*3.1. Screening Participation*

From 2003 to 2017, reported BC screening participation generally declined among women aged 40–49, with the largest decrease (21.4 percentage points) occurring in New Brunswick (Table 1). Even in comparator jurisdictions, some screening occurred among women aged 40–49, ranging from 10.0% in Yukon in 2003 to 51.8% in Newfoundland and Labrador in 2008.

#### *3.2. Stage Distribution of BC Related to Screening Guidelines for Women 40–49 Years Old*

The rate of diagnosis of stage I disease decreased by a non-statistically significant average of 1.5% per year from 2011 to 2017 (*p* = 0.147) for women 40–49 years old (Figures 4 and 5). The corresponding rate of diagnosis of stage II BC in women in their forties increased by an average of 2.1% per year (*p* = 0.001). The increasing trend for stage II cases was more pronounced among screening jurisdictions (3.0% annual average) than among comparators (1.6% annual average). Stage III BC incidence rates decreased by an average of 2.5% per year (*p* = 0.012) over this same time period. This reduction was driven by a 5.7% annual decrease in the rate of diagnosis of stage III cases in screener jurisdictions (*p* = 0.007) (Figure 5A). Stage IV BC rates increased by an annual average of 1.8% per year (*p* = 0.103); 2.4% among comparators (*p* = 0.244) and 1.4% among screeners (*p* = 0.707).

**Figure 4.** (**A**). Stage-specific female breast cancer incidence rates, ages 40 to 49 years, Canada excluding Quebec, 2010 to 2017. (**B**). Stage-specific female breast cancer incidence rates, 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. Source: Canadian Cancer Registry (1992 to 2018) at Statistics Canada [42].



(**B**) comparator

screener

\*

overall

**Figure 5.** (**A**). Stage–specific female breast cancer incidence rate trends by jurisdictional screening status, ages 40 to 49 years, Canada excluding Quebec, 2011 to 2017. (**B**). Stage-specific female breast cancer incidence rate trends by jurisdictional screening status, ages 50 to 59 years, Canada excluding Quebec, 2011 to 2017. Note: Quebec is excluded because cases diagnosed in Quebec from 2011 onward had not been submitted to the Canadian Cancer Registry. Screeners: Alberta, British Columbia, Nova Scotia, Northwest Territories, and Prince Edward Island; Comparators: Manitoba, New Brunswick, Newfoundland and Labrador, Nunavut, Ontario, Saskatchewan, Yukon. The vertical error bars indicate 95% confidence intervals. Asterisks indicate the trend is significant at the *p* < 0.05 level. Source: Canadian Cancer Registry (1992 to 2018) at Statistics Canada [42]; provincial and territorial screening practices [49,50].

#### *3.3. Stage Distribution of BC in Women 50–59 Related to Screening Guidelines for Women 40–49 Years Old*

The incidence of stage I BC remained stable in women aged 50–59 over the same time period from 2011 to 2017 (Figures 4B and 5B). A significant average annual increase of 1.1% in the rate of diagnosis of stage II BC was observed (*p* = 0.047). Among stage III BC cases, there was a significant average decline of 2.5% per year (*p* < 0.001) which was mainly influenced by an annual reduction of 5.8% per year in the screener jurisdictions (*p* = 0.001). The overall trend for stage IV was not significant, but there was a significant average annual increase of 1.7% in metastatic disease among comparator women 50–59 (*p* = 0.025). This corresponded to a 10.3% increase in stage IV BC in these women over the six years.

#### *3.4. Impact of Provincial/Territorial Screening Status on BC Stage at Diagnosis in Women Aged 40–49*

BC stage distribution at diagnosis in women aged 40–49 was significantly different in screener versus comparator jurisdictions (Figure 6A). Higher proportions of stage I BC were diagnosed among screener jurisdictions (33.3% vs. 39.9%, *p* < 0.001), while comparators had proportionately more BC diagnosed at stage II (43.7% vs. 40.7%, *p* < 0.001), III (18.3% vs. 15.6%, *p* < 0.001) and IV (4.6% vs. 3.9%, *p* = 0.001). Among the BC cases that were coded as unstaged, 96.9% were diagnosed in the comparator province of Ontario (data not shown). As a consequence, there were significantly more unstaged BC cases among comparators than screeners, with an incidence rate ratio of 2.4 (data not shown).

Regression of provincial and territorial screening participation with an incidence of the stage at diagnosis revealed a significant relationship between screening and stage I BC (*p* = 0.010). An increase of 10 percentage points in the screening participation of women aged 40–49 was linearly associated with a 6.6 per 100,000 increase in the stage I incidence rate (Figure 7A) and with a non-significant 1.2 per 100,000 decrease in the rate of stage IV or metastatic disease (*p* = 0.186) (Figure 7B).

**Figure 6.** *Cont*.

**Figure 6.** (**A**). Stage-specific distribution of female breast cancer cases by jurisdictional screening status, ages 40 to 49 years, Canada excluding Quebec, 2010 to 2017. (**B**). Stage-specific distribution of female breast cancer cases, ages 50–59 years, by jurisdictional screening status for women 40–49 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. Screeners: Alberta, British Columbia, Nova Scotia, Northwest Territories, and Prince Edward Island; Comparators: Manitoba, New Brunswick, Newfoundland and Labrador, Nunavut, Ontario, Saskatchewan, Yukon. Source: Canadian Cancer Registry (1992 to 2018) at Statistics Canada [42,44]; provincial and territorial screening practices [49,50].

(**A**) 

**Figure 7.** *Cont*.

**Figure 7.** (**A**). Incidence rate of stage I female breast cancer during the 2011 to 2013 period by provincial screening participation rate in 2012, ages 40 to 49 years, selected provinces. (**B**). Incidence rate of stage IV female breast cancer during the 2011 to 2013 period by provincial screening participation rate in 2012, ages 40 to 49 years, selected provinces. Note: Orange dots denote comparators and blue dots denote screeners. Quebec is excluded because cases diagnosed in Quebec from 2011 onward had not been submitted to the Canadian Cancer Registry. The three Canadian territories (Yukon, Northwest Territories, and Nunavut) were excluded from the stage I and IV analyses, and Prince Edward Island from the stage IV analysis, because the number of incident cases was too small to be reliably compared against other jurisdictions. Source: Canadian Cancer Registry (1992 to 2018) and Canadian Community Health Survey [42,44]: Annual Component (2012) at Statistics Canada; provincialandterritorialscreeningpractices[49,50].

#### *3.5. Impact of Provincial/Territorial Screening Status for Women Aged 40–49 on BC Stage at Diagnosis in Women Aged 50–59*

The stage distribution of BC cases diagnosed among women aged 50–59 differed according to the screening practices—screener or comparator—used in their jurisdiction for 40–49 year-olds (Figure 6B). There was a lower proportion of stage I in women aged 50–59 at diagnosis (44.5% vs. 46.8%, *p* < 0.001), and higher proportions of stage II (37.2% vs. 36.0%, *p* = 0.003) and stage III (13.6% vs. 12.3%, *p* < 0.001) in the comparator than in the screener jurisdictions. No significant difference was observed between comparator and screener jurisdictions in terms of the proportion of cases diagnosed at stage IV.
