Once rare, the use of mammograms and breast examinations has led to a rising number of early tumours being discovered. The most common type of early breast cancer is a small tumour that has invaded the breast tissue but not advanced in any obvious way to the lymph nodes of the armpit, the place breast cancer cells spread first. In the USA, early tumours accounted for 80,000 cases in 1990, compared with 35,000 cases in 1975. The vast majority of such breast cancers can be cured. The usual procedure is a lumpectomy, followed by radiation therapy. Sometimes chemotherapy is administered for at least six months after surgery to mop up any cancer cells that might have spread to distant parts of the body.
An even earlier type of breast cancer, called carcinoma-in-situ, consists of a self-contained cluster of malignant cells that have not yet broken through a boundary membrane into adjacent normal tissue. In 1980, only 3% of cancers were detected at this stage. In 1990, some studies show that the figure is 20 to 30% (and up to 40% in the case of women above 50 years of age). Mastectomy (breast removal) is the recommended and usual treatment for carcino-in-situ because historically it has almost always cured the disease; newer methods, such as lumpectomy or drugs are poorly tested by comparison. This leads to the bizarre position of sacrificing the breast to treat non-invasive disease and saving the breast when the cancer is more advanced or invasive and makes it difficult to counsel women on treatment.
According to a 1993 report, the death rate from breast cancer has altered little since the 1940's. It is the leading cause of cancer death among women aged 15-54, and accounts for over one quarter of all cancer cases among women, around 60% of cases in women over 60. Breast cancer has an incidence of 1 in 12 in Britain, 1 in 9 in Canada and 1 in 8 in the USA as of 1988. One US woman in 90 will have had breast cancer by the age of 45; the mortality rate is 1 in 28. Rates are 4 to 7 times lower in China, Japan and the Philipines, but within 2 generations of migrating to the USA, the incidence among the immigrants' offspring reaches local levels, leading to speculation that diet and environmental factors are a cause.
Cancer risk increases with age and varies with ethnicity. A 30-year-old woman has a 2% (1 in 50) chance of being diagnosed with breast cancer in the next 20 years. This risk increases to 4.5% (1 in 20) chance for a 40-year-old, and from 1 in 15 to 1 in 133, depending on her ethnicity, for a 50-year-old. For women currently aged 50, the estimated risk of developing invasive breast cancer within 5 years varied from 0.8% (1 in 133) among Hispanics to 1.3% (1 in 75) among Caucasians. Within 20 years, estimated risks increased to 6.6% (1 in 15) among Caucasians, 5% (1 in 20) among African Americans, 3.9% (1 in 26) among Asian/Pacific Islanders, and 3.7% (1 in 27) among Hispanics.
After the age of 50, the risk rapidly increases. Men can also contract breast cancer, although the incidence is 100 times less than in women. The incidence of male breast cancer in the UK was 1400 cases in 1998, of whom 20% died.
Prior to 1993 research, women with family histories of breast cancer were supposed 4-6 times more likely to develop malignant breast tumours than the average woman. Recent studies, however, suggest that while women with familial ties to breast cancer are at higher risk, they are only 1.5-2.5 times more susceptible than the average woman. (In a 1993 US study of 2,389 women with breast cancer, only 2.5% of cases were attributed to a family history of the disease.) The summary of research in 1997 was that fewer than 10 per cent of breast cancers are due to inherited genetic defects, and sporadic cases cannot be explained by known risk factors. Aside from radiation, most established breast cancer risk factors are related to total lifetime exposure to estrogen. The key component is the estrogen receptor, which is linked to the transcription of a variety of important genes, some of which control tumor induction and tumor growth. Xenoestrogens are environmental agents that function as estrogens and can influence the development of breast cancer and other hormonally related diseases. Some xenoestrogens (e.g., in certain soy products) are protective, while others (e.g., in certain insecticides) greatly increase breast cancer risk.
A powerful predictor of breast cancer risk appears to be bone strength. The suspected link is high life-time exposure to (pre-menopausal) oestrogen, which ensures healthy bones but may also be implicated in breast cancer; it was reported in 1997 that older women with the strongest bones have almost four times the breast cancer risk of women with weak bones.
According to a 1993 National Cancer Institute report, one in three lesbians may develop breast cancer by the age of 85, three times the risk faced by heterosexual women. As lack of childbearing increases the chance of breast cancer by 80% and an estimated 70% of lesbians are childless, their risks are greatly increased. Excess body weight, cigarette smoking and heavy alcohol drinking additionally increase the risks.
Since one proven somatic cause of breast cancer is ionizing radiation, a large fraction of future breast cancers could be prevented by reducing radiation doses. At least 50 percent of breast cancers of the last 30 years have been caused by medical x-ray examination, used both in diagnosis and as therapy in areas outside the cancer.