Polish J. of Environ. Stud. Vol. 15 No. 4A (2006), 207-209
Doses in Computed Tomography
W. ĝlusarczyk-Kacprzyk, W. SkrzyĔski
Medical Physics Department, Maria Skáodowska-Curie Memorial Cancer Centre and Institute of Oncology,
ul. Roentgena 5, 02-781 Warszawa
Abstract
In this study we analyzed doses for a group of 484 patients who underwent an examination on
a GE HiSpeed CT scanner in Centre of Oncology in Warsaw. Patient doses (CTDIw and DLP) have been
compared against reference values published by the Polish Ministry of Health. We found that typical patient
doses do not exceed reference values. As reference dose levels are defined only for a standard-sized patient,
sometimes they may be exceeded for a properly done examination. Polish reference dose levels are not based on
up-to-date data.
Keywords: CTDI, DLP, doses for diagnostic patient, doses for planning patient
Introduction
Patient dose in an X-ray examination should always be
kept as low as achievable while maintaining acceptable
image quality. This rule is extremely important in computed
tomography, which gives a major contribution to collective
dose from medical X-rays (i.e. 47% in UK, [1]).
Several dosimetric quantities are used in computed
tomography, measurement methods are well-established
[2]. CTDI (Computed Tomography Dose Index) is defined
as an integral of dose distribution profile (measured along
a line parallel to the axis of rotation of the lamp) divided by
the nominal slice thickness, it is usually measured with
a dedicated “pencil” ionization chamber. Other dosimetric
quantities in wide use are DLP (Dose Length Product),
CTDIw, (Weighted Computed Tomography Dose Index). It
is recommended that dosimetric information for an examination is displayed on a CT console [3].
Decree on safe use of ionizing radiation in healthcare
issued by the Polish Ministry of Health [4] provides a clear
guidance on the doses in CT. Values of CTDIw and DLP
should not exceed reference dose levels, which are set for
several regions of body. Polish reference dose levels have
been adapted from guidelines published by the European
Commission [2]. Originally they were set at the third
quartile values of the distributions of mean doses observed
for a large number of CT scanners in survey in UK [5]. This
work presents patient doses for a CT scanner installed in
Centre of Oncology in Warsaw. The doses are compared
with the reference dose values.
Experimental procedures
In this study we investigated doses for a group of 484
patients who underwent an examination on a single slice
GE HiSpeed CT scanner in Centre of Oncology in Warsaw.
Patients have been divided into 9 groups depending on the
examined region of body (Table 1). As the scanner serves
radiotherapy department, each group includes both
diagnostic examinations and examinations for radiotherapy
treatment planning.
Data collected for each patient included: X-ray beam
parameters (kVp, mA), scan time, nominal slice thickness,
slice interval or pitch, CTDIw value displayed on the
console. DLP values have also been calculated for each
patient.
Measurements of CTDIw have been performed for
selected examination protocols. We used PTW Unidos
dosimeter, ionization chamber PTW 77336 (calibrated in
Secondary Standard Dosimetry Laboratory in Centre
of Oncology in Warsaw) and phantoms constructed in
workshops of Centre of Oncology in Warsaw. Measured
ĝlusarczyk-Kacprzyk W., SkrzyĔski W.
208
Table 1. Values of CTDIw and DLP for each for each region of body.
Region
of body
Number
of patients
brain
DLP (mGy u cm)
CTDIw (mGy)
min.
max.
aver.
ref. [4]
min.
max.
aver.
ref. [4]
57
5.3
26.8
19.2
60
110
499
254
1050
head/neck
47
6.9
28.5
16.4
60
129
690
294
1050
chest
107
4.4
14.4
10.7
35
149
770
330
650
breast
43
4.1
9.1
6.9
35
82
216
162
650
abdomen
26
5.9
11
8.8
35
204
485
344
780
stomach
14
4.4
9.1
8.2
35
94
293
207
780
pelvis
62
6.8
17.8
9.1
35
53
590
239
570
prostate
113
7.1
9.5
9.5
35
106
176
157
570
thigh
15
9.1
14.4
12.1
-
273
571
439
-
doses have been compared with values displayed on the CT
console. Collected dosimetric data for each region of body
have been analyzed, mean and maximum values of CTDIw
and DLP have been compared against Polish reference dose
values.
Results
The values of CTDIw displayed on the CT scanner
console agree with results of the measurements within
±10% tolerance. Table 1 presents observed values of CTDIw
and DLP compared against the reference levels.
We compared average DLP values for patients that had
diagnostic examination and patients that had examination
for radiotherapy treatment planning. Doses in diagnostic
examinations are generally higher, the difference of average
DLP ranges from 5 to 37%. The largest differences of mean
DLP was noted for groups: "stomach" and "thigh", but these
groups are the least numerous, so the result might be not
representative. In more numerous groups differences do not
exceed 12%.
Discussion of results
Average CTDIw and DLP values do not exceed
reference values in any region of body. Reference values for
CTDIw have not been exceeded. However, in a few cases
CTDIw was significantly higher than needed for high
quality image. In this cases CT operators have not
optimized exposure parameters (kV, mA). Reference values
for DLP have been exceeded for a few patients. We found
that all of them significantly exceeded standard dimensions
of 70kg and 170cm, for which reference values are defined.
In each of the cases the examination parameters have been
chosen properly.
It should be stressed that the Polish reference values are
based on the data published in 1999 (results of the UK
survey [5]). During last 15 years typical patient doses for
single slice CT scanners have significantly decreased.
Recently a new CT dose survey has been performed in UK
and, consequently, new British reference dose levels have
been set (separately for single-slice and multi-slice
scanners) [6]. If we compare CTDIw for two unites types
Siemens Somatom DRH (was an axial-only scanner, it was
replaced by the GE HiSpeed scanner in 2000) and
GE HiSpeed we can see how much the doses are less. For
brain region 46,3 for Siemens, 19,2 for GE (chest: 25 –
Siemens, 10,7 – GE; abdomen: 25 – Siemens, 8,8 – GE).
This is why in UK the new reference dose levels was
established. The new British reference levels seem to be
more appropriate in this case.
Conclusions
Typical patient doses for a GE HiSpeed scanner in the
Centre of Oncology in Warsaw do not exceed reference
values.
Reference dose levels are defined only for a standardsized patient, therefore sometimes they are exceeded for
a properly done examination.
Polish reference dose levels are not based on up-to-date
data, their revision should be considered.
References
1.
2.
Hart D., Wall B.F.: UK population dose from medical
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European Commission, Report EUR 16262: European
Guidelines on Quality Criteria for Computed Tomography, 1999.
Doses in Computed Tomography
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International Electroctechnical Commission. Medical
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Shrimpton P.C., Jones D.G., Hillier M.C., Wall B.F.,
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