This retrospective study on a screening population adheres to local patient confidentiality requirements. Breast density data was obtained from screening mammograms obtained from March 2013-September 2014. All mammograms were performed on Hologic Selenia and Dimensions machines (2011).
Screening Parameters
At the time of the study, the United Kingdom,
standard screening age was 50-70.
After age 70,
women must self-refer to undergo screening.
Below age 50 (in practice,
below age 49),
women who are screened are from high risk groups.
Density Measurement
Mammogram densities were measured using Volpara Density software (VolparaSolutions Wellington,
NZ),
a fully automated volumetric method of density assessment which has recently been validated against other methods of measuring density (2). Average density scores were calculated using the Volpara algorithm (the Macro) using all the available views (CC and MLO) and averaged right and left breast densities.
Approximately half of all screening mammograms were able to be assessed with Volpara software based on raw data transfer capability.
Volpara Breast volume, fibroglandular tissue volume,
and Volapara breast density% were obtained.
The % percent volumetric density was divided into preassigned categories (Volpara Grades) as follows:
0 to 4.5%
≥4.5 to 7.5%
≥7.5 to 15.5%
≥15.5%.
These divisions accord to the four BI-RADS breast composition categories:
1-fatty (VDG 1)
2-scattered (VDG 2)
3-heterogeneous (VDG 3)
4-extremely dense (VDG 4)
(Fig.
1 and Table 1.)
Demographic Data
Patient age and self-reported ethnicity were obtained from the database.
As well,
socioeconomic circumstances were approximated using The Indices of Deprivation 2010,
a deprivation index at the small area level,
created by the British Department for Communities and Local Government (DCLG) and released on 24 March 2011.
The IMD is made up of 7 domains (income,
employment,
health deprivation and disability,
education skills and training,
barriers to housing and services,
crime,
and living environment) combined into one score.
Each Lower Super Output Area is then ranked by overall score with the lowest score being the most deprived.
The range of values is from 1 to 32,000. These have been divided into quintiles.
Data Analysis
Data analysis was done using the R statistical system (3).
The data was loaded from Excel using openxlsx (4).
The statistical charts were generated using the lattice package for R (5).
Age distribution for the overall study population was analysed and categorized by incident and prevalent screens.
Breast volume, fibroglandular tissue volume (FGV) as well as breast density% with attendant Volpara Grades were analysed in relationship to cohort age.
Density was also evaluated in relation to ethnic origin code.
Finally,
density was analysed with respect to deprivation index quintiles (1-5,
1 is the most deprived and 5 is the least deprived).
Significance for ethnic and deprivation density differences was assessed via pairwise t-test using R.