Evaluation of Dosimetric Properties in Full Field Digital

Mammography (FFDM)

Development of a New Dose Index

Chiara Sottocornola

1,2,5

, Antonio Traino

, Patrizio Barca

1,5

, Giacomo Aringhieri

1,4

, Carolina Marini

Alessandra Retico

, Davide Caramella

and Maria Evelina Fantacci

1,5

Dipartimento di Fisica “E. Fermi”, Università di Pisa, L.go B. Pontecorvo 3, 56127 Pisa, Italy

U. O. Fisica Sanitaria, Azienda Ospedaliero-Universitaria Pisana, via Roma 67, 56122 Pisa, Italy

S. D. Radiologia Senologica, Azienda Ospedaliero-Universitaria Pisana, via Roma 67,56122 Pisa, Italy

Dipartimento di Ricerca Traslazionale e delle Nuove Tecnologie in Medicina e Chirurgia, via Savi 10, 56126 Pisa, Italy

Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Pisa, Italy

Keywords: Mammography, Dose-Index in Mammography, Average Glandular Dose, Breast Absorbed Dose, Dosimetry

in Mammography.

Abstract: According to the World Health Organization (WHO), breast cancer is the most common cancer in women,

constituting 29% of all cancers related to the female population. In this context, Full Field Digital Mammog-

raphy (FFDM) is the reference imaging technique for breast cancer early detection and diagnosis and it is

widely employed in screening programs. Therefore, the absorbed radiation dose for each examination shall

be evaluated in order to ensure proper radiation exposures for the patient. In addition, the new European

Directive 59/2013/EURATOM requires that dosimetric data referred to the radiation exposure should be in-

serted in the radiological report. For these reasons, we designed a multidisciplinary research project with the

intention of realizing and validating a new method for calculating the Average Absorbed Breast Dose (2ABD)

by the patient during a mammography procedure. The innovative aspect regards the availability of a quanti-

tative and personalized dosimetric parameter, providing an index that is patient-specific rather than related to

the X-ray machine output, directly related to the risk of radiation. Specifically, in this work we present our

scientific approach as well as the initial results.

1 INTRODUCTION

Breast cancer is the most common cancer among

women both in developed and developing countries

and it is also the principal cause of death from cancer

among women (De Santis et al., 2014; Ferlay et al.,

2010). It affects 1 in 8 women in their lifetime, and

represents 29% of all cancers related to the female

population. Based on these data it is of fundamental

importance to both do an early diagnosis and submit

the patients suffering from this pathology to periodic

checks, in order to offer appropriate treatments with

the goal of reducing mortality.

FFDM (Full Field Digital Mammography) is a

non-invasive high sensitive method for early stage

breast cancer detection and diagnosis, and represents

the reference imaging technique to explore the breast

in a complete way (Dance et al., 2014). Hence,

mortality from breast cancer can be reduced by

mammographic screening (Myers et al., 2015).

However, in a mammographic screening program

healthy people are exposed to ionising radiation.

Besides, the breast is a significant radiosensitive

organ, so special care is required in the evaluation of

the patient exposure (EUREF 2006). Additionally, the

new European Directive 59/2013/EURATOM

highlights the importance of controlling the doses

delivered during radiological procedures and requires

that a dosimetric data referred to exposure should be

inserted in the radiological report

(59/2013/EURATOM). The weight factor for breast

tissue increased from 0.05 to 0.12 in the new

directive, following the ICRP recommendations.

212

Sottocornola, C., Traino, A., Barca, P., Aringhieri, G., Marini, C., Retico, A., Caramella, D. and Fantacci, M.

Evaluation of Dosimetric Proper ties in Full Field Digital Mammography (FFDM) - Development of a New Dose Index.

DOI: 10.5220/0006644302120217

In Proceedings of the 11th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2018) - Volume 1: BIODEVICES, pages 212-217

ISBN: 978-989-758-277-6

In order to satisfy these necessities, this

experimental work was accomplished with the

ambition of identifying a quantitative and

personalized dosimetric index to evaluate the

radiation dose absorbed by each patient during a

mammographic procedurebased on dosimetric

measurements and mathematical calculations.

The current dosimetric index employed in

estimating the radiation dose in mammography is the

Average Glandular Dose (AGD) that is representative

of the dose absorbed by glandular tissue. The most

common algorithms for the AGD evaluation in

mammography are based on the works of Dance

(Dance et al., 1990) and of Wu (Wu et al., 1994). Both

methods are based on incident air kerma (k

a,i

)

measurements and the AGD is obtained through an

empirical expression by applying tabulated factors

(Dance et al., 1990; Wu et al., 1994). The only

measurable quantity in the AGD computation is k

a,i

which is an X-ray output related quantity rather than

a patient dose related quantity.

Thus, we propose the Average Absorbed Breast

Dose (2ABD), defined as the mean value of the

energy imparted per unit of mass in a considered

volume of interest, which could represent a more

suitable physical quantity to evaluate patient

exposure in a mammographic procedure.

In this paper, we present the 2ABD method, show

the preliminary results obtained and discuss its

potential impact on radiological workflow, further

development of the whole project and other possible

applications of the same method.

2 MATERIALS AND METHODS

The absorbed dose in mammography depends mainly

on the quality of the beam and the breast thickness.

Following the mathematical definition, the 2ABD

(mGy) for a specific total breast thickness t (cm) can

be expressed as:

 























(1)

where μ

is the energy absorption coefficient(cm

-1

)

and k

a,i

is the incident air kerma at the breast surface

(mGy).

While the tube voltage (kVp), mAs, anode-filter

combination and t are supplied by the mammographic

device, k

a,i

and μ

have to be assessed in order to

compute the 2ABD. It is important to notice that μ

depends on kVp and anode-filter combination, while

a,i

depends on kVp, tube current-exposure time

product (mAs), anode-filter combination and focus-

to-breast surface distance. Therefore, an experimental

evaluation of these dependences is required to ensure

a reliable assessment of the 2ABD value.

In this first stage we employed a square water-

equivalent phantom (1 g/cm

) for breast tissue

simulation. The phantom is composed of a set of

slabs, which allow selecting a specific thickness.

A solid-state detector coupled to a Piranha

multimeter (RTI-Electronics AB

) and calibrated

termo-luminescent (TL) dosimeters have been

employed in our measurements. The measurements

were performed on the GE Senographe DS machine

(GE Healthcare, Waukesha, WI, USA).

The first step of our work was the beam

characterization in order to evaluate k

a,i

as a function

of kVp, mAs, t and Focus-to-Image Distance (FID).

We set a wide range of nominal kVp (24-34) and mAs

(10-100) values and we measured k

a,i

, kVp and mAs

by placing the solid-state detector 6 cm from the chest

wall edge in the centre of the flat support plate (t=0)

with the compression paddle between X-ray tube

focus and the detector. We chose the Rh-Rh anode-

filter combination for all measurements.

We found the following relationship between k

a,i

and the other parameters:























(2)

where the terms a and b were estimated fitting our

experimental data. The dependence of the k

a,i

on kVp

is linear as a first approximation and this relationship

is valid only in the considered energy range. Repeated

measurements of k

a,i

varying the kVp value were

performed keeping fixed the value of mAs. Each

measurement of k

a,i

was repeated five times and the

average value was considered. The parameters a and

b were obtained by fitting the experimental

measurements to Eq. 2.

The second step was to determine the energy

absorption coefficient μ

. A set of experimental

measurements was performed varying the kVp (range

22-34) and setting 40 mAs for the Rh-Rh anode-filter

combination. The TL dosimeters were placed

between the phantom slabs at specific depths to

evaluate the absorbed dose for different phantom

thicknesses (Fig. 1).

In order to estimate μ

for each kVp value, the

data were fitted according to the exponential

attenuation law:









 





















(3)

in which D(t) represents the absorbed dose measured

by TL dosimeters at depth t. Repeated measurements

Evaluation of Dosimetric Properties in Full Field Digital Mammography (FFDM) - Development of a New Dose Index

213

varying the kVp value were performed in order to

evaluate a possible dependence of the energy

absorption coefficient μ

from kVp.

Figure 1: Experimental setup of water equivalent phantoms

and TLDs: four different dosimeters were placed each time

at the centre of the area irradiated by the beam. At each ir-

radiation the thickness over the TLDs was increased, up to

the maximum thickness of 5.5 cm.

Once k

a,i

and μ

were derived as functions of the

input parameters described above, we computed the

2ABD values in different clinical conditions and we

compared them to the AGD values computed (for the

Rh-Rh anode-filter combination) through the Dance

and Wu methods. Uncertainties in AGD (using Dance

and Wu methods) were estimated considering an

overall 20% error (Hauge et al., 2013). The

uncertainty in 2ABD was estimated considering the

error propagation on a, b, kVp, t, FID and μ

. The

comparison between AGD and 2ABD took into

account the overlap between data within their

uncertainties.

3 RESULTS

In Fig. 2 we show the X-ray tube yield (i.e. k

a,i

/mAs)

as a function of kVp for the Rh-Rh anode-filter

combination (Eq. (2)). As we said previously, the tube

yield is approximately linear with respect to kVp in

the energy range of interest. The repeatability of the

kerma measurements (based on five repeated

measurements for each value of kVp) is < 2%

(standard deviation). a and b values allow to estimate

the incident air kerma k

a,i

for any breast thickness t,

mAs and kVp values (Eq. 2).

In Table 1 we compare k

a,i

measured values and

a,i

computed values. The good agreement between

the measured and calculated values allows the

evaluation of k

a,i

by Eq. 2 for each mammographic

equipment. Notice that the determination of a and b

permits to obtain k

a,i

without directly measure it.

In Fig. 3 we show D(t) as a function of depth t for

different kVp values. Data were fitted with Eq. (3)

and the μ

for each kVp was obtained. The

experimental data confirm the exponential trend of

the beam intensity as a function of the phantom

thickness.

Numerical results are presented in Table 2. The

energy absorption coefficients μ

vary slightly with

kVp. For this reason, the average value was

considered, as shown in Table 2

Once evaluated a and b values and the energy

absorption coefficient μ

the 2ABD can be calculated

according to Eq. 1.

In Table 3 we compare the 2ABD and AGD values

in clinical situations. Five mammograms were

selected from the PACS (Picture Archiving and

Communication System) and the data needed to

compute the AGD through the Dance and Wu

methods were extracted. The most used kVp values

were chosen for each anode/filter combination. For

each kVp value the most frequent thickness was

considered. From Tab. 3 we observe a good

agreement between 2ABD and AGD in every

considered conditions. The two methods provide

results that are consistent within the uncertainties.

4 DISCUSSION

The 2ABD, defined as the mean value of energy

imparted per unit mass in a considered volume of

interest, represents a suitable physical quantity to

evaluate the patient exposure in a mammography

procedure. In fact, we notice a good agreement

between the 2ABD values and the AGD values

computed through the Dance (Dance et al., 1990) and

Wu (Wu et al., 1994) methods for the clinical

situations considered in this work (Table 3).

Our method requires kVp, mAs and breast

thickness values as input parameters, for a specific

anode-filter combination. These parameters can be

The difference between 2ABD computed by considering the kVp

dependence of μ

and the 2ABD computed considering the

average value of μ

was negligible.

BIODEVICES 2018 - 11th International Conference on Biomedical Electronics and Devices

214

easily obtained or selected by the operator before the mammographic exam. Therefore, the 2ABD index

Figure 2: X-ray tube yield (







) as a function of kVp for the Rh-Rh anode-filter combination. Data were fitted using Eq.(2)

with a=(0.0078±0.0002)





and b=(-0.149±0.006)





. Five measurements were averaged and the standard deviation

was computed in order to evaluate the precision of our experimental data.

Table 1: Comparison between k

a,i

(mGy) measured and calculated for different experimental settings. Five measurements

were averaged and the standard deviation was computed in order to evaluate the precision of our experimental data.

Anode-filter

FID (cm)

t (cm)

kVp

mAs

a,i

calculated

a,i

measured

Rh-Rh

63.5

4.65 ± 0.6

4.80 ± 0.01

Rh-Rh

63.5

2.67 ± 0.4

2.83 ± 0.01

Rh-Rh

63.5

3.57 ± 0.5

3.70 ± 0.01

Figure 3: Absorbed radiation dose at different depths in the water-equivalent phantom for different kVp values.

Table 2: μ

values for different kVp and average value of μ

for the Rh-Rh anode-filter combination.

kVp

en_avg

(cm

-1

)

(cm

-1

)

0.74±0.05

0.72±0.05

0.69±0.09

0.66±0.04

0.63±0.06

0.69±0.06

Evaluation of Dosimetric Properties in Full Field Digital Mammography (FFDM) - Development of a New Dose Index

215

Table 3: Comparison between 2ABD and AGD values in different clinical situations.

Age (y)

Glandularity (%)

t (cm)

kVp

mAs

AGD (Dance)(mGy)

AGD (Wu) (mGy)

2ABD (mGy)

1.0±0.2

1.1±0.2

1.4±0.3

1.5±0.3

1.6±0.3

1.5±0.3

1.6±0.3

1.5±0.3

1.7±0.3

1.6±0.3

could be easily computed and employed as dosimetric

index for each mammographic procedure (i.e. for

each patient) and recorded in the radiological report.

In addition, the computation could be conveniently

automated and this could be advantageous in order to

comply with the European Directive

59/2013/EURATOM. Notice that k

a,i

can be

computed directly from Eq. 2 once kVp, mAs, FID

and t are known and therefore, to assess 2ABD, a

direct measurement of k

a,i

can be

avoided.Furthermore, Table 3 does not show evident

discrepancy between AGD and 2ABD values,

although a different set of radiation exposure and

patient-specific parameters was involved in each

mammographic procedure. Thus, 2ABD could be

employed as surrogate of AGD.

However, our model has some limitations. In fact,

this model could be improved taking into account the

X-ray tube yield variations for different anode-filter

combinations among different mammographic

devices, which can affect the k

a,i

evaluation.

Moreover, breast composition should be considered

and a correction factor might be applied for the μ

assessment. In addition, different phantoms with

different shapes could be used so as to better simulate

the breast. Besides, in order to comprehensively

validate this model, the method should be tested in

different clinical conditions on different

mammographic devices. Moreover, breast

composition should be considered and a correction

factor might be applied for the μ

assessment. Breast

density is being studied in many epidemiological

works also related to screening programs (Freer 2015,

Berg 2016). We are also realizing image processing

software able to automatically analyse clinical images

to evaluate the breast composition, based on both a

classical approach (comprising a pre-processing step,

a pattern recognition step, a classification step and a

segmentation step) and novel approaches based on

machine learning methods.

As a further development, this method could be

also applied to the tomosynthesis, an advanced

imaging technique that allows the reconstruction of a

three-dimensional view of the breast, overcoming the

projective (two-dimensional) imaging approach

limitations.

5 CONCLUSIONS

In conclusion, in this work we proposed the 2ABD as

a reproducible and easily computable dose index to

assess the radiation dose absorbed by the patient

during a mammography procedure. According to our

preliminary results, the 2ABD could be employed as

dosimetric index to be inserted into the radiological

report as required by the European Directive

59/2013/EURATOM. The development of this new

dose index is a part of a whole project finalized to

optimize the dose in mammographic procedures, give

a correct information about the risk related to ionizing

radiation and maintain high adherence to screening

programs.

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