Japanese Journal of Radiology

, Volume 32, Issue 9, pp 561–567

Feasibility study of a breast density measurement within a direct photon-counting mammography scanner system

Authors

    • Diagnostic Imaging CenterKameda Kyobashi Clinic
    • Department of Diagnostic Radiology and OncologyTokyo Medical and Dental University, Graduate School of Medicine
  • Mitsuhiro Tozaki
    • Diagnostic Imaging CenterKameda Kyobashi Clinic
    • Breast CenterKameda Medical Center
  • Tamiko Yoshida
    • Diagnostic Imaging CenterKameda Kyobashi Clinic
  • Ai Saita
    • Diagnostic Imaging CenterKameda Kyobashi Clinic
  • Mari Yakabe
    • Philips Electronics Japan, Ltd.
  • Kanae Nii
    • Philips Electronics Japan, Ltd.
Technical Note

DOI: 10.1007/s11604-014-0333-x

Cite this article as:
Machida, Y., Tozaki, M., Yoshida, T. et al. Jpn J Radiol (2014) 32: 561. doi:10.1007/s11604-014-0333-x

Abstract

Purpose

To evaluate the clinical feasibility of breast density measurements by a new application within a direct photon-counting mammography scanner system.

Materials and methods

A retrospective study of consecutive women who underwent mammography using a direct photon-counting mammography scanner system (MicroDose mammography SI; Philips Digital Mammography Sweden AB) was performed at the authors’ institution between September and December 2013. Quantitative volumetric glandularity measurements were performed automatically for each acquired mammographic image using an application (Breast Density Measurement; Philips Digital Mammography Sweden AB). The quantitative volumetric glandularity of each breast was defined as the average values for the mediolateral oblique (MLO) and craniocaudal (CC) mammogram views.

Results

Of the 44 women who underwent bilateral mammogram acquisitions, the breast density measurements were performed successfully in 40 patients (90.9 %). A very good to excellent correlation in the quantitative breast density measurements acquired from the MLO and CC images was obtained in the 40 evaluable patients (R = 0.99).

Conclusion

The calculated volumetric glandularity using this new application should correspond well with the true volumetric density of each breast.

Keywords

MammographyBreast density measurementApplicationBI-RADS

Introduction

Breast density has been reported to be a risk factor for breast cancer. Previous studies from Western countries have shown that women with dense tissue in 75 % or more of their breasts have a risk of breast cancer that is four to six times as great as the risk among women with little or no dense tissue [19]. Reports from Asian countries, mainly from Japan, have also identified dense breast tissue as a risk factor for breast cancer [1012].

Quantitative methods of assessing fibroglandular tissue in the breast have been developed. The first generation of these methods involved the review of 2D projection images of compressed breasts [13, 14]. Because 2D methods have several disadvantages, such as a lack of consideration of breast thickness, their accuracy was limited. While more recently proposed methods have attempted to measure the fibroglandular and adipose thickness in each pixel and enable an increased accuracy [1518], they still bore measurement errors for volumetric breast density. Studies have shown that the accurate measurement of breast density can be achieved using dual-energy imaging [19, 20], although the additional acquisition of images using high-energy X-rays were indispensable for this measurement, resulting in an increased glandular dose as well as the risk of misregistration artifacts produced by patient movements.

A direct photon-counting mammography scanner system (MicroDose mammography SI; Philips Digital Mammography Sweden AB, Solna, Sweden), which is a scanning multi-slit full-field digital mammography system with a photon-counting silicon stripe detector, is capable of sorting photons into low- or high-energy categories, acquiring spectral energy data without additional dose exposures or the risk of misregistration, and enabling the volumetric glandularity of each breast to be calculated automatically by a new application within the system (Breast Density Measurement; Philips Digital Mammography Sweden AB, Solna, Sweden). Although a high accuracy of quantitative breast density measurements by this new application using computer simulations and physical phantoms have been reported [21], the clinical feasibility or validation of this new technology has not yet been reported. The purposes of this study were to evaluate the clinical feasibility of breast density measurements and to assess its correlation with visual assessments performed by a physician.

Materials and methods

Patients

A retrospective study of 45 consecutive women who underwent mammography using a direct photon-counting mammography scanner system (MicroDose mammography SI; Philips Digital Mammography Sweden AB) was performed at the authors’ institution between September and December 2013. One woman was excluded from the analysis because a comparison between the right and left breasts was unavailable because of a previous unilateral mastectomy for breast cancer. The remaining 44 women who underwent bilateral mammogram acquisitions were included in this study.

Correlation of breast density measurements between mediolateral oblique (MLO) and craniocaudal (CC) images

Both MLO and CC mammogram views were acquired for each woman. Quantitative volumetric glandularity measurements were performed automatically without any additional procedures for each acquired mammographic image using an application (Breast Density Measurement; Philips Digital Mammography Sweden AB). The breast density data were stored in the Digital Imaging and Communication in Medicine (DICOM) header and as a DICOM Structured Report and were displayed in the mammography diagnosis monitors.

The quantitative volumetric glandularity of each patient was compared between the MLO and CC views of the mammograms.

Correlation between age and quantitative breast density measurements

The quantitative volumetric glandularity of each breast was defined as the average values for the MLO and CC mammogram views. The quantitative volumetric glandularity of each patient was then compared according to age.

Correlation between quantitative measurements and visual assessments

Qualitative mammographic breast density assessments for each woman were performed by one radiologist (who had 7 years of experience in diagnostic radiology, who was engaged in breast screening and diagnosis, and who had an AS-ranked license in mammography interpretation guaranteed by The Japan Central Committee on Quality Assurance of Breast Cancer Screening, a nonprofit organization) who was blinded to the quantitative volumetric glandularity or any other clinical information. The qualitative visual mammographic breast density assessments were performed according to the Breast Imaging-Reporting and Data System (BI-RADS) reporting criteria for breast density [22] as follows: BI-RADS 1, the breast is almost entirely fat (<25 % glandular); BI-RADS 2, there are scattered fibroglandular densities (~25–50 % glandular); BI-RADS 3, the breast tissue is heterogeneously dense, which could obscure detection of small masses (~51–75 % glandular); BI-RADS 4, the breast tissue is extremely dense. This may lower the sensitivity of the mammography findings (>75 % glandular). The quantitative volumetric glandularity of each breast was defined as the average values of the MLO and CC images. The average of the quantitative volumetric glandularity for each BI-RADS breast density score group was calculated.

Statistical analysis

The correlation coefficient was calculated for the results of the quantitative volumetric glandularity measurements acquired from MLO and CC images of each breast and for the age and quantitative volumetric glandularity measurements. When interpreting the value of the correlation coefficient, R values from 0 to 0.25 were regarded as indicating the absence of a correlation, R values from 0.25 to 0.50 were regarded as indicating a poor correlation, R values ranging from 0.50 to 0.75 were regarded as indicating a moderate to good correlation, and R values from 0.75 to 1.0 were regarded as indicating a very good to excellent correlation between the variables [23]. The statistical analysis was performed using Microsoft Excel 2007 software (Microsoft Corp, Redmond, WA, USA).

Results

Patient populations

Out of the 44 women who underwent bilateral mammogram acquisitions, comprehensive breast density measurements were unavailable in four patients for the following reasons: in one patient, the thickness of the breast compressed for image acquisition was below 20 mm, which was the lower limit of the breast density measurement software; in three patients, the breast density measurements for the MLO view was unavailable after successful image acquisition. Among these latter three patients, the breast density measurement of the MLO view was unsuccessful because the unilateral pectoralis major muscle was not within the field of view after positioning in two patients, and the breast glandular tissue overlapped with the unilateral pectoralis major muscle in one patient. The breast density measurements were performed successfully in the remaining 40 of the 44 patients (90.9 %). The average glandular dose of these mammographic images ranged from 0.42 to 1.45 mGy (mean 0.74 mGy).

Correlation of breast density measurements between MLO and CC images

A very good to excellent correlation in the quantitative breast density measurements acquired from the MLO and CC images was obtained in the 40 evaluable patients (R = 0.99, Fig. 1).
https://static-content.springer.com/image/art%3A10.1007%2Fs11604-014-0333-x/MediaObjects/11604_2014_333_Fig1_HTML.gif
Fig. 1

Correlation of breast density measurements between MLO and CC images. A very good to excellent correlation in the quantitative breast density measurements acquired from the MLO and CC images was obtained in the 40 evaluable patients (R = 0.99)

Correlation between age and quantitative breast density measurements

The relationship between the quantitative volumetric glandularity of each breast and the age of the 40 patients is shown in Fig. 2. A poor correlation between the two variables was seen (R = –0.34).
https://static-content.springer.com/image/art%3A10.1007%2Fs11604-014-0333-x/MediaObjects/11604_2014_333_Fig2_HTML.gif
Fig. 2

Correlation between age and quantitative breast density measurements. A poor correlation between the two variables was seen (R = –0.34)

Correlation between quantitative measurements and visual assessments

The average quantitative volumetric glandularity of each BI-RADS breast density score group was 9 % for BI-RADS 1 (n = 2) (Fig. 3), 21.7 % for BI-RADS 2 (14–38 %, n = 10) (Fig. 4), 34.3 % for BI-RADS 3 (17–54 %, n = 36), and 64.3 % for BI-RADS 4 (37–85 %, n = 32) (Figs. 5, 6), respectively. Distribution of volumetric glandularity for each BI-RADS breast density score is shown in Fig. 7.
https://static-content.springer.com/image/art%3A10.1007%2Fs11604-014-0333-x/MediaObjects/11604_2014_333_Fig3_HTML.jpg
Fig. 3

A 69-year-old woman with breasts that were assessed to be almost entirely fat (BI-RADS 1). Volumetric glandularity was calculated to be 9 % in both breasts. The average glandular dose was 0.60 mGy

https://static-content.springer.com/image/art%3A10.1007%2Fs11604-014-0333-x/MediaObjects/11604_2014_333_Fig4_HTML.jpg
Fig. 4

A 40-year-old woman with breasts that were assessed to have scattered fibroglandular densities (BI-RADS 2). Volumetric glandularity was calculated to be 38 % in the right breast and 35 % in the left breast. The average glandular dose was 0.50 mGy. The volumetric glandularity was exceptionally high in this case among the BI-RADS 2 cases

https://static-content.springer.com/image/art%3A10.1007%2Fs11604-014-0333-x/MediaObjects/11604_2014_333_Fig5_HTML.jpg
Fig. 5

A 28-year-old woman with breasts that were assessed to be extremely dense (BI-RADS 4). Volumetric glandularity was calculated to be 59 % in the right breast and 58 % in the left breast. The average glandular dose was 1.10 mGy

https://static-content.springer.com/image/art%3A10.1007%2Fs11604-014-0333-x/MediaObjects/11604_2014_333_Fig6_HTML.jpg
Fig. 6

A 64-year-old woman with breasts that were assessed to be extremely dense (BI-RADS 4). Volumetric glandularity was calculated to be 37 % in the right breast and 41 % in the left breast. The average glandular dose was 0.57 mGy

https://static-content.springer.com/image/art%3A10.1007%2Fs11604-014-0333-x/MediaObjects/11604_2014_333_Fig7_HTML.gif
Fig. 7

Box-and-whisker plot of volumetric glandularity was estimated using the BI-RADS breast density score

Discussion

Venturini et al. [24] evaluated a breast-screening program for 40- to 49-year-old women living in the town of Segrate, Milan, Italy, using a previous version of the MicroDose mammography system (with the same dose reduction ability, but without the availability of breast density measurements) by Sectra Mamea (Solna, Sweden), which is now part of Philips Women’s Healthcare. In their study, they reported that the average glandular doses for CC and MLO projections were 0.73 mGy (range 0.33–1.90 mGy) and 0.76 mGy (range 0.33–1.90 mGy), respectively. In this study, our results regarding the average glandular dose were comparable to those of this previous study, indicating that a similar dose reduction can be achieved among both Asian and European populations.

In previous studies, the breast density value was defined as the average of results from MLO and CC images [25, 26], and this new application for breast density measurement also adopts this consensus. However, no previous studies have assessed the clinical feasibility of this new technology or provided any validation of the technology; consequently, the validity of adopting the average value as a representative value for each breast has not been investigated.

In this study, 9.1 % (4/44) of the patients were excluded from the breast density measurement analysis even though mammographic images were acquired, and the MLO breast density measurement was unavailable in 75 % (3/4) of these cases. Furthermore, the excellent correlation of the quantitative breast density measurements between the values acquired from the MLO and CC images of each breast (R = 0.99) indicates that breast density information acquired from only the MLO or CC image and that calculated from the average of the MLO and CC images are comparable. Given the reported high accuracy of this system using computer simulations and physical phantoms [21], this excellent correlation also indicates that the calculated value, whether it is from a single MLO or CC image or an average of the two images, seems to correspond well with the true volumetric density of each breast. Even though adopting the calculated value from the CC image may be promising, MLO images are more widely used for screening mammography. Therefore, adopting the average values acquired from the MLO and CC mammogram views may be an appropriate procedure.

Although breast density is known to decrease with age [27], Asian women are more likely to have dense breasts than white women, and this tendency becomes more obvious in older populations [28]. In this study, there was a tendency for breast density to be high, and the tendency persisted even in older patients, though there was a weak tendency for breast density to decrease with age. These results are in accordance with those of previous studies.

The average quantitative volumetric glandularity for each BI-RADS breast density score group was 9 % for BI-RADS 1, 21.7 % for BI-RADS 2, 34.3 % for BI-RADS 3, and 64.3 % for BI-RADS 4. These results indicate that, with an exceptional case as shown in Fig. 4, this new method of volumetric breast density measurement is thought to be comparable with the qualitative visual assessment, which is the current standard for breast density assessments. However, we assessed the correlation between quantitative measurements and qualitative visual assessments for breast density in only 40 patients, and this small population was a serious limitation of the present study.

We conducted a preliminary study of this new application, which has not been previously reported. This new application is easy to use without physicians’ evaluation and may have possibilities to select women for whom screening ultrasound will be beneficial. The significance of breast density measurements using a direct photon-counting mammography scanner system for clinical concerns (e.g., its influence on breast cancer risk or the sensitivity of mammography results) needs to be evaluated in a larger population in the future.

Acknowledgments

There is no grant support.

Conflict of interest

The authors declare no conflict of interest.

Copyright information

© Japan Radiological Society 2014