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According to Korde et al. (2010):

Male breast cancer accounts for less than 1% of all cancers in men and less than 1% of breast cancers.

This raises the question: Why do most breast cancers occur in women?

Two plausible explanations I can think of:

  • A male is less likely to get breast cancer for anatomical reasons (such as a smaller quantity of breast tissue, or breast tissue that is less susceptible to cancer),

  • Women have higher significantly levels of estrogen, which is linked to mutations that cause breast cancer (see Cavalieria et al. (2006)).

Although, I have no evidence to suggest that either of these is predominant factor.

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I agree that estrogen plays a very important role (among other hormonal cues). Another significant explanation that follows from your first example is the sheer quantity of female breasts to male breasts. While only some men have fatty breast tissue, all women do, and so the diagnoses are largely skewed by the anatomy. Something else to keep in mind is that women have breasts from puberty through the rest of their lives, while many men only develop them later in life. –  leonardo Sep 11 '12 at 23:43
    
There is another problem with breast cancer in men: Its usually not detected and searched for. Its probably happening more often than doctors think. –  Chris Feb 22 at 18:47
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2 Answers 2

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Yes, this is mostly about estrogen. Most breast cancers rely on endogenous estrogen to sustain proliferation.

Some general reading: Cancer Medicine, Chapter 18

More in-depth reading: Endogenous Hormones as a Major Factor in Human Cancer

Requested summary of mentioned readings:

First of all, there is an established link between breast cancer cell proliferation and concentration of estrogens and progesterone, which is logical, because normal breast cells divide in response to those hormones (e.g. puberty, pregnancy, even luteal phase of the menstrual cycle). Secondly, the incidence of breast cancer in women correlates with major changes in their hormonal profile - girls and elderly women (i.e. women with lower levels of sex hormones) don't get breast cancer.

Many factors, that influence the risk of developing breast cancer are in fact tightly connected to the hormones' levels. For example - early age of menarche (or, more importantly, first ovulation, beacause physical activity at young age disturbs ovulation AND is protective agains breast cancer) and Hormone Replacement Therapy raise the risk, early age of first full-term pregnancy or any form of artificial menopause (such as preventive oophorectomy for women with mutations in BRCA1 or 2) reduce the risk.

The first table from the book chapter lists known risk and preventive factors. The review article explains the same ideas, but connects them to other types of cancer (e.g. ovarian cancer) and suggests mechanisms, which might be the cause of those risk changes.

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Can you summarize some important points of those links? –  nico Sep 20 '12 at 14:40
    
estrogen and prolactin are hormones that stimulate mammary development and growth of the breast tissue. They greatly enhance the chance of cancer since that tissues is proliferating at a great rate. Males do not typically experience these developmental phases. –  shigeta Sep 21 '12 at 19:15
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For two reasons: Women are

  1. exposed to estrogen,¹ and
  2. the lobules in a woman's breast go through 4 stages:

Type 1: prepubescent & Type 2: pubescent

types 1 & 2 lobules

Type 3: reproductive & Type 4: lactation lobules²

types 3 & 4 lobules

The sooner a woman progresses to Type 3 and Type 4 lobules, the lower her risk of cancer. An interruption of the progression from Type 1 to Type 4 lobules (such as with a late-term miscarriage or induced abortion) increases the risk of cancer, as cancer generally arises in Type 1 and Type 2 lobules.³

References

  1. Miller, Katherine. Estrogen and DNA damage: The silent source of breast cancer? J Natl Cancer Inst 2003: 95:100-102.
  2. Schwartz, Shires, Spencer, Principles of Surgery (McGraw Hill) Russo J, Tay LK, Russo IH. Differentiation of the mammary gland and susceptibility to carcinogenesis. Breast Cancer Res & Treatment 1982;2:5-73.
  3. Daling JR, Malone DE, Voigt LF, White E, Weiss NS. Risk of breast cancer among young women: relationship to induced abortion. J Natl Cancer Inst 1994;86:1584-92.
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