 Uncertainties in the annual eye lens dose estimations
The estimations of annual doses have higher uncertainties than the dose values recorded by
the TLDs during the measurement period. The higher the number of procedures performed,
with the headband and PDM during the measurement period (nm), the more reliable the
annual dose estimation based on equation (2.2) is. With a high nm the probability of the
“dose per procedure” during the measurement period being representative for the “dose per
procedure” the rest of the year is higher. For example, if the procedure with the largest
dose contribution all year would take place during the measurement period for an operator
with nm=2, the annual dose could be severely overestimated. Also the estimation based on
29
equation (2.3) is more reliable with a higher nm (given that nm is still representative for the
number of procedures performed per month the rest of the year).
The two different annual dose estimations (for a given operator) are relatively similar for
operators with nm larger than 10, with exception for operator 1. It can be seen in Table 3.6
that the average number of procedures per day during the measurement period is not
representative for the rest of the year (dm=28 for operator 1). The work load of operator 1
was lower than usual during the measurement period. The annual lens dose estimation
based on equation (2.2) is therefore considered to be the most accurate for operator 1.
The annual lens dose estimations for operator 2, 10 and 13 (who had a low values of nm)
have large uncertainties, which is reflected in the difference of using the two equations.
Operator 2 performed 33 procedures during the measurement period but wore the PDM at
only seven of them. For this operator the estimation based on equation (2.2) is considered
to be the most reliable. The PDM of operator 13 had only three procedures registered, one
with a dose contribution of 2.9 mSv (which is about 10 times higher than the average).
This means that there is a substantial possibility that the annual lens dose estimation for
operator 13, based on equation (2.2), is an overestimation.
4.3.3 Recommendation of lead glasses
The estimated annual eye lens doses of operators are high enough to cause concern. It is
possible that the actual annual eye lens doses exceed the new ICRP recommendation, due
to the uncertainties and the limited length of measurement period. As mentioned in section
1.4.1 , the possibility of cataract being a stochastic effect is not excluded and if so is the
case, it means that even if the actual annual eye lens doses are below 20 mSv, they should
still be minimized. Operators working at the Catheterization Laboratory and at the division
of Peripheral Interventional Radiology might be recommended to wear lead glasses in the
future. The lens doses of nurses on the other hand are generally much lower than those of
the operators. Wearing lead glasses is not as crucial for nurses as it is for operators.
The use of ceiling suspended shield was not closely examined during the present study. It
is possible that the eye lens doses could be significantly reduced only by using these
shields more correctly and/or more often.
4.3.4 Using the PDM as an eye lens dose estimator
As can be seen in the results, both from the phantom measurements and the personnel dose
30
assessments, the ratios between the eye lens dose and the dose recorded by the PDM are
strongly dependent on the distance from the irradiated body part of the patient, staff
member height, head angle and individual working method of the staff member. For
operators, who reported lens doses relatively close to the ICRP recommendation,
estimating an eye lens dose using only the PDM is therefore difficult. For operators at the
two departments, the PDM should instead serve their original purpose, to work as a
radiation protection instrument rather than a dosimeter.
The fact that the ratios differed is not as crucial for nurses, as it is for operators, due to
their lower lens doses. With a few exceptions the ratios were close to 1 for nurses. The
lowest and highest ratio reported were 0.54 and 1.37, respectively. The eye lens doses
reported by nurses at the two departments are sufficiently far below the ICRP
recommendation for this under- and overestimation to be acceptable for future monitoring.
However, with such large uncertainties caution should be taken if a nurse reports an
equivalent dose of more than even about 10 mSv (after extension to annual dose). It should
also be mentioned that nurse 1 (ratio 1.37) reported the lowest accumulated doses, both
from TLD and PDM, of all staff members and therefore the ratio of nurse 1 contains larger
uncertainties than the ratios reported by other staff members.
When approaching a new department were the personnel eye lens doses are unknown, the
PDM can also used. For medical staff with PDMs reporting values of approximately 5-10
mSv (after extension to annual dose) or more, a more precise eye lens dose assessment
should be made. Also the ratios between lens dose and PDM recorded dose of these staff
members are unknown. However if a PDM of a staff member reports a value of less than 5
mSv it is extremely unlikely that the annual lens dose of that staff member exceeds 20
mSv.
4.3.5 Lens doses in relation to current dose limits
The annual eye lens dose estimations are close to or above the new ICRP recommendation
of 20 mSv in a year. However, the current dose limit according to Swedish regulations set
by SSM is 150 mSv in a year. All personnel eye lens doses measured are far below this
dose limit.
4.3.6 Consistency with other studies
The eye lens doses assessed within the present study are consistent with those of both the
ORAMED project [10] and a similar study at two hospitals in Denmark by the National
31
Institute of Radiation Protection in Denmark [28]. Also a study carried out at all hospitals
in Norway where interventional cardiology procedures are performed reported similar eye
lens doses per procedure as reported in the present study [26].
5 CONCLUSIONS
Based on the results of the present study, operators might be recommended to wear lead
glasses in the future. Lead glasses should be individually tested to minimize the gap
between the cheek and the glasses. One should keep in mind that wearing glasses does not
reduce the lens dose with 100%. The phantom measurements indicate that this reduction in
practice should be somewhere between 30% and 88%, depending on working method and
on how well they fit. If this is not considered the glasses can give a “false” sense of
security; minimizing the eye lens dose is still important also when lead glasses are worn.
Estimating eye lens dose from the dose recorded by the PDM is difficult. This method
includes large uncertainties. The PDM should instead be used to indicate whether a more
precise measurement is necessary. With a sufficiently low dose recorded by the PDM, the
annual eye lens dose being lower than 20 mSv can be guaranteed, without a more precise
measurement.
To get higher accuracy in the lens dose estimations the measurement period needs to be
extended. For example, during the measurement period (of one month) four operators each
performed less than six procedures. Only with a measurement period of one year the true
annual eye lens dose can be assessed.
32
6 ACKNOWLEDGEMENTS
First off, I would especially like to thank my supervisors Charlotta Lundh, Pernilla
Jonasson and Åke Cederblad for all your guidance, feedback and encouragements.
Many more people have helped me in many different ways during my work with this
project and I would also like to thank:
•
Mikael Bergfjord at Unfors Instruments for all your help with the
DoseAware system and for your input about the results from my phantom
measurements. It has been very much fun getting to know you.
•
All personnel at the departments of Cardiology and Radiology. You have all
been very helpful and interested in my work, which of course made
everything much easier.
•
Robert Eklund at Scanflex and Ann-Sofie Forsten at Mediel for letting me
borrow and evaluate several different models of lead glasses.
•
Janne, Mats and Anders at our local workshop here at Sahlgrenska for your
inventive solutions to my unexpected problems during the phantom
measurements.
33
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