**3. Results**

Female BC incidence rates increased with age until a peak in the 70–74 year age group (Figure 1). Between 2010 and 2017, less than half (46.1%) of all new primary BC cases were diagnosed in women aged 50–74, the age range recommended by current screening guidelines. This percentage increased by almost 9 percentage points to 54.6% when women

in their forties are included. The number of BC cases diagnosed in women aged 40–49 was equivalent to 18.5% of the total number of BC cases in screened women aged 50–74. The incidence rate ratio between cases diagnosed in the 40s and those diagnosed in their 50s was 0.63.

**Figure 1.** Age–specific female breast cancer incidence rate, by five–year age group, Canada excluding Quebec, 2010 to 2017 period. Note: Quebec is excluded because cases diagnosed in Quebec from 2011 onward had not been submitted to the Canadian Cancer Registry. Solid lines display current ages included in screening; dashed line indicates the age threshold where screening is being explored. Source: Canadian Cancer Registry (1992 to 2018) at Statistics Canada [42].

The proportion of women diagnosed with stage I BC was observed to be greater among those aged 50–74 at diagnosis—for whom screening is currently recommended— than among those diagnosed outside of this age range (Figure 2). Canadian women aged 50–74 had a proportionally higher incidence of stage I BC, and lower relative proportions of stage II and III BC, compared with younger and older women who fall outside the CTFPHC recommended screening age.

**Figure 2.** Stage–specific distribution of female breast cancer cases by age group at diagnosis, Canada excluding Quebec, 2010 to 2017 period. Note: Quebec is excluded because cases diagnosed in Quebec from 2011 onward had not been submitted to the Canadian Cancer Registry. The ages between the dashed lines represent those age groups for which mammogram screening is currently recommended. Source: Canadian Cancer Registry (1992 to 2018) at Statistics Canada [42].

The distribution of BC stage at diagnosis was significantly different between women aged 50–59 and their 40–49 year counterparts not targeted by screening programs (Figure 3, Table 2). Except for stage IV, Canadian women aged 40–49 had BC diagnosed at significantly later stages than those aged 50–59 who had higher relative proportions of stage I BC (45.3% vs. 35.7%; *p* < 0.001), and lower proportions of stage II (36.7% vs. 42.6%, *p* < 0.001), and III (13.1% vs. 17.3%, *p* < 0.001) BC. The proportion of stage IV at diagnosis in women aged 40–49 was significantly lower than in women aged 50–59 years old (4.4% vs. 4.8%, *p* = 0.005).

**Figure 3.** Stage–specific distribution of female breast cancer cases, ages 40 to 49 years versus ages 50 to 59 years, Canada excluding Quebec, 2010 to 2017. Note: Quebec is excluded because cases diagnosed in Quebec from 2011 onward had not been submitted to the Canadian Cancer Registry. Vertical error bars indicate 95% confidence intervals. Source: Canadian Cancer Registry (1992 to 2018) at Statistics Canada [42].

**Table 2.** Number and proportion of women diagnosed with breast cancer by stage at diagnosis, ages 40 to 49 years versus 50 to 59 years, Canada excluding Quebec, 2010 to 2017 period.


NA = not applicable, CI = confidence interval. Note: Quebec is excluded because cases diagnosed in Quebec from 2011 onward had not been submitted to the Canadian Cancer Registry. Counts have been randomly rounded to a multiple of five in accordance with Statistics Canada's disclosure-avoidance guidelines. Source: Canadian Cancer Registry (1992 to 2018) at Statistics Canada [42].
