Quality control in Mammo

1. Master in Medical Physics 2015 to 2016
Quality control in Mammography
Francisco J.Hernández Flores∗
International Centre for Theoretical Physics
franciscohernandez_f2010@hotmail.com
August 3, 2015
Abstract
This report it is about the quality control in mammography, the importance in
accurately diagnosing breast cancer, particularly in earlier stage cancers, reducing
its high mortality rate in women, it is essential that all mammograms be performed
and interpreted with the highest possible quality standards. The existence of and
strict adherence to quality assurance (QA) and quality control (QC) measures
and guidelines must be practiced in all mammography facilities in order to assure
the most accurate diagnoses for all patients. The following report will discuss
current QC and QA measures in mammography, including the Phantom exposure
for determine the mean glandularly dose, determination of incident air Kerma
and determination of Half Value Layer. In the case of mean glandular dose
was evaluated for the manufacturer this measure was compared with the mean
glandular dose obtained in practice using ki and HVL, the difference obtained
was 14.2%.the value obtained for HVL during the practice was 0.64 mmAl using
28KVand 74mAs, the incident air kerma was 5.026 mGy.
I. Introduction
Breast dosimetry is an important
part of mammographic quality con-
trol and is an essential element of
the optimization of ionizing-radiation-
based breast imaging procedures. For
conventional projection mammogra-
phy there are various standard proto-
cols for the estimation of breast dose
which provide conversion factors to
relate the incident air kerma at the up-
per surface of the breast to the mean
dose to the glandular tissues within
the breast (mean glandular dose). The
determination of the mean glandu-
lar dose and HVL was made using x-
ray spectra from a W/Ag target/filter
combination at 28 kV using ionization
chamber and electrometer PTW for
obtain the measured and compared
with the manufacturer value.
∗Physics of diagnostic X ray 2
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2. Master in Medical Physics 2015 to 2016
II. Theory
I. Half-Value Layer
The half-value layer (HVL) is defined
as the thickness of material required
to reduce the intensity (e.g., air kerma
rate) of an x-ray or gamma-ray beam
to one half of its initial value. The
HVL of a beam is an indirect mea-
sure of the photon energies (also re-
ferred to as the quality) of a beam,
when measured under conditions of
narrow- beam geometry. The HVL of
a diagnostic x-ray beam, measured in
millimeters of aluminum under nar-
row beam conditions, is a surrogate
measure of the penetrability of an x-
ray spectrum. It is important to un-
derstand the relationship between µ
and HVL. In Equation 1, N is equal
to No/2 when the thickness of the
absorber is 1 HVL. The HVL can be
easily calculated from the linear atten-
uation coefficient, and vice versa. the
mathematically form of evaluation it
is show in the equation 2 [3]. The
condition for obtain the HVL in mam-
mography during the measurements
is take into account compression plate
Select the target, filter and the value
for tube voltage and typical tube load-
ing that would be used for the routine
clinical examination of the breast be-
ing simulated (see figure 1 Scheme of
measurement HVL in reference condi-
tion).
N0
2
= N0 × e−µ(HVL)
(1)
HVL =
ln2
µ
(2)
Figure 1: Measurements scheme HVL [4]
II. Mean glandular dose
The Mean Glandular Dose (MGD)
is the special dose quantity used in
mammography. It is defined as the
mean, or average, dose to the glan-
dular tissue within the breast. The
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3. Master in Medical Physics 2015 to 2016
assumption is that the glandular tis-
sue, and not the fat, is the tissue at risk
from radiation exposure. Obviously, it
is just about impossible to determine
the actual dose to the glandular tissue
during a specific mammographic pro-
cedure because of variations in breast
size and distribution of glandular tis-
sue within the breast. The MGD is
based on some standard breast param-
eters. MGD values are determined by
following a standard two-step proto-
col.
• The first step is to determine the
entrance surface exposure, or air
kerma, to the breast. This can
be measured directly with small
dosimeters placed on the breast
or calculated from the know cali-
bration factors for the mammog-
raphy equipment.
• Then, the MGD is determined
by multiplying the surface ex-
posure value by published dose
factors. The dose factor values
are tabulated according to breast
size and composition and the
penetrating characteristics of the
x-ray beam as determined by the
anode material, filtration, and
KV.
For comparison of imaging tech-
niques, evaluation of equipment per-
formance, general dose management,
and regulatory and accreditation pur-
poses, the MGD to a "standard" breast
is used. The standard is a 4.2cm thick
compressed breast consisting of 50%
glandular tissue and 50% fat. This
corresponds to the standard phantom
that is used for image quality evalua-
tion and comparative dose determina-
tions.
Conversion coefficient to estimate
mean glandular dose from incident air
kerma are dependent on the glandu-
larity of each breast. Two approches
are possible: Assume glandularity of
50% estimate the effective glandular-
ity.
DG = CDG50,ki,PMMA × S × Ki (3)
Where CDG50,ki,PMMA is the Conver-
sion factor, in mGy/mGy, used to cal-
culate the mean glandular dose to
breast of 50% gladularity from inci-
dent air kerma, S is the factor for
different mammographic target/filter
combinations, Ki is the incident air
kerma. [4]
Estimation of PMMA equivalence:
for the dosimetry protocols under
consideration, MGD may be deter-
mined using exposures of slabs of
PMMA which are equivalent to spec-
ified model breasts. The equivalent
thickness of PMMA is the thickness of
PMMA that gives the same incident
air kerma at its upper surface as that
for a model breast of specified thick-
ness and composition. [4]
Figure 2: mounting for MGD
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4. Master in Medical Physics 2015 to 2016
III. Incident Air Kerma
The incident air kerma, Ki, is the
kerma to air from an incident X ray
beam measured on the central beam
axis at the position of the patient or
phantom surface (Fig. 1). Only the
radiation incident on the patient or
phantom and not the backscattered ra-
diation is included. Unit: J/kg. The
name for the unit of kerma is gray
(Gy). [2]
Figure 3: mounting for Ki
—————————————————————————————-
III. Methods
I. Evaluation of different
points in quality control of
mammography
In this practical exercise we use the
mammograph giotto, chamber paral-
lel plate PTW, phantom PMMA and
meter measure, for measurement the
distance focus chamber.
I.1 Evaluation of MGD use the
manufacturer value
The standard method of estimating
the MGD dose on patients undergoing
mammography X-ray examinations
is based on the incident air kerma
measurements without backscatter
and the conversion to glandular dose
using appropriate conversion factors
depending on the type of phantom
used. The air kerma value may be
determined either for patients or for a
standard breast phantom; polymethyl
methacrylate (PMMA) is normally
used as a breast substitute phantom
(see figure 2).
The first measurement was about
the phantom exposure, we positioning
the PMMA phantom with two differ-
ent: thickness, filters, KV and mAs at
the same distance in the equipment of
mammography giotto, these parame-
ter it is show in table 1.
I.2 Determination of incident air
Kerma
In this case was evaluate X-ray tube
output values of the free in air inci-
dent air Kerma to the chamber for a
mammography system’s target and fil-
ter combination, kV, mAs, and source
to chamber distance (see figure 3). As-
sume that a mammography system
with a tungsten target and silver fil-
ter, uses 28 kV and 74 mAs for a Fo-
cus Chamber Distance of 56.5cm. The
chamber was positioning down the
compressor plate the follow parame-
ter use are expressed in the table 2.
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5. Master in Medical Physics 2015 to 2016
I.3 Determination of Half Value
Layer
The this part was determined of the
HVL added different thickness of
mmAl betwen the radiation beam and
ionization chamber, we went collected
each incident air kerma obtained for
different thickness, After these mea-
sures were plotted incident air kerma
vs thickness and interpolate to find
the value of HVL the value was ex-
pressed in graph 4
IV. Results and discussion
Focus table distance 61.7cm 61.7cm
Phantom thickness 4.5cm 2cm
Anode Tungsten Tungsten
Filter Ag Rh
KV 28 25
mAs 74 40
Table 1: Determine the mean glandular dose for the manufacturer value
The data shown in this table were
used to determine mean glandular
dose using the manufacturer values,
in this case we evaluate the mean glan-
dular dose for two different thickness
of PMMA using two different com-
bination of anode filter, for the com-
bination tungsten anode with silver
filter for standard thickness to 45mm
PPM the mean glandular dose was
1.6mGy, for the case when we com-
bine tungsten anode with Rh filter,
20 mm PMMA thick we obtained 0.8
mGy of mean glandular dose.
Focus table distance 56.5cm
Anode Tungsten
Filter Ag
KV 28
mAs 74
Polarization of electromiter 200volt
RQR M2
Factor of chamber calibration 4.385 × 1008cGy/C
reading I 5.025mGy
reading II 5.015mGy
reading III 5.035mGy
reading IV 5.030mGy
mean of the reading 5.026mGy
Table 2: Determination of incident Air Kerma
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6. Master in Medical Physics 2015 to 2016
The graph 4 shows the half-value
layer, in which the relation of the in-
tensity of the kerma in air with thick
of aluminium, the result of HVL is
0.64mmAl was obtained by interpola-
tion.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
3.00
4.00
5.00
Thickness (mmAl)
Ki(mGy)
Fit HVL(mmAl) = 5.0002e−1.075x
Figure 4: Determination of half value layer; adding mm aluminum filter
The result of the Mean Glandular Dose was find using these follow value:
HVL=0.64 mm,Ki = 5.026mGy and CDG50,ki,PMMA = 0.349mGy/mGy using the
equation 3 MGD=1.83 mGy.
V. Conclusion
• The difference of MGD between
manufacturer and MGD ob-
tained for measure value during
the practice was 14.2% this out-
come is high because the condi-
tion take into account during the
measure was not the adequate,
according to the specific condi-
tions for determining HVL.
References
[1] AAPM report Number 49, Equipment requirements and Quality Control
for Mammography,
[2] IAEA TRS 457Dosimetry in Diagnostic Radiology: An international
of code of practice, IAEA , 2007
[3] Jerrold T Bushberg the Essential Physics of Medical Imaging,second edition,
Lippincott Williams-Wilkins, 2012
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7. Master in Medical Physics 2015 to 2016
[4] Paola Bregant , Lecture Physics of Diagnostic with x-ray 2, ICTP Trieste Italy,
2015
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