California women who have dense breasts on their mammograms are currently receiving written notification that they have “dense breasts.”  Some institutions are notifying all women of their density – both high density and low density – whereas some institutions are notifying only women with dense breasts, the minimum that the law requires.  The law does not stipulate any action that a physician must take, but the letter raises more questions than it answers.

Does breast density correlate with density on clinical examination?

No!  Breast density – both in the context of SB-1538 and as a risk factor discussed below – is a specific finding revealed only by mammograms.  Dense breasts on clinical breast examination may be dense either 1. because of dense gland tissue as seen in radiographically dense breasts, or 2. because of accumulated fatty tissue that can exist in breasts that are almost entirely fat on mammograms, i.e very non-dense.  It is not possible to determine whether tissue or fat causes the palpable density by clinical breast examination alone.

How many women have dense breasts?

Half of women have dense breasts on their mammogram.  This “half” breaks down as approximately 10% of all women with extremely dense breasts and 40% with heterogeneously dense breasts [click here, Table 1].  The breakdown is important because only the 10% of women with extremely dense breasts have the maximum increase in risk, and only one-fifth of women who are told they have dense breasts will have a very high risk.  

For comparison, about 10% of women have almost entirely fatty breasts and 40% have scattered densities.

What is the risk with extremely dense breasts?

Independent of other risk factors, a woman with extremely dense breasts (10% of all women) has roughly twice the risk of developing breast cancer as a woman with scattered densities (40% of all women and the majority of women with non-dense breasts) [click here, Table 5]. 

Similarly- as an approximation – breast density divides any risk group of women – for example those with family history, higher alcohol use, etc. – into higher risk (dense breasts) or lower risk (non-dense breasts) sub-groups.  However, the ability to detect truly low-risk versus truly high-risk groups by mammographic density is limited because the ratio of risk between the highest and the lowest groups is only about two fold.

For example, a woman with atypical duct hyperplasia has about an 8% chance of cancer over 10 years.   This can be divided into higher and lower risk groups, but the ratio between low and high is always two to one and they must average to 8%.  This limits how high the high risk really is, but more importantly, it also limits how low the risk is for lower risk women.  These are real differences, but it is not clear how to apply this information since women with low-density mammograms can still develop breast cancer.  [See previous post on Breast Density.]

Why study breast density?

Wolfe in 1967 reported that breast parenchyma patterns could identify women at high risk of developing breast cancer.  Breast cancer increased with the progression from N1 through P1 and P2 to DY parenchyma patterns.  These patterns correspond roughly – though not exactly – to current breast density classifications ranging from almost entirely fatty to extremely dense.

Predicting risk from parenchyma patterns is complicated because many women do not fit one category or another.  Measuring breast density has the same problem.  Computerized systems help, but there is an unavoidable arbitrariness in selecting the cut points between density groups.  Even using arithmetic quartile groups assumes that the point of separation between quartiles has biological significance. 

Cost cutting is the new goal.

Cost cutting – the opposite of risk identification – is the reason for recent research on breast density.   Instead of finding high-risk women, the goal has become to identify a group of women with an absolute low-risk of cancer – or a low enough risk – for whom mammograms can be done less frequently or omitted altogether [click for abstract]. 

Kerlikowski et al expand on this reasoning advocating biennial (every two years) instead of annual screening for most women [click for abstract].  However, they also note that women 40 to 49 years of age – with extremely dense breasts – are more likely to be diagnosed with advanced breast cancer if they have biennial rather than annual mammograms. 

They contend this risk of more advance cancer is acceptable because annual mammograms for ten years are also associated with a significant possibility of a false positive mammogram with attendant stress and unnecessary testing, perhaps even a benign biopsy. 

Women who undergo any breast diagnostic workup experience stress.  They ignore, however, the likelihood of greater stress from remorse for a woman with delayed cancer diagnosis after skipping her mammogram for a year or more.  Such stress has not been considered in their arguments.

What is recommended for women with dense breasts in 2013?

A group of radiologists representing Stanford, UC Davis, UC Irvine, UC Los Angeles, UC San Francisco, UC San Diego, California Pacific Medical Center, and Alta- Bates Summit Medical Center in Berkeley have created a website with recommendations for representative patient scenarios.

They advise physicians to base screening recommendations on breast density only when density is considered along with other factors

Their recommendations have a general theme.  If a woman has dense breasts – and after risk assessment of family history and/ or other factors – is at increased risk for reasons in addition to breast density alone, they recommend screening MRI.  Otherwise, mammography should be recommended.  Other modalities, e.g. screening ultrasound and 3-D mammography, are investigational as of March 2013.

New tests are being developed.

Berg and colleagues have evaluated adding hand-held ultrasound to screening for women with at least heterogeneously dense breasts on mammograms [click here].  Ultrasound significantly increased cancer detection by an additional 4.2 cases per 1000 exams, but the false positive rate increased from 4.4% for mammograms alone, to 8.1% for ultrasound alone, and 10.2% for combined mammograms and ultrasound.  Ultrasound helps, but at the cost of more false positive evaluations.

Computerized tomosynthesis or 3-D mammography processes two breast exposures to create a set of multiple tomographic images of the breast.  In preliminary studies, tomosynthesis  detected 27% more cancers with a simultaneous 15% decrease in false positives [click for abstract].  The tomographic study itself has only a little more radiation exposure than a screening mammogram, but interpretation is based on both standard mammograms and tomosynthesis together – done with two different exposures at the same time – so currently the total radiation dose is doubled.  Computer algorithms to use one pair of exposures to create both tomosynthetic and standard whole thickness images are being developed to resolve the problem of increased radiation exposure. 


Would you walk 30 minutes every day to live a year longer?

That ‘s the starting benefit of moderate exercise.  You gain more if you do more strenuous exercise. 

Exercise is measured in METS.

METs – “metabolic equivalents” – are the way exercise physiologists compare energy used by a person doing different activities.  By definition, one “metabolic equivalent” (1.0 MET) equals sitting quietly for an hour.  At the other extreme, for example, running at a speed of 10 miles per hour is 18 METS (if you keep it up for a whole hour).  This means you use 18 times more energy to run at a speed of 10 miles per hour than to sit in a chair.

If the METs are multiplied by the number of minutes you do an exercise, they are called MET minutes.  For example, sitting for one hour is 1 MET energy expenditure for 60 minutes or 60 MET minutes.  Or, 3 METS an hour for 60 minutes equals 180 MET minutes (3 MET hours x 60 minutes/ hour = 180 MET minutes).  MET minutes can be added up to measure exercise for a whole week.

Note: an activity counts only if it is done for a minimum of 10 minutes or in an extremely intense short burst, such as a sprint race.  Doing something for 5 minutes does not have the same clear benefit because it does not make you breathe faster.

METs are measured by how much oxygen you use.

We need oxygen to extract energy from sugar and fat.  Physiologists determine energy use by measuring the oxygen a person uses as they breathe in and out during exercise.  To measure the oxygen a person uses, they measure the oxygen left in the air when the person breathes out.  The amount of oxygen the person used is the difference between what the person breathed in and what is left in the air when they breathe out.   

A person with normal lungs – and normal breathing response – can estimate the relative number of METS they are using by observing how hard or how fast they are breathing.  3.0 METS an hour means you can still carry on a conversation, but you are breathing hard enough that you can’t sing.  7.0 METS an hour means you are breathing hard enough that you can’t talk more than gasping a few words at a time.

MET minutes per week of exercise benefits longevity.

A team of researchers from Canada, Boston, Baton Rouge, and Columbia, South Carolina have used data from NHANES (National Health and Nutrition Examination Survey), the Health Interview Study, and US Life Tables to estimate 5-year survival according to how much a person exercises in their leisure time. 

They found that people who exercised regularly- but not enough to add up to 500 MET minutes per week – had some benefit [click for abstract].  People who did more exercise got more benefit.  As an approximation, 500 METs per week is equivalent to 2½ hours of moderate activity such as brisk walking, or 1¼ hours of vigorous activity such as running hard.

Caucasians and African Americans gained about 1 ½ to 2 years of life for being somewhat active every week (a little more for younger persons and a little less for persons over 60 years old).  If they exercise more than the basic 500 MET minutes per week, they will gain about an additional year.  For reasons that were not clear, neither Hispanic men nor Hispanic women gained life years from increased leisure time activity in this study, but there was no measure of how much activity they already had in their daily lives, so they may have been more active all the time.

Women who are active have a lower risk of getting breast cancer.

The Nurses Health Study previously found that women who get 9 MET hours per week or more of exercise – including those who start exercise after menopause – have about a 10 percent reduction in breast cancer [click here for complete article].   [9 met hours x 60 minutes per hour= 540 MET minutes, about the same as the 500 MET minutes that increases longevity].  Again, this is 30 minutes of brisk walking every day.  Note, if you walk briskly for 45 minutes, you can walk fewer days to get the same number of METs.  More intense exercise provides the same benefit in less time.

Calculate your METs per week.

There is a US Government website that discusses METs, but they only list a limited variety of examples [click here].  A year 2000 article in Medicine and Science in Sports and Medicine gives a wide variety of examples, but you need to pay for the whole article [click here for the abstract and the 2011 update]. 

To help you, I have selected representative activities and calculated the MET minutes for an hour and half an hour of each activity [click here].

To calculate the number of METs you personally expend in a week, list each activity, and how long you do it for.  You get credit for the intensity of each activity, but only for the length of time you do it.  For example, if you swim hard for an hour, that is 480 MET minutes (or 6 METs).  If you only do it for half an hour, that is 240 MET minutes, as in the table.  But if you only do it for 15 minutes, that is half of the 240 or 120 MET minutes. 

The health benefit you receive comes from the total of all of your leisure time activity.  If you swim hard for 30 minutes two days a week, that is 240 MET minutes x 2 or 480 MET minutes.  Suppose you also do an hour of Tai Chi for 240 MET minutes, and take a one-hour, brisk walk on Saturday morning for 228 MET minutes.  Your total would be 948 MET minutes or a total of 15.2 METs (948 MET minutes/ 60 minutes per hour = 15.2 METs) in that week.   

Again, note that you receive the basic benefits of 500 MET minutes a week by walking only 30 minutes per day. 

The benefits of vigorous activity also apply to men.