I. Epidemiology of breast cancer
(I) Globally, North America and Northern Europe are the high incidence regions of breast cancer, Southern Europe and South America are intermediate incidence regions, and most Asian and African countries are low incidence regions. The incidence and mortality rates in large coastal cities in China are higher than those in inland areas. In terms of urban-rural distribution, the incidence rate is higher in urban than in rural areas. The regional distribution of breast cancer mortality is basically the same as the incidence rate, and the high mortality areas are still Europe and North America.
The incidence of breast cancer is negatively correlated with the intensity of sunlight. From the viewpoint of the regions of the incidence of breast cancer in the world, its low incidence area is near the equator, and its incidence rate increases with the increase of the earth’s latitude, and the incidence rate of breast cancer in the northern half of the United States is 1.5~2.0 times of that in the southern half, and the incidence rate of breast cancer in China is also higher in the northern region than in the south.
(B) Population distribution Breast cancer is more common in women and less common in men, and only about 1 in men. Among adult women in the same age group, unmarried women have a higher incidence than married women.
In terms of age group, the incidence rate increases with age and decreases slightly at the age of 55 years in the female population. The incidence of breast cancer in China increases steeply with increasing age after 25 years of age, and is more stable until around menopause, and can be slightly reduced after menopause.
(III) Racial characteristics: There are some racial differences in the incidence of breast cancer, with whites in the United States having a higher incidence than blacks, and Han Chinese in China having a higher incidence than ethnic minorities, while the mortality rate is lower among Mongolians and Tibetans.
(iv) Immigration relationship: Women from low incidence countries who migrate to high incidence countries have higher incidence rates than their birthplace and lower incidence rates than their place of migration. The incidence rate of Chinese women in San Francisco is 4 times higher than that of Shanghai women and lower than that of local women; and the incidence rate of the second generation is close to that of locals.
(E) The epidemiological trend of breast cancer incidence, according to the trend of breast cancer incidence to younger people, breast cancer will become a common disease.
Research on the etiology of breast cancer;
Many epidemiological and laboratory advances have been made by domestic and foreign scholars, but so far, the etiology has not been fully understood and the role of various risk factors in the development of breast cancer is still being explored. The purpose of studying breast cancer and its related factors is to find the causes of its development, suggest high-risk factors, and monitor the high-risk group, so as to make the three early detection, early diagnosis, and early treatment and intervention control, and open up new ways for the prevention and treatment of breast cancer.
Most scholars believe that early onset of menstruation, late age at first birth, late age at menopause, family history of breast cancer, history of benign breast disease, and the contralateral breast of breast cancer patients are high-risk factors for the development of breast cancer. Other factors associated with breast cancer are marriage, feeding, diet, lifestyle habits, obesity, certain medications, psychological factors and viral factors. Therefore, breast cancer is the result of a combination of factors under certain conditions.
(i) Menstruation and marriage.
Early menarche is an important risk factor for breast cancer, and it is believed that the risk of breast cancer increases more than 4 times if the age of menarche is before 12 years old than if it is after 13 years old. This may be related to the increase in the incidence of breast cancer.
In addition, the length of the menstrual cycle reflects the number of changes in hormone levels that a person experiences during his or her lifetime, and the shorter the menstrual cycle, the greater the number of changes and the greater the number of times the breast gland is stimulated by estrogen. The risk of breast cancer is increased by late menopause age. Some people have calculated that the risk of breast cancer is reduced by 50 for those who have menopause at the age of 45 compared to those who have menopause at the age of 55. Before menopause, the risk of breast cancer is large, while the risk of breast cancer after menopause is smaller, and the risk of breast cancer after menopause is only 1/6 of that of breast cancer before menopause. The incidence of breast cancer in the breast after artificial menopause is reduced.
Women with long menopause and long irregular menstrual periods have an increased risk of breast cancer. Being unmarried is a risk factor for breast cancer and it has been proven that the incidence of breast cancer is higher in unmarried women, women who marry late and those who are married for a short duration. It has been learned that young age at menarche, late age at menopause and long duration of menstruation are each independent risk factors for breast cancer.
(b) Whether the number of births and breastfeeding births are factors affecting breast cancer
The results are not entirely consistent. The risk of breast cancer can be reduced by more births, and the protective effect of higher births on breast cancer may be due to the large amount of estriol produced by the placenta, which has a protective effect on women. It has been suggested that a high number of breastfeeding months has a protective effect on the occurrence of breast cancer, which is considered to be caused by the confounding of the number of births, and in recent years, some studies have concluded that breastfeeding is an independent protective factor, especially for premenopausal women. However, more births require more breastfeeding opportunities, and more breastfeeding cannot be considered as an important protective factor for breast cancer.
(C) Whether benign breast disease cystic hyperplasia of the breast is a precancerous lesion or not
In the 1980s, the risk of breast cancer increased 3-6 times due to benign breast diseases, with cystic hyperplasia and breast fibroids being the most important ones. However, recent studies suggest that they tend to be risk factors for breast cancer.
(iv) Endogenous factors.
Breast cancer is an estrogen-dependent tumor and its development is closely related to endocrine dysfunction. The main source of estrogen is the ovaries, which secrete estrone, estradiol and estriol, which mainly act on the ducts of the breast. When ovaries secrete too much hormone and act on sensitive breast tissue for a long time, it can lead to proliferation of breast cells and cancer.
Checking testosterone and dihydroandrosterone in the blood and urine of breast cancer patients, the mean values of androgens are higher than those of the control group. The prognosis of breast cancer patients with hypothyroidism or thyroid disease is poor, and thyroid surgery on stable breast cancer patients may cause sudden spread of cancer.
(v) Exogenous factors.
WHO conducted a collaborative study and found that there is a relationship between contraceptive drugs and breast cancer, with the relative risk of breast cancer occurring during childbearing years being higher than that of infertile years, and the relative risk of breast cancer occurring in lower social classes being higher than that of higher social classes. It was observed that those who did not use it for several years after the first dose did not have an increased risk of breast cancer, while those who took it continuously or recently could have an increased risk of breast cancer, and those who took it before the age of 35 would have an increased risk of breast cancer.
WHO also analyzed the relationship between different types of birth control pills and different breast cancers by tissue type. The risk of breast cancer increased for those without ovaries taking estrogen, for those with ovaries taking estrogen for a short period of time was not associated with breast cancer, and for those taking it for more than 5 years for a long period of time, the risk of breast cancer increased. The relationship between daily doses and cumulative monthly doses and breast cancer in those with ovaries is not uniformly understood. Selenium levels have been examined and are lower in smokers than non-smokers, and lower in those with first menstruation less than 13 years old than those older than 13 years old.
Another analysis showed that manganese and chromium levels in the hair of breast cancer patients were higher than in normal subjects. Breast cancer tissue contains several times more potassium than normal tissue. Whether these elements are the cause of breast cancer or the result of lesions needs to be further explored.
(vi) Lifestyle habits.
High-fat diets can increase the incidence of breast cancer. The reasons for the effect of high-fat diets on breast cancer risk may be
1. Long-term high-fat diet can change the status of intestinal bacteria, which can transform steroid substances from bile into carcinogenic estrogens through metabolism.
2. A high-fat diet can increase the secretion of prolactin, which in turn increases the secretion of estrogen in the body.
3. Fat can increase weight or even obesity, and the greater the weight, the higher the risk of developing breast cancer.
4. Excessive nutrition can lead to early menarche and delayed menopause, and estrogen is derived from fatty tissue after menopause. In short, high-fat diet can cause early menarche and obesity, which can increase the risk of breast cancer. It has been studied that alcohol consumption increases the risk of breast cancer by 1.5 to 2.0 times. Biological studies suggest that ethanol affects the permeability of cell membranes and its metabolites have an irritating effect on the breast, but the risk of breast cancer caused by ethanol has not yet been determined.
(vii) Virus: In 1936, Bittner discovered a factor, mammary factor, in the milk of mice with breast cancer that could be passed on to their offspring. 1958, someone discovered this substance on a section of a murine breast cancer and classified it into two types, A and B, which is murine mammary tumor virus (MuMTV). 10 years later, Bermbaro proved that MuMTV is the type B virus particle, i.e. type B RNA virus. Schlom et al. (1971) also found type B RNA virus particles with similar morphology to murine mammary factor MuMTV in the breast milk of breast cancer patients.
Axel et al. (1972) reported the discovery of an RNA-dependent reverse transcriptase in the breast milk of breast cancer patients that could only be found in the milk of B particles.Hageman (1978) isolated four antigenic substances from patient breast cancer tissues that were associated with MuMTV antigens, which strongly suggests the presence of MuMTV in human breast cancer tissues with MuMTV associated virus in human breast cancer tissues.
(H) Hereditary breast cancer: its prevalence in families has long been statistically proven that the incidence of breast cancer in those with a family history of breast cancer is three to five times higher than that of the general population. It is often seen that mothers and daughters or sisters have breast cancer at the same time or successively, and the age of onset is 10 to 20 years earlier in the second generation. It is obvious that breast cancer has a tendency to run in families. The heritability of breast cancer induced by mother’s milk transmission in mice has been demonstrated in experiments with mice.
Genetic evidence for human breast cancer is gradually increasing, and gene linkage analysis has identified chromosomal long and short arm exchange loci that may be associated with breast cancer sensitivity genes. Gene segregation analysis has revealed that breast cancer genotypes are transmitted in the same manner as chromosomal dominant inheritance. However, most breast cancer patients do not have a family history and most twins do not have the disease at the same time, suggesting that genes are not the sole cause of the disease.
(ix) Body type: The incidence of breast cancer does not increase with age in menopausal women with thin body type, but some countries reported that the age of the beginning of obesity is related to breast cancer, age below 50 years old obesity is mostly unrelated to breast cancer, while for every 10Kg increase in weight above 60 years old, the risk of breast cancer increases by 80. long-term physical exercise. Preventing weight gain and obesity can prevent the occurrence of breast cancer.
(J) Radiation: The survivors of the atomic bombings in Japan and the people exposed to medical X-rays have shown that high doses of radiation can increase the risk of breast cancer. The magnitude of the risk of breast cancer depends on the age and dose of radiation received. Generally, the age of 10-30 years is the most sensitive to the effects of radiation exposure during the active mitotic phase, and the risk is less after 30 years of age. The risk of breast cancer from radiation exposure in the first pregnancy is higher than either before or after this period, and in women who have not given birth, the risk of breast cancer from radiation exposure is higher than in women who have given birth.
In conclusion, women are sensitive to radiation during menstruation and pregnancy. Regarding the latency period of breast exposure to radiation, it is estimated to be as short as 5 years and generally 10 to 15 years, with a longer latency period in young people than in older people. Low-dose radiation is used for breast screening, and the risk of breast cancer is very small.
(xi) Years of education: The longer the years of education, the higher the risk of breast cancer. It is generally believed that the high risk of breast cancer in people with long years of education is a combination of factors, and these people tend to marry late, give birth late, have fewer births, take oral contraceptives, have a high economic level, and have a good nutritional status, which all contribute to the occurrence of breast cancer.
(XII) Mental role: When the nerves are strongly stimulated by anxiety and tension or depression, acting on the central nerve of cerebral cortex, which disrupts the autonomic function and suppresses the immune function, the immune mechanism to resist cancerous tumors can be inhibited. If the cerebral cortex is repeatedly stimulated by strong stimulation, so that the body is always in a state of tension, resulting in an imbalance of the internal environment of the body, which will eventually affect the function of the body’s anti-cancer mechanism, and studies have shown that the increased risk of breast cancer is related to emotional disorders.