Breast Density classifications according to the BI-RADS lexicon.
Breast density is thought to be a factor for an increase in the risk of breast cancer. The idea was originally met with scepticism, but the consensus now is that it is indeed a true risk element and of interest to screening mammography.
Essentially, the density of the breast is a comparison of the relative amounts of fat versus fibroglandular tissues in the breast.
Women with ‘dense‘ breasts have a higher percentage of fibrous and glandular tissue and less fat tissue. (Fibroglandular parenchyma, basically means breast tissue).
The BI-RADS classification system identifies four levels of breast density in keeping with relative increases in the number of levels of fibro-glandular tissue. These are:-
BI-RADS type 1
The first ‘type’ classification is whereby the breast is composed of almost entirely fat. Glandular tissue is less than 25%.
BI-RADS type 2
In type 2 there is a scattering of fibroglandular tissues, ranging from 25% to 50% of the breast.
BI-RADS type 3
Specialists term the breast tissue in type 3 as ‘heterogeneously dense’. The parenchyma ranges from 51% to 75% of the breast tissue. ‘Heterogeneous’ means something contains many different items and has many different variations. With respect to breast density, it implies that the fibrous tissue is prevalent throughout the breast, but not clustered together.
BI-RADS type 4
The highest category is ‘type 4’. This means that the breast contains more than 75% glandular and fibrous tissue. At this level, the density of the breast may even reduce the sensitivity of the mammogram.
What exactly is ‘breast density’ measuring?
There are many different types of tissues in the breast. With breast cancer development we often speak of malignant cells originating in duct ‘epithelial’ tissue.
Fibroglandular parenchyma: What is that?
Inside the breasts, there are ‘glandular tissues’ So, glandular tissues are collections of cells that secrete fluids. Within the breast, the main glandular tissues are, obviously, those that produce milk and also those that secrete sweat.
“Fibrous” tissues are essentially collections of cells that give strength and resilience. Connecting and joining tissues between different elements are also ‘fibrous’.
“Stroma” is another term we often hear in association with breast cancer and breast tissues. Stroma basically refers to fibrous tissues, but specifically, those which ‘function’ to support the organs and glands, which ‘hang’ from them, more-or-less.
So, What is High Breast Density?
Well, high breast density refers to larger than average amounts of fibroglandular cells and tissue. Indeed, more than one would normally expect to see.
The amounts of fibrous and glandular tissues also tend to be quite proportionate. So, imagine a tree with the fibrous tissue as branches and the glandular tissue as leaves. High breast density is like a tree with many branches and lots of bushy leaves. In comparison, low density is like a tree with fewer branches and fewer leaves.
Breast density is really looking at the ‘parenchymal‘ tissues. This is an old Greek term which means ‘poured-in’. So, this gives a clue as to what we are specifically addressing. The ‘parenchymal’ tissues of the breast are those essential for the functioning. This tissue is distinct from structures that encompass the breast (lining tissues), or support it (stroma).
In the case of a breast, parenchymal tissues would include both glandular and fibrous tissues. However, an overabundance occurs when there is more of this tissue than necessary for the breast to function.
“Odds ratio” for high breast density
The American College of Radiology adopted and modified the four categories of breast density. The categories are part of an ‘integrated’ risk assessment and staging platform (BI-RADS).
However, BI-RADS considers much more than just breast density. It is difficult to isolate breast density in terms of an odds or risk ratio. This is because, in order to be reliable, so many other variables need to be taken into consideration.
However, even an inconclusive study can give general trends that can still help gauge cancer threat and plan for follow-up and treatment.
This page is a little out of date and we have more up-to-date information on breast density HERE, with all the latest risk ratios.
The Risk of Breast Cancer and Breast Density
When comparing the least dense breast tissue categories with the densest, studies suggest the chances of developing breast cancer ranges from an odds ratio of 1.8 all the way up to 6.0.
That is quite a bit of variability, but the majority of research still shows an odds ratio of 4 or higher. ( An ‘odds ratio’ of 4.0 means that a woman with dense breasts is four times more likely to develop breast cancer than a woman without dense breasts.)
There are a couple of plausible explanations of why high breast density leads to an increase in the risk of breast cancer. Firstly, the development of premalignant lesions such as atypical ductal hyperplasia is more likely in an ‘elevated growth’ environment.
Secondly, a woman may have naturally overactive aromatase. This enzyme increases estrogen production within the breast and may stimulate the growth of latent tumour cells.
The effect of Hormone Levels on Breast Density
Studies agree that hormone levels can affect breast density in women. Predicting the influence of both natural hormones and supplemental hormone therapies is always a bit of a guessing game.
After menopause, women’s breasts become fattier, in other words, ‘less dense’. In terms of breast cancer risk and breast density, this ought to be a positive thing. However, some hormone treatments work against this.
Women who take estrogen to counter the effects of menopause run the risk of an increase in breast density and faster breast cancer growth if a tumor is already developing.
Aromatase is an enzyme that synthesizes estrogen, and the biosynthesis of aromatase can stimulate the growth of fibro-glandular cells in the breast. To counter these processes, doctors will sometimes prescribe ‘aromatase inhibitors’ to block the synthesis of estrogen.
Is ‘breast density’ a modifiable risk factor?
Breast density, together with ‘obesity’, is a condition common in a certain percentage of women with breast cancer. So researchers speculate whether these factors are preventable.
Unlike obesity, research studies show that breast density actually increases the chance of developing cancer. Whereas obesity in relation to breast cancer is, in almost all cases, a concurrent observation.
The question remains, however, is high breast density something a woman can influence through her behavior or is it simply genetic?
Although results are still inconclusive, there is some evidence to support a genetic predisposition towards dense breasts. Indeed, some researchers even claim to have found one or two specific genes responsible. It is far too early to say what percentage of women may be genetically predisposed to dense breasts, but it could be over 50%.
Obesity means more Fat in the Breast
Obesity seems to have an inverse effect on breast density. This is because women with a high body mass index (BMI) tend to have an overabundance of fat in their breast tissue and hence, a very low breast density.
So, conclusions as to the genetic effects on high breast density would have to be in comparison somehow with the genetic predisposition towards obesity. The one cancels the other out, so to speak.
Can you change your Breast Density?
In terms of a women’s voluntary behavior, aside from taking hormones, there is some speculation that diet and alcohol intake can influence breast density.
Whilst there may be minimal evidence to support these suppositions, the differences in density between those that restrict their diet or alcohol intake versus those who do not is so very small, it is not statistically relevant.
There has also been some speculation that women who live in urban areas may, in general, have a higher breast density than women who live in rural areas. However, it may be the case that women living in urban centres are generally more likely to take hormone supplementation than women in rural areas. So this can possibly lead to the suggestion of an increase in breast density in some studies.
It is worth noting, however, that the increase in the risk of breast cancer remains high for at least 5 to 10 years after the mammogram first shows dense breasts. In order to measure any effects of ‘voluntary lowering’ of breast density, one would have to follow the study over a 5 to 10 year period.
Breast Density in Layman’s terms
After a mammogram, a woman may receive her report with the breast density in plain English rather than the BI-RADS types.
- ‘Very dense’: on a report is probably type 4, meaning 75% to 100%
- ‘Moderately‘ or ‘somewhat’ dense, most likely refers to category III, or 50% to 75% density
- Somewhat fatty or moderately fatty refers to type II, 25% to 50% density
- Partially fatty probably refers to type I or the 0 to 25% range
It used to be, that radiologists did not describe the mammographic density in their reports. I think that in the USA now it is a requirement, but not in all countries.
Are there any other ways to determine breast density, aside from the mammogram?
With the advent of computer processing, specialists are constantly devising more specific ways of calculating and classifying breast density. In addition to the basic mammogram X-ray, experiments are underway that calculate density on the basis of the overall two-dimensional area of fibro-glandular tissue in the image. In addition, there are ‘textural’ models that examine the statistical distributions of parenchymal tissues.
Essentially, these more-detailed analyses create many sub-categories within the four basic ‘types’ of increasing breast density. It is not immediately clear as to what purpose is served by attempting to sub-classify, or more precisely, to ‘quantify’ mammogram densities.
Indeed, the four types are already very useful and only part of an overall risk assessment. However, over time, following the outcomes of highly specific breast density measurements may lead to statistically meaningful trends, and to earlier, more focused breast cancer treatments.
- Common Mammogram Findings
- BIRADS 2, 3, 4 and 5: What do they Mean?
- BIRADS 4 and 5
- Do You Have Dense Breasts? How to Find Out?
- Breast Cancer Risk Factors that you Can NOT Change
- Example Mammograms of Dense Breasts
- Index of ALL our Articles on Breast Cancer Screening
Return to Homepage
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