Breast Cancer in Canada

• Breast cancer is the most frequently diagnosed cancer in Canadian women.

• Incidence rates are increasing among women over 50, while mortality rates are beginning to decrease among women under 70.

• The most important risk factors, age and sex, are non-modifiable.

• Mammographic screening for women aged 50-69 has been proven to reduce breast cancer mortality.

Breast cancer is the most frequently diagnosed cancer in Canadian women, accounting for about 30% of all new cancer cases each year. It is estimated that in 1999, 18,700 cases will be diagnosed and 5,400 women will die of this disease¹. While breast cancer is the most frequently diagnosed cancer among women, lung cancer has now surpassed breast cancer as the leading cause of cancer deaths in women. In 1995, breast cancer accounted for 97,000 potential years of life lost for Canadian women¹. Male breast cancer accounts for less than 1% of all breast cancer.

Trends

Since 1984 breast cancer incidence rates have increased steadily for women overall (Figure 1), while mortality rates have remained relatively stable (Figure 1). Incidence rates have remained steady over time for women under age 50 (Figure 1). In contrast, women over 50 have had an increasing risk of breast cancer. Since 1990 there has been a decrease in mortality rates among women 50 to 69 years of age, with the most dramatic decrease - 15% - occurring among women 60 to 69 years of age². Similar trends of decreasing breast cancer mortality rates have been observed in the United States, the United Kingdom, and Australia¹ and have been attributed to screening and improved treatment.

Breast cancer incidence and mortality rates vary between the provinces and territories of Canada (Figure 2). There are many reasons why breast cancer rates may vary between areas of Canada including differences in the characteristics of women living in these jurisdictions and differences in the way the disease is reported. Figure 2 indicates that British Columbia, Manitoba, Saskatchewan and Alberta have higher incidence rates while having lower mortality rates when compared with the Canadian average.

Primary Prevention

One in nine Canadian women is expected to develop breast cancer in her lifetime and one out of every 25 is expected to die from it¹. The likelihood of a woman developing breast cancer in the next 5 years increases rapidly with age³ making age the most significant risk factor for breast cancer (Table 1).

Other risk factors for breast cancer include a family history of breast cancer, a history of breast cancer in one breast, a history of certain types of benign breast disease, and high levels of radiation exposure to the chest⁴. Weak but well-established risk factors include obesity in post-menopausal women and various reproductive risk factors such as never having had children, being 30 or more years of age at first full-term pregnancy, having an early onset of menstruation, and a late onset of menopause. Other demographic factors that increase the risk of developing breast cancer include living in an urban area, belonging to a higher socioeconomic class, and being born in North America or Northern Europe⁴.

Most of these established risk factors are not modifiable, and there is insufficient evidence to advocate primary preventive action⁵. The evidence linking the use of oral contraceptives to the risk of breast cancer remains controversial. There is also suggestive evidence linking post-menopausal estrogen replacement therapy, high intake of dietary fat, alcohol use and physical inactivity to the risk of breast cancer⁴.

Tamoxifen

There is tremendous interest in the use of tamoxifen for the prevention of breast cancer because of its demonstrated effect in decreasing the risk of breast cancer recurrence. The results of three tamoxifen chemoprevention trials have recently been published. The National Surgical Adjuvant Breast and Bowel Project (NSABBP) (P-1) Study⁶ found that tamoxifen lowers the risk of breast cancer by 45% among women considered at high risk for breast cancer. The Royal Marsden Hospital trial⁷ did not find any benefit of tamoxifen among women who had a family history of breast cancer. A trial conducted in Italy⁸, which enrolled healthy women between the ages of 35 and 70, did not find any benefit for tamoxifen.

Figure 1
Age-standardized incidence and mortality rates for breast cancer in Canadian women, by age-group

Note: Rates are standardized to the age distribution of the 1991 Canadian population.
Source: Cancer Bureau, Laboratory Centre for Disease Control, Health Canada, based on data supplied by Statistics Canada.

Tamoxifen did cause significant side effects such as blood clots and uterine cancer in all of these trials. Because tamoxifen has only been tested in a select group of women, and because of its potential side effects, tamoxifen is not recommended, at this time, for the prevention of breast cancer in all healthy women. Discussion of the relevance and generalizability of the NSABBP trial results is underway.

Genetics

The topic of genetic testing and breast cancer is generating an increasing amount of attention. Although there are many genes that are involved in the development of breast cancer, most discussion focuses on the recent discovery of the BRCA1 and the BRCA2 genes. It is estimated that about 1% of women carry the BRCA1 or BRCA2 gene for breast cancer⁹ and these genes are believed to be responsible for 3-8% of all breast cancers¹⁰. Having the BRCA1 or BRCA2 gene may raise a women's risk of developing breast cancer from about 11% to anywhere from 50 to 85% during her lifetime¹⁰.

Unlike other types of tests, genetic testing only predicts the likelihood of developing breast cancer and does not mean that the disease will occur. This type of 'predictive' testing separates high-risk family members into those truly at high risk who will likely benefit from enhanced early detection or prevention strategies from those at the same risk as the general population. There is little known about the effectiveness of prevention strategies for those identified to be at high-risk. Additional research is required before BRCA1 or BRCA2 testing becomes recommended as part of routine practice.

Secondary Prevention

At present, the only proven strategy to reduce breast cancer deaths is early detection through mammography in women over 50. There is clear evidence from population-based trials that screening mammography can reduce mortality from breast cancer by approximately 30% in women aged 50-69¹¹.

Recommendations for breast cancer screening in Canada have been made by the Canadian Task Force on the Periodic Health Examination (CTPHE). The CTPHE recommends, for women aged 50-69 years, annual screening with clinical breast examination (CBE) and mammography⁵ and that, wherever possible, screening should be done at centres dedicated to this purpose. A 1988 Canadian workshop recommended mammographic screening be offered every 2 years in a dedicated breast cancer screening centre¹².

Figure 2
Age-standardized incidence and mortality rates for breast cancer by province and territory

* p-value < 0.05 ** p-value < 0.01

Note: Rates are standardized to the age distribution of the 1991 Canadian population.
Source: Cancer Bureau, Laboratory Centre for Disease Control, Health Canada, based on data supplied by Statistics Canada.

Screening mammograms are available through organized screening programs, which have been implemented in 11 provinces/territories. Some provincial/territorial screening programs include a clinical breast exam and the teaching of breast self examination in the screening visit¹³. Screening mammograms can also be obtained through a physician in hospitals or clinics under the general fee for service system.With the expansion of organized breast screening programs in the provinces, more and more women are being screened in an organized approach where the effectiveness of screening is monitored. LCDC, in collaboration with the provincial screening programs, has developed and implemented a national breast cancer screening database that collects information on screening that occurs in provincially organized breast cancer screening programs. The Canadian Breast Cancer Screening Database is used to describe both the quality and the outcomes of screening in these programs. Efforts are being made to monitor and assess mammography for screening purposes occurring outside of screening programs.In the 1996-1997 National Population Health Survey, 79% of Canadian women 50-69 years of age report ever having had a mammogram (Figure 3). The proportion of women in this age group reporting ever having had a mammogram is between 75% and 82% in all provinces except Nova Scotia (64%) and Newfoundland (54%) (Figure 3). Eighty-five percent of those who had a mammogram in the last 2 years report having had one for the following reasons: routine check-up, family history, age, or hormone replacement therapy (LCDC, unpublished data). Low education and income are associated with fewer mammograms of any type¹⁴.Recommendations for screening among women 40-49 years of age differ from those for women 50-69 years of age. For women between 40 and 49 years of age, differing interpretation of the population-based studies have resulted in two points of view regarding the effectiveness of mammography screening in this age group. One group of scientists argues that there are no benefits, while the other group argues that analyses to date demonstrate benefits of screening in this age group. The CTPHE recommendation that women younger than 50 not be screened⁵ is currently under review. The National Cancer Institute's Cancernet Web site at http://cancernet.nci.nih.gov/health.htm has an in-depth review of screening recommendations for women under 50.

Figure 3
Proportion of women 50-69 reporting ever having had a mammogram

Treatment Breast cancer is highly treatable by surgery, radiation therapy, chemotherapy, and hormonal therapy. The choice of treatment is influenced by the amount of spread to distant sites in the body, the type of tumour including the hormone receptor status of the cancer, and the age, menopausal status and general health of the woman¹⁵.Early stage cancers (Stage 1) are often managed with breast-conserving surgery followed by radiotherapy; however, the exact management, such as the addition of hormone therapy or chemotherapy may be modified by specific patient factors¹⁵. The overall average 5-year survival rate at this stage is estimated at more than 85%². Late stage cancers are often managed with chemotherapy or hormone therapy. Surgery may be indicated in specific cases. Survival for these cancers is poorer but depends on numerous factors.In recent years, there has been a steady increase in the proportion of breast cancer cases diagnosed as ductal carcinoma in situ (DCIS). DCIS is very early cancer, found prior to any deep tissue invasion. It is unclear whether all DCIS lesions progress to cancer, and therefore whether they require treatment. There are presently differing opinions on the treatment of DCIS.Under the Canadian Breast Cancer Initiative, Clinical Practice Guidelines for the Care and Treatment of Breast Cancer¹⁶ were produced to help guide physicians and women in many aspects of the management of a breast cancer patient. There is one version designed for physicians and one for patients. Single copies of the document are available from the Canadian Cancer Society (888-939-3333) and multiple copies from Publications - Health Canada (613-954-5995) or available on the Canadian Medical Association Web site. The Canadian Breast Cancer Research Initiative has summary information regarding the evidence on the effectiveness of alternative therapies for breast cancer on their Web site at www.breast.cancer.ca.
Current Canadian InitiativesThe Canadian Breast Cancer Initiative, Phase I (1993-1998), was a partnership of the Canadian Cancer Society, Health Canada, the Medical Research Council of Canada, and the National Cancer Institute of Canada. The goal of this initiative was to reduce the incidence and mortality of breast cancer among women, and to improve the lives of women living with the disease. The major components of this initiative were clinical practice guidelines, a screening initiative, a professional education strategy, an information exchange pilot project, and a research initiative.Building on the strengths identified in the evaluation of Phase I, the Renewed Canadian Breast Cancer Initiative has recently been approved for stable, ongoing funding at $7 million a year. The Renewed Canadian Breast Cancer Initiative has identified the following points for emphasis: research; prevention, early detection, and quality screening; surveillance and monitoring; quality approaches to breast cancer diagnosis, treatment and care; community capacity building; and co-ordination and evaluation.
Future DirectionsThe Canadian Breast Cancer Initiative sets forth future directions for breast cancer research and programs in Canada. Activities in this initiative will emphasize the issues outlined above with a special orientation towards quality assurance and consistency in the prevention, care and treatment of breast cancer. Synthesis and analysis of new and current knowledge in primary prevention, genetics, alternative therapies and environmental risk factors for breast cancer is also an important emphasis.
SummaryBreast cancer is a common disease in Canada and, consequently, an important public health issue. It is evident that there is a benefit from screening for women aged 50-69. There are many questions that remain about modifiable risk factors. There is a strong multisectoral commitment to addressing the problem of breast cancer in the future through the Canadian Breast Cancer Initiative.

References

1. National Cancer Institute of Canada. Canadian cancer statistics, 1999. Toronto, Canada, 1999.

2. Gaudette LA, Silberberger C, Altmayer CA, Gao RN. Trends in breast cancer incidence and mortality. Health Reports 1996:8(2):29-37.

3. Bryant HE, Brasher PMA. Risks and probabilities of breast cancer: Short-term versus lifetime probabilities. Can Med Assoc J 1994:150(2):211-16.

4. Kelsey JL, Bernstein L. Epidemiology and prevention of breast cancer. Ann Rev Public Health 1996:17:47-67.

5. Canadian Task Force on the Periodic Health Examination. The Canadian guide to clinical preventive health care. Ottawa: Minister of Public Works and Government Services Canada, 1994 (Cat. No. H21-117/1994E).

6. Fisher B, Costantino JP, Wickerham DL, Redmond CK, Kavanah M et al. Tamoxifen for prevention of breast cancer: Report of the National Surgical Adjuvant Breast and Bowel Project P-1 Study. J Nat Cancer Institute 1998:90(18):1371-88.

7. Powles T, Eeles R, Ashley S, Easton D, Chang J et al. Interim analysis of the incidence of breast cancer in the Royal Marsden Hospital tamoxifen randomised chemoprevention trial. Lancet 1998:352:98-101.

8. Veronesi U, Maisonneuve P, Costa A, Sacchini V, Maltoni C et al. Prevention of breast cancer with tamoxifen: preliminary findings from the Italian randomised trial among hysterectomised women. Lancet 1998:352:93-7.

9. Easton DF, Bishop DT, Ford D et al. Genetic linkage in familial breast and ovarian cancer: results from 214 families. Am J Human Genetics 1993:52(4):678-701.

10. Brody LC, Biesecker BB. Breast cancer susceptibility genes. BRCA1 and BRCA2. Medicine 1998:77(3):208-26.

11. Fletcher SW, Black W, Harris R et al. Report of the International Workshop on Screening for Breast Cancer. J Nat Cancer Institute 1993:85(suppl. 21).

12. The Workshop Group. Reducing death from breast cancer in Canada. Can Med Assoc J 1989;141:199-201.

13. The Canadian Breast Cancer Screening Database. Preliminary report on breast cancer screening in Canada. (1995).

14. Snider J, Beauvais J, Levy I, Villeneuve P, Pennock J. Trends in mammography and pap smear utilization in Canada. Chron Dis Can 1996;17 (3/4):108-17.

15. Hortobagyi GN. Treatment of breast cancer. N Engl J Med 1998:339(14):974-84.

16. Health Canada and the Canadian Medical Association. Clinical practice guidelines for the care and treatment of breast cancer, a Canadian consensus document. Can Med Assoc J 1998;158 (suppl. 3).

Acknowledgements

Data were provided to Health Canada from the Canadian Cancer Registry, formerly the National Cancer Incidence Reporting System, at Statistics Canada. The cooperation of the provincial and territorial cancer registries that supply the data to Statistics Canada is gratefully acknowledged.Mortality data were provided to Health Canada by Statistics Canada. The cooperation of the registrars of vital statistics in the provinces and territories of Canada who make mortality data available to Statistics Canada under federal-provincial agreements is gratefully acknowledged.

Contributors: Rosemarie Ramsingh, Dena Schanzer, Judy Snider, Francoise Bouchard and Janet Beauvais.

The Cancer Bureau is part of the Laboratory Centre for Disease Control,
Health Protection Branch, Health Canada.

Our Mission is to help the people of Canada maintain and improve their health.
Health Canada