What exactly is mammographic breast density?

What you read is correct. There is no relation between how your breasts feel and mammographic density.

That said, what your doctor told you is probably also true. This is because at the age of 22, it's almost certain that your breasts are dense. That's because the vast majority (80%+) of young women have dense breasts. This is why screening mammograms are not recommended for younger women - because mammos aren't that great at 'seeing through' dense breast tissue and almost all young women have dense breast tissue. When younger women have a breast concern, they are sent for an ultrasound first, not a mammogram.

It is true that dense breast tissue can increase breast cancer risk, but the concern is when breast tissue is denser than expected for one's age, and if high density remains as the woman ages and enters menopause. A 22 year old who has dense breasts has perfectly normal breasts (for a 22 year old) and the same breast cancer risk as an average 22 year old. Because high breast density is expected for someone this age, it's already factored into the risk numbers. According to this site, the risk that a woman will develop breast cancer anytime over the entire 10 years of her 20s (i.e. from age 20 to 29) is 0.06%.

As for your breasts feeling lumpy, that's normal too. It's a misconception that breasts are supposed to feel soft and smooth with no lumps. That's true for some women, but 60% of women have fibrocystic breasts (which is different than dense breasts) that often can feel lumpy. What's important to learn what feels normal for your breasts, so that you can identify changes or new lumps that develop.

Antonia,

First, to my earlier post, the appearance and feel of your breasts have NOTHING to do with density.

This is mammographic density. It's how your breast tissue looks when imaged by a mammogram machine, not how your breasts look to you or how they feel to you.

Second, you may have read that 7%-10% of women have extremely dense breast tissue. This data is based on screening mammograms, which most women only start to get sometime between the ages of 40 and 50. Any time I have seen this "10%" figure, it has specified that it refers to women aged 40 and over. It is not relevant to someone your age.

As I said in my earlier post, younger women have denser breasts, and younger women are not included in the 10% figure. Take a look at this chart. It doesn't even go down to your age group, but for women in their 30s, over 90% have dense breasts. The figure would be even higher for women in their 20s.

This chart includes both "heterogeneously dense" and "extremely dense" in the dense category, but I seem to recall reading that over half of younger women (20s and 30s) have extremely dense breasts - the highest category of density.

So if at age 22 you have extremely dense breasts, you are not in the 10% group but for your age, you are in the majority.

Worry about breast density only if you still have extremely dense breasts 30+ years from now, when you are at least 5 years into menopause.