Breast density is a known risk factor for breast cancer (1,6,7).
A variety of studies have suggested that certain demographic factors may be predictive of breast density and,
therefore,
of breast cancer risk (8-12).
Our study focuses on teasing out the relationship between density and demographic characteristics which are readily obtainable through our screening program records,
specifically,
age,
ethnicity,
and socioeconomic status/deprivation index.
Importantly,
this study is not a longitudinal study; instead,
it offers a snapshot view of density in a screening population at one moment in time.
It is also worth noting that we did not obtain data for the population on factors which are known to influence risk,
including hormone replacement therapy (HRT) status,
parity,
previous history of atypia or high risk lesion,
history of previous breast cancer,
etc. However,
such information may not always be available in a real time clinical scenario; our purpose was therefore to evaluate the feasibility of acquiring and analysing population density in relation to demographic information that was readily available to us as part of the routine screening examination.
Age and Density
An overall negative association between age and breast density has been shown in other studies (13).
Our study population demonstrates overall little difference for breast volume with respect to age particularly in the postmenopausal age group. A slight negative trend was noted when FGV was evaluated versus age as would be expected. In addition,
the downward trend is visible only before age 56,
a finding that may arise from a perimenopausal/postmenopausal divide. Analysis of percentile density measurements with respect to age showed that older women generally had less dense breasts than younger women,
again as would be expected,
but there was more variability towards the upper range of measurements.
The proportions of women with lower density breasts (Volpara Grade 1 and 2),
remain the same compared with the older women. However,
the proportion of women with dense breasts is decreased in older compared to younger women. It is,
however,
also important to note that there are women with very high breast density (Volpara Grade 4) in all age groups,
even in older age groups.
This has been noted in other studies and may have implications for a woman's lifetime risk of breast cancer (13).
These findings may also result from the characteristics of our particular study population.
UK screening parameters entail a narrower standard screening band than many screening programs; normal risk women are invited for screening between ages 50-70 (although in practice a woman may be 49 at her initial screen).
Women younger than 49 would usually be of higher than normal risk and therefore on specialized screening protocols.
Women older than 70 would be self-referred and represent a relatively small percentage of the overall screening population.
There may also be specific historical lifestyle influences at play.
For example,
it has been suggested that post-World War II rationing,
which persisted through the 1950's in Britain,
may have impacted on factors such as fat consumption which could affect breast density in the older cohort (14). Contraceptive use,
on the other hand,
with its potential impact on breast density,
would only apply to the younger members of the cohort (15).
HRT was introduced in the 1940's and became widely used in the 1960's,
but after the 2002 US Women's Health Initiative randomised clinical trial and the 2003 UK Million Women observational study brought up concerns regarding HRT safety,
HRT use decreased (16).
These changes may also have had impact on our cohort.
Ethnicity and Density
In our study, density was noted to be linked to ethnicity,
compatible with the literature (8-10). In particular,
women of Chinese ethnicity had higher breast density but this largely reflects their lower breast volume compared to other ethnic groups.
Black women had higher breast volume range than other ethnic groups,
but %density was not higher.
Age did not impact on ethnic group densities.
Deprivation and Density
Women in the most deprived quintiles (Quintile 1) tended to have larger and less dense breasts and women in the least deprived quintiles had denser breasts. This is consistent with other studies which have identified a positive association between socioeconomic status and breast density for women of highest educational level and for those living in the most affluent areas. This is generally attributed to the lower BMI of an affluent population (11). Our obsevation that there is a decrease in proportion of women with fatty breast density in the older population may represent a preferential self-referral rate from more affluent socioeconomic groups.
Limitations
Our study was limited by several factors.
Volpara analysis was not available for all screening sites during our study period.
As stated above,
we were not able to ascertain specific factors with impact on density (use of HRT,
menopausal status,
etc.,). Again,
these factors may well be obtainable in a research context,
but such data collection is not necessarily feasible within the confines of a mass population and a large screening practice.
Another limitation is the lack of information regarding body mass index.
Some demographic and density studies have taken into account body mass index,
either by collecting this data from participants or by performing BMI estimation,
for example,
using the volume of non-dense tissue as a proxy for BMI (8,12).
We have not used this information,
although we did evaluate overall breast volume.
However,
again,
we submit that in a practical context,
it is difficult to acquire BMI data in a population screening program.
Our study is therefore valuable as a practical investigation of what information may be readily available in a real time setting.
Conclusion and Future Directions
Our study suggests that age,
ethnicity,
and socioeconomic status may have impact on breast density with relevance for identification of future cancer risk.
Interestingly,
our study identified a group of women in the older age group range with extremely dense (Volpara Grade 4) breast tissue.
It would be worth evaluating cancer occurence in those women whose breast density is high and does not diminish with time compared to women whose density decreases with age.
Future investigations should focus on longitudinal density changes in the screening population,
in particular,
a comparison of relative risk in women with persistent dense breasts compared to those with initially dense breasts and decreasing density over time.
Statistical analysis performed by Toby Thurston,
Thurston Information Ltd, [email protected].