Download Establishing national diagnostic reference levels for digital

Establishing national diagnostic reference levels
for digital mammography
Avril Weir, Caoimhe McIntyre
Medical Physics, Royal Infirmary of Edinburgh
Aim and introduction
Method
Breast cancer is the most common cancer in the UK; the lifetime risk of
breast cancer is 1 in 8 for women in the UK. However, more than 90%
diagnosed at the earliest stage survive their disease for at least five
years [1].
A comprehensive dose audit was carried out following the national dose
audit protocol [6]. Clinical data from 100 consecutive patients was
recorded at all four symptomatic mammography units in NHS Lothian
and NHS Fife.
Mammography uses x-rays to image the breast. This imaging technique
allows the early detection of breast cancer, thus improving the prognosis
of the disease. However, it carries the risk of radiation-induced breast
cancer.
Radiation dose parameters, including the air kerma, were measured at
each unit by Medical Physics. The air kerma quantifies the amount of
radiation entering the body.
Images of a mammography unit are shown in Figure 1 and images of the
breast are shown in Figure 2.
The MGD was calculated for six CBT ranges by Medical Physics using
published equations [7]. Specifically, the air kerma was converted to
MGD through the use of published conversion factors, which depend on
patient demographics and the x-ray characteristics [7]. A DRL for each
CBT range was proposed based on the average MGD for each unit.
Results
Compression
plate
Figure 3 displays the average MGD for each unit relative to the proposed
DRL for each CBT range. This can be compared to the current national
DRL of 3.5 mGy [8]. The error bars represent the 95% confidence
interval.
Figure 1: Schematic diagram of a mammography unit [2] (left) and
photograph of a clinically used mammography unit (right).
MGD (mGy)
The MGD increases with CBT as higher x-ray exposure factors are
required to image thicker breasts and also because the breast
composition changes with CBT.
3.5
3
Unit 1
2.5
Unit 2
2
Unit 3
1.5
Unit 4
Proposed
DRL
1
0.5
0
<40
[40-50)
[50-60)
[60-70)
[70-80)
>=80
CBT (mm)
Figure 3: Mean glandular dose (MGD) for each compressed breast
thickness range (CBT), for each mammography unit, plus proposed
diagnostic reference level.
Conclusion
Figure 2: Mammogram of the breast (left) and a schematic diagram of
the breast [3] (right).
The probability of radiation-induced breast
L cancer is directly proportional
to the dose delivered to the breast. The dose metric used
α ∝to estimate the
risk is the Mean Glandular Dose (MGD), measured in1/a
milligray (mGy).
The risk is approximately 1 in 100,000 per mGy [4].
It is a requirement of the Ionising Radiation (Medical Exposure)
Regulations 2000 that diagnostic reference levels (DRLs) are established
for radio-diagnostic procedures [5]. If good and normal practice is
followed, the radiation dose should not exceed the DRL for the particular
examination.The use of DRLs helps to optimise the risk-benefit of the
procedure (i.e. detection of cancer versus cancer induction).
The current national DRL is over 10 years old and is only applicable for
patients with a compressed breast thickness (CBT) between 50-60 mm.
Over the years, technique and technology have improved, meaning that
this value is now out-of-date.
The aim of this work is to establish new DRLs for a range of CBTs,
incorporating recent advances in technology and technique.
The proposed DRLs are lower than the current national DRL for all
CBTs. This reflects recent improvements in clinical practice and
highlights the need for new national DRLs. Use of the proposed DRLs
will help to optimise dose for all CBTs, reducing the incidence of
radiation-induced cancer without impacting on the diagnostic
acceptability of the images.
For the CBT range of 50-60 mm, the DRL would reduce by 60%.
Equivalently, the risk of radiation induced breast cancer arising from
mammography examinations carried out with an MGD equal to the DRL
will reduce from 1 in 29,000 to 1 in 67,000.
A Scottish-wide dose audit will now be conducted based on this
work, taking into consideration any feedback from radiographers.
References
[1] http://www.cancerresearchuk.org/
[2]
Blausen.com
staff.
"Blausen
gallery
2014".
Wikiversity
Journal
of
Medicine
DOI:10.15347/wjm/2014.010. ISSN 20018762.
[3] http://commons.wikimedia.org/wiki/File:Illu_breast_anatomy.jpg
[4] NHS Breast Screening Programme Publication Number 54, UK, February 2003
[5] Ionising Radiation (Medical Exposure) Regulations, (SI 2000 No 1059), UK:HMSO, 2000
[6] Dosimetry Working Party of the Institute of Physical Sciences in Medicine. National protocol for
patient dose measurements in diagnostic radiology. Chilton: NRPB, 1992.
[7] D. Dance, C. Skinner, K. Young, J. Beckett and C. Kotre, “Additional factors for the estimation of
mean glandular breast dose using the UK mammography dosimetry protocol”, Phys. Med. Biol. vol
45, pp. 3225-3240, 2000.
[8] Institute of Physics and Engineering in Medicine, IPEM Report 88: Guidance on the Establishment
and Use of Diagnostic Reference Levels for Medical X-Ray Examinations, York, UK, 2004. Ch 5.5
pp30-31.