Why is breast tissue so susceptible to cancer when it isn't exposed to toxins?

With most other forms of cancer, there’s some form of repetitive harm or poison involved - lung cancer, for instance, is because of repeatedly inhaling toxins (i.e smoking,) colon cancer is because of unhealthy diet, skin cancer is because of exposure to sunlight UV, leukemia can happen because of toxins repeatedly getting into the blood, etc.

But what specifically about breast tissue makes it so likely to turn cancerous? Same for bone cancer, and every other non-toxin-caused cancer too.

This is your first fallacy. While toxins/harm can increase the risk of cancer it is not the sole cause of cancer.

Evert time a cell divides there is a risk of the process being less than perfect and errors occurring during the copying of DNA. Cancer happens when enough mutations of the right sort occur, taking the brakes off cellular reproduction and other control systems. That is why age, all by itself, increases the risk of cancer - the cells in the body of someone 80 have divided/reproduced so much more than the cells in someone say… 20.

But this sort of mutation disaster can happen at any age. There are even instances of babies being born already with cancer (fortunately, that is rare but it can and does happen). Early cancers include retinoblastoma and Wilms’ tumor.

Incorrect. A bad diet increases the risk of colon cancer but does not cause colon cancer.

Exposure to UV light is only one potential cause of skin cancer. Contact with certain chemicals can also do it.

Not sure where you get that one, either.

Another thing that does not cause cancer but can contribute to its spread is anything that suppresses the immune system.

Hormones, mostly, at least for breast tissue. Sex hormones cause cells in the gonads to go through various cycles of reproduction, over and over, and thus over the course of a lifetime those cells, due to so many divisions, are more prone to cancer than some others that divide less. That’s why one of the treatments for breast cancer is administering drugs that suppress sex hormones, so those cells are less stimulated and in many cases it can slow down cancer. Hormones are also why men become more and more prone to prostate cancer as they age… unless they’re castrated, something most men would rather not do. Again, one of the treatments for prostate cancer is hormone suppression.

Bone cancer is due, again, to cell turnover/reproduction. Your bones are not static, their components are constantly rebuilt and reinforced over and over a lifetime, so while bones may appear to be inert they aren’t, they’re very much alive.

For breast cancer, the existence of BRCA genetic susceptibility explains much of it. Because breast cancer tends to occur after age 50 there isn’t really a direct evolutionary selection against it, and in pre-modern medicine women would often die in or due to complications from childbirth at some point in their child-bearing age. There is some evidence that breast cancer may be exacerbated by hormonal disruptions and other environmental and dietary factors but the reality is just that there is a population of women who are just very prone to developing the various cancers of the breast tissues, and thus the focus on developing effective treatments for it.

Tobacco, alcohol, non-organic contaminants, ionizing radiation, and transmissible viruses are responsibly for about half of all non-breast-related cancers. These are all essentially maladies of the agricultural and industrial age, as are endocrine-disrupting chemicals, sedentary lifestyles, and obesity that are credited with causing or exacerbating many other cancers. Sarcomas (cancers of the bone and joint tissues) are relatively rare and generally related to genetic susceptibility although they can also be caused by environmental factors; when a ‘cluster’ of such cancers occur, particularly in children, it is a strong indication of some kind of significant industrial contamination.

Since in the pre-Industrial and pre-modern medicine era child mortality was very high, this probably masked a lot of germ cell cancers, blastomas, and acute lymphoblastic leukemia. Similarly, cancers that tend to affect the elderly, such as prostate cancer (which occurs in many men but progresses at such a slow rate that it may not present symptoms until the patient dies of other natural causes), are a feature of modern medicine extending life expectancy into the seventies and beyond.


There are also several other genes associated with an increased risk of breast cancer. The Wisdom Study, which is trying to determine if there are better screening regimens than “mammogram every year” is looking at these nine mutations:


Plus other genetic stuff.

I ageee with the above posts, with the quibble that cancer is in some sense a disease of time, rather than age. It just so happens that age comes along with the passage of time.

All our cells have various mechanisms in place that either tell them to stop dividing in mature individuals (neurons and muscle cells are the best examples) or to only divide to the extent needed to keep the various tissues in a stable state. Once enough of those mechanisms are disrupted, a cell will become malignant.

Think of it as flipping a certain number of coins at the same time, let’s say 10 just for this example. If they all come up tails, then you have cancer. For cells in breast tissue, having the BRCA 1 or 2 mutation is like that person now only flipping 9 coins instead of 10, because one of the coins is automatically tails. Add in a toxins, radiation, and the other known risk factors, and now you’re only flipping 8, or 7, or just 1 coin in someone with many risk factors / severe exposures to thing like radiation / etc. Age is a factor because, as @Broomstick noted, the older one is, the more times all those coins have been flipped, so the greater chance tails will figuratively come up, which in the real world means a mutation in one of the several genes that help to prevent unregulated cellular division. Unfortunately this means, as also noted, that with a bit of bad luck, even very young children can get cancer if all the figurative coins happen to come up tails the first time or two that they’re flipped.

Genetic susceptibility, and HORMONES.

Breast tissue can certainly be exposed to potentially toxic agents, some of which have been linked to breast cancer.

As an aside to the above excellent posts.
A risk factor is not the same thing as a causative agent. Risk factors are correlated with disease. Correlation is not causation. Might be, but that is a separate question.
Probably the biggest risk factor in general health problems is being poor. The biggest risk factor for breast cancer is being female. Men can and do die of breast cancer, but it isn’t nearly as common.
The genetic links are big in breast cancer. There you see both correlation and causation.

Would this reduce to: the biggest risk factor is the amount of breast tissue?

No, the amount of hormone cycling. Breast tissue is mostly fat.

As a breast cancer survivor (2021) my thoughts are. “Toxins” don’t just come from outside. If you drink alcohol, that metabolizes inside your body and probably gets everywhere. The same may happen with toxins that you breathe in.

The hormonal thing seems key. My tumor was “estrogen responsive” meaning the cells would grow faster in the presence of estrogen. So I’m on an estrogen blocker for 5 years. Since I was overdue for menopause anyhow it wasn’t a big deal other than more of the same (hot flashes).

I have also heard that as women age, they are prone to uterine cysts, which I have. If you get a lot of cysts, that can actually stimulate estrogen production so I’ll have to watch those as well.

To add to my last post. I noticed the tumor after being on a soy-milk kick for several months prior. The Silk brand chocolate soy milk was so delicious and a quick protein boost when I was constantly stressed and had a hard time eating anything at all. Soy is said to be an “estrogen mimic”. It’s possible that w/o that, my tumor would have stayed small and I never would have noticed it.

My docs pooh-poohed this suggestion but I still think there is a connection, so now I don’t eat soy except for tamari sometimes. No more tofu or soy milk. Sad face.

Glands in general tend to be prone to cancer.

Well, it’s possible, but we can’t really know things like this without detailed, systematic studies of many, many people.

So some cancers are definitely exposure related. Bladder and lung cancer for example. For each a majority of cases are felt to be due to tobacco or workplace toxin exposure or both.

And more are due to the total number of stem cell divisions.

Question on the apparent frequency of cancer predisposing genes: do they have any evolutionarily offsetting beneficial impacts?

These are for cancers that take away mothers when they are still of age to be contributing to the possible reproductive success of their children, if not their children’s very survival. How did they become so relatively common in some sub populations? (For example one in 40 women of Ashkenazi Jewish descent carry BRCA.)

Yeah, that annoyed me sometimes when my spouse was dying of bladder cancer - every so often I’d run into a doctor or other medical type who kept pressing me on my spouse’s smoking - he NEVER smoked - or what his job was, looking looking looking for a REASON he had the cancer. It’s like they couldn’t accept he was one of the outliers without those risk factors.

Some people who never smoked in their life get lung cancer. Some people with no known risk factors get bladder cancer. Or some other cancer.

Or sometimes there’s a risk factor we aren’t aware of - a couple months after my spouse died my sister-the-doctor found some medical research that indicated that people with spina bifidia were prone to a fast, highly aggressive bladder cancer from their mid-40’s onward. It’s preliminary, not a huge sample size, but food for thought. (If so - it wouldn’t have made a difference. The spouse was already on a lot of urinary monitoring due to frequent bladder infections (…hmm… aren’t repeated infections/damage a risk factor…?) and that would be the only screening recommendation. Which, as I said, was already being done for other reasons.)

Maybe one day those same annoying doctors will be pressing the families of patients to admit to their loved one’s spina bifida diagnosis when there actually isn’t one…

I’m sorry you went through this with doctors! I hear it all the time from friends and colleagues, but would expect better from doctors.

People always want to find the “reason” or “cause” for cancer or any bad health outcome (including Covid). They look for some way that it can be the fault of the victim, even if they’re not consciously or explicitly casting blame.

I think t’s a way of defending against the tragic and terrifying fact that terrible things can happen to anyone, anytime. If you can somehow believe “well, that happened to her because she did X, or didn’t do Y,” then you can believe that you are going to be OK because you never do X, or always do Y.

Sadly, the universe just doesn’t work that way. You can do everything “right” and never ever let yourself be exposed to any “toxins.” And. you can still get cancer. It happens all the time. And that’s awfully hard to accept.

Are we really sure that this is the case? at least if I understand your distinction?


Point of the piece being that “time” is the metric that tracks with accumulated chances to have mutations, just by number of reproductions, or triggers like viruses and toxins and sunlight, etc. “Age” though is the loss of the monitor and repair functionality. Which is the bigger reason for the sudden rise in cancer rates as we hit 60s and above?

We all presumptively go through time at the same speed, but we do not necessarily all age at the rate. Exercise and certain nutrition patterns (and possibly some medications like metformin) do not impact “time” but do possibly impact “age”. …

You are right to question this. I think the evidence points to cancer correlating much more closely to aging than to clock time.

And a major source of mutation is oxidative stress, which correlates more to metabolic rate and rate of aging rather than to clock time.

We know a lot about cancer in mice, since they are the principal experimental model for cancer in humans. The incidence of different types of cancer is not identical, but mice do develop many similar cancers. And about one third of wild type lab mice develop some type of cancer toward the end of their ~3 year lifespan, a similar proportion to humans toward the end of our normal lifespan.

Daniel E. Lieberman addresses this question in his book The Story of the Human Body. To directly quote his book:

Cancer, for example, is actually an aberrant evolutionary process going on within a body. Every time a cell divides, its genes have a certain chance of mutating, so cells that divide more frequently … have a greater chance of accidentally acquiring mutations that cause them to divide out of control, forming tumors.

[A] woman’s chance of developing breast, ovarian, or uterine cancer increase significantly with the number of menstrual cycles she experiences and decrease with the number of children she bears. … During each menstrual cycle, levels of estrogen rise (as do other related hormones, such as progesterone), causing cells that line the wall of the uterus to multiply and enlarge in preparation for a fertilized embryo to implant. These surges also stimulate breast cells to divide. Thus, when women cycle they repeatedly experience high doses of estrogen, which cause reproductive cells to proliferate, each time increasing the chances for cancerous mutations to occur and increasing the number of copies of any mutant cells. However, when a woman becomes a mother, by getting pregnant and then nursing, she lowers her risk of breast cancer and other reproductive tissue cancers by reducing her exposure to reproductive hormones. Breast-feeding may also help flush out the lining of the mammary ducts, removing potentially mutant cells.

The issue is a bit more complicated than this. There are three types of breast tissue: fibrous tissue, glandular tissue, and fatty tissue. And there is a correlation between the type of breast tissue you have and your likelihood of developing cancer. Dense breasts, meaning breasts with more fibrous/glandular tissue, are more likely to be cancerous than breasts with more fatty tissue. Medical research has uncovered this fact, but has not yet (to my knowledge) discovered why this is so.

From a previous thread