Chapter 4: Cancer incidence in Canada: trends and projections (1983-2032) – Breast cancer (in females) - HPCDP: Volume 35, Supplement 1, Spring 2015

Chapter 4: Projections by Cancer Site

10. Breast cancer (in females)

Breast cancer is the most frequently diagnosed cancer and the second leading cause of cancer death in females in Canada. The lifetime risk of developing breast cancer is estimated to be 1 in 9, and the lifetime probability of dying from the disease is 1 in 29.^{Endnote 1} The average annual number of new cases of breast cancer in females in 2003–2007 was 20 110 (Table 4.10.1), accounting for 13.0% of all new cancer cases in Canada and 27.1% of the cancer cases in females (Figure 3.9). Breast cancer risk is strongly connected to age, with 88% of cases occurring in women 45 or older in the same period and 67% of cases diagnosed in the age groups between 45 and 74.

The incidence rate increased steeply with age up to age 65, followed by a less marked increase to a plateau at age 75–84 (Table 4.10.2). The 5-year relative survival rate was 88% in 2006–2008.^{Endnote 1}

Breast cancer incidence in Canada increased through the 1980s and 1990s and has decreased since then (Figure 4.10.1). Similar patterns occurred in women aged 45 to 74, despite fluctuation in the 65–74 age group (Figure 4.10.2). The rise in overall breast cancer incidence rates until 1998–2002 was seen primarily in women aged 55 to 74 and was more evident in women aged 55 to 64, who are in the targeted age group (50–69 years) of the provincial/territorial organized breast cancer screening programs. While incidence rates have remained steady over time in the youngest females, the rates have decreased in the oldest age group. During the last 10 observation years, breast cancer ASIRs decreased significantly by 0.7% per year (Figure 3.2). Inter-regional comparison illustrates that female breast cancer incidence rates appear to be fairly consistent across the country (Figure 4.10.1).

Extending the current 10 years trend into the future gives predicted relatively stable rates in Canada (Figure 4.10.1). The ASIRs are projected to increase slightly in British Columbia and Ontario but decrease slightly in other regions. The generally consistent regional rates of breast cancer in females are expected to continue.

The age-specific comparison indicates that the primary trend of cancer incidence in Canada in each age group is expected to be static (Figure 4.10.2). From 2003–2007 to 2028–2032, the ASIRs for breast cancer are projected to be stable and will be 98.7 per 100 000 by the end of that period (Table 4.10.2). Over the same period, the annual number of new cases is predicted to increase by 55%, from 20 110 to 31 255.

Comments

Breast cancer incidence in Canada rose steadily during the 1980s and 1990s and has since decreased. The most prominent increases until the early 1990s may reflect the introduction of organized provincial screening programs and increasing uptake of breast cancer screening, leading to a transient additional increase in incidence due to the detection of a prevalent pool of undiagnosed cancers.^{Endnote 1} Screening may have resulted in the more recent decrease because of the exhaustion of undiagnosed prevalent cases. In 2008, 72% of Canadian women aged between 50 and 69 reported having had a screening mammogram in the preceding 2 years, an increase from 40% in 1990.^{Endnote 142} The increase occurred from 1990 to 2000/2001, and mammography utilization rates have since stabilized.

In 2011, the Canadian Task Force on Preventive Health Care released its recommendations on breast cancer screening for women aged 40 to 74 at average risk of breast cancer.^{Endnote 143} The Task Force recommends that women aged 50 to 74 be routinely screened with mammography every 2 to 3 years (where previously it was 1 to 2 years), and that women aged 40 to 49 not be routinely screened with mammography. The Task Force notes a gap in knowledge exists about the benefits and harms of screening using mammography for females aged less than 40 and more than 74. In 2013, the Canadian Association of Radiologists (CAR) published new practice guidelines which differ from the Task Force's recommendations.^{Endnote 144} The CAR recommends annual screening of women aged 40 to 49 and annual or biennial screening of women aged 50 to 74.^{Endnote 144}

Some of screen-detected breast cancers are indolent and would never present clinically; this is referred to as over-diagnosis. In other research, in cohorts screened for breast cancer, there was an increase in the incidence of early-stage disease without a subsequent decrease in late-stage tumours, which is evidence of over-diagnosis in this disease.Endnote 145 Miller et al.Endnote 146 reported that 22% of screen-detected breast cancers were over-diagnosed in the most recent Canadian study with up to 25-year followup of women aged 40 to 59. The extent of over-diagnosis might be underestimated, as ductal carcinoma in situ was not included in the study. Miller et al.Endnote 146 also observed that mammography does not reduce breast cancer mortality. While the screening can detect small cancers, it is associated with exposure to x-rays, false positive results, complications of extra breast cancer diagnosis (such as biopsy), and treatment for breast cancer that would not have caused any problem in a woman's lifetime. Mammography screening for average-risk women should be considered based on a discussion between a woman and her physician weighing the benefits and risks.

The trend in breast cancer incidence rates is likely linked to changes in hormonal factors. Studies indicate that early menarche, late menopause, delayed first full-term pregnancy and no full-term pregnancy are associated with increased risks of breast cancer.^{Endnote 147} A larger number of births and breastfeeding have protective effects.^{Endnote 147} The long-term decrease in fertility rates in Canada would be expected to result in an increase in breast cancer rates, but fertility rates have stabilized.^{Endnote 148, Endnote 149} Both oral contraceptives and hormone replacement therapy increase the incidence of breast cancer.^{Endnote 147, Endnote 150, Endnote 151} Following a 2002 Women's Health Initiative trial report about increased breast cancer risk associated with hormone replacement therapy, its use fell dramatically in Canada and elsewhere, which appears to have contributed to a temporary decrease in breast cancer incidence rates.^{Endnote 152, Endnote 153} Obesity increases breast cancer risk in postmenopausal women, but may be protective in premenopausal women.^{Endnote 85, Endnote 147} Breast density is strongly and independently associated with risk of breast cancer, with approximately 4-fold increased risk for highest (≥75%) versus lowest density (<10%).^{Endnote 154} Breast density is mainly influenced by genetic factors, and is also inversely associated with age, menopausal status and parity.^{Endnote 154, Endnote 155, Endnote 156} Breast density makes detecting cancer by mammography difficult and increases risk of advanced tumour stage at diagnosis.^{Endnote 157, Endnote 158} IARC classified "shift work that involves circadian disruption" as a probable human carcinogen for breast cancer.^{Endnote 47}

Approximately 27% of breast cancer cases diagnosed in the United Kingdom (UK) in 2010 are attributable to mostly modifiable lifestyle and environmental factors.^{Endnote 159} A systematic analysis of 48 studies shows a modest (15%–20%) risk reduction for physically active females, with a stronger link for postmenopausal women (20%–80%).^{Endnote 160} The analysis also indicated a 6% reduction in breast cancer risk for each additional hour of physical activity per week. In the Canadian context, light intensity activity did not reduce breast cancer risk.^{Endnote 161} The occurrence of breast cancer is causally related to the consumption of alcoholic beverages.^{Endnote 47, Endnote 162} According to the IARC, the evidence that tobacco smoking causes breast cancer is limited.^{Endnote 163} However, considerable evidence has suggested a potentially casual role for active smoking and early smoking initiation. ^{Endnote 164, Endnote 165} The association between passive smoking and breast cancer is still a topic of some debate, although there has been a suggestion for an elevated risk for premenopausal breast cancer.^{Endnote 147, Endnote 165} Xradiation and gamma-radiation are associated with increased risk of breast cancer. ^{Endnote 47} The younger the age of exposure, the greater the excess risk.^{Endnote 151} Changes in the modifiable factors may have also influenced the trend of breast cancer.