ARCHIVED - Organized Breast Cancer Screening Programs in Canada - Report on Program Performance in 2005 and 2006

2005 AND 2006 RESULTS

This report presents statistics for the 2005 and 2006 calendar years using data submitted up to March 2010. Further, the outcomes presented in this report are based on the 2007 report by the Evaluation Indicators Working Group except where otherwise indicated.^{[Note 5] (4)} Data submission is staggered and may impact the completeness of cancer-related data for some programs. Unless otherwise noted, the summary statistics include data from all 10 provinces and the Northwest Territories and apply to women aged 50 to 69. These results are based on the experiences of Canadian organized breast cancer screening programs (Appendix A) and not on opportunistic breast cancer screening.

Participation in Organized Breast Cancer Screening Programs

Participation Rate

Adequate participation in breast cancer screening is essential for reductions in mortality to occur in the target population. Based on principles of screening and extrapolation from randomized controlled trials, Canadian programs have established 70% as the target participation rate.⁽⁴⁾ The participation rate presented is calculated over a 30 month time period, which is similar to international reporting but results for the traditional 24 months are reported for comparison to previous reports.

Participation rates include all 10 provinces and the Northwest Territories. Overall, 1,588,699 Canadian women between 50 and 69 (Table 6. pg29 ), and 2,031,754 women of all ages (40+) ^{[Note 4]} , received a screening mammogram through a Canadian organized screening program in 2005 and 2006 (Table 3. pg15 ). Since the inception of the first Canadian organized screening program in British Columbia, over 8 million screening mammograms have been performed.

Although these numbers appear high, the targeted program participation rate of 70% among women 50 to 69 years for biennial screening is far from being reached through organized programs. In 2005 and 2006, 43.9% of the target population received a screening mammogram through an organized program over 30 months and 40.0% over 24 months. The participation rate varies among organized programs from 10.4% to 59.2% over 30 months
(Figure 3a. below ) and 9.1% to 53.3% over 24 months (Figure 3b. pg13 ).

Participation among women 50 to 69 years is influenced by the proportion of women outside of this age group who are screened. Although there is relative consistency among programs on acceptance of women outside of the 50 to 69 year age group (Table 1. pg5 ), the proportion of screening occurring in the target age group (50 to 69) varies considerably from 36.7% to 100% (Figure 4. pg14 ).

Importantly, these rates do not include women who receive their breast cancer screening outside of an organized program. Results from population health surveys suggest that close to 63% of women between 50 and 69 years received a screening mammogram within the past two years (Figure 5a. below ). This figure is self-reported and may be slightly inflated as survey respondents tend to overestimate desirable behaviours, however, it is more closely aligned with the target of 70% set by the Evaluation Indicators Working Group Report. When attendance to mammography through opportunistic screening ^{[Note 6]} , in addition to organized screening, is considered screening mammography utilization substantively increases and becomes very similar to self-reported screening mammography from the same period (Figure 5b. below ). Data for mammography utilization includes women 50-69 years old with bilateral mammography (including screening mammography in organized programs, screening mammography outside of organized programs, and bilateral diagnostic mammography in provinces that included this in their mammography billing code). The range of screening mammography utilization shows little variation among provinces (60.0%-64.6%), but large variation in the proportion of utilization attributable to organized screening (14.5%-85.2%).

Retention Rate

Optimal benefits from screening programs are achieved when regular participation in screening occurs. Two targets have been set based on an understanding of participation rates, sojourn time, screening interval studies and randomized controlled trials.^(5,6,7) The first, for women undergoing their initial screening mammogram, states that ≥75% of women should return within 30 months. The second states that ≥90% of women undergoing a subsequent screen should return within 30 months. The retention rate for women aged 50 to 67 excludes women who did not return because of death, breast cancer, or age limit (greater than 67 years). This indicator differs from that reported in the Evaluation Indicators Report and has been updated to censor women greater than 67 years to allow more accurate comparison of provinces where screening is strictly limited to between ages 50 and 69.

Overall, most women aged 50 to 67 who received a screening mammogram between 2002 and 2003 were rescreened within 30 months up to 2006. Among women who received their first screening mammogram in the 2002 and 2003 calendar years, 68.9% returned for a subsequent mammogram within 30 months. Among women aged 50 to 67 who received a subsequent screening mammogram in the 2002 and 2003 calendar years, 81.3% returned for a subsequent mammogram within 30 months. (Tables 6-8. pg29-35 )

In general, younger women (40 to 49 years) were more likely to return for subsequent screening within 30 months compared to older women (70+ years) regardless of whether it was an initial (65.2% and 45.5% respectively) or subsequent screen (83.6% and 63.6% respectively) (Table 7. pg32 ). Women aged 40 to 49, who choose to have a screening mammogram, are usually recommended for annual screens. Most women, aged 50 to 67, returned for subsequent screening between 21 and 27 months after their 2002 to 2003 screen but women between age 40 and 49 were more likely than older women to return between 12 and 15 months (Figure 6. pg19 ).

Results of Organized Breast Cancer Screening Programs

The goal of organized screening programs is to identify disease in asymptomatic women and at the same time minimize the number of healthy women who receive abnormal screening results and associated follow-up tests. Both the abnormal call rate and the positive predictive value offer insight into the process of accurately identifying asymptomatic women with breast cancer.

Abnormal Call Rate

The abnormal call rate refers to the percentage of all women screened who are referred for further testing because of abnormalities found during the screening mammogram and is one way to measure the quality of a screening program. The Canadian target is <10% for women undergoing their first screen and <5% of women undergoing their subsequent timely screens.

Among women 50 to 69 years, the abnormal call rate for women receiving their first screening mammogram is 12.2% and for a subsequent screening mammogram is 6.0% (Table 6. pg29 ). Radiologist inexperience and/or low reading volumes can contribute to unnecessarily high abnormal call rates, as can delays in rescreening. For all age groups, the abnormal call rate rises after a screening interval of 30 months indicating the importance of regular screening intervals (Figure 7. above).

Positive Predictive Value

The positive predictive value is determined by the proportion of women with an abnormal call who go on to be diagnosed with invasive or in situ cancer. A high positive predictive value reflects the minimization of unnecessary follow-up procedures. The Canadian target is ≥5% for first screens and ≥6% for subsequent timely screens.

Among women aged 50 to 69 years, and based on detection by mammography alone, the positive predictive value meets these targets for subsequent screening (7.8%) and is close to meeting the target for initial mammograms (4.7%). It is worth noting that positive predictive value is sensitive to the age distribution of the screened population, which is among the reasons why the Canadian targets are only intended for women age 50 to 69. The positive predictive value increases dramatically with age: it is as low as 2.0% for women between 40 and 49 years undergoing their initial screening mammogram and as high as 13.9% in women over 70 years (Tables 6-8. pg29-35 ).

Diagnostic Process Used by Organized Breast Cancer Screening Programs

As suggested by the positive predictive value, most women who receive abnormal screening results do not actually have breast cancer; however, additional assessment is required to determine the definitive diagnosis. The provision of timely, well coordinated, and minimized follow-up assessment has been shown to reduce fear and anxiety associated with abnormal results.⁽⁸⁾ Women who receive abnormal screening results require additional radiological or surgical assessment including diagnostic mammography, ultrasonography, core or open biopsy, and/or fine needle aspiration.

In 2005 and 2006, 75.3% of women who received an abnormal screen were followed-up with additional breast imaging only. Breast imaging includes diagnostic mammography, ultrasound, or magnetic resonance imaging (MRI). A further 13.6% received breast imaging combined with core biopsy or fine needle aspiration; similar to the 13.0% in the previous reported interval (2003 and 2004) (Figure 8. pg21 ). Lastly, there was a shift from the use of open biopsy to core biopsy in 2005 and 2006 compared to the previous reported interval (2003 and 2004) . Core biopsy increased from 12.3% (13,648 women) to 13.5% (16,513 women) and open biopsy decreased from 5.6% (6,188 women) to 4.3 % (5,303 women) (Table 4. pg22 ).

Diagnostic Interval

The diagnostic interval is defined as the duration of time from the abnormal screen to its resolution. Excessively long diagnostic intervals can have negative psychological impact and potentially worsen prognosis.⁽⁸⁾ The Canadian target is ≥90% of abnormal screens will be resolved with 5 weeks if no tissue biopsy is required and ≥90% within 7 weeks if a tissue biopsy is ever required during diagnostic follow-up. This methodology differs from that reported in the Evaluation Indicators Report and has been updated to close the diagnostic interval after 6 months of benign test results.

Nationally, 75.0% of women not requiring a tissue biopsy received resolution within five weeks and 46.7% of women requiring tissue biopsy received resolution within seven weeks. The proportion of women who did not require tissue biopsy and received resolution within five weeks has been showing gradual improvement, and has now stabilized at approximately 75%. The proportion of women requiring at least one tissue biopsy who received resolution within seven weeks has been relatively stable over time (Tables 6-8. pg29-35 ).

Benign Open Surgical Biopsy Rate

The rate of open surgical biopsy can provide an indication of the quality of pre-surgical assessment but no target has been set for this indicator.

In 2005 and 2006, the benign open surgical biopsy rate was 3.4 and 2.1 per 1,000 screens (initial and subsequent screens respectively). The biopsy rate is lower among older women (70+ years) undergoing their first screening mammogram compared to younger women. The rates among women undergoing subsequent screening mammograms shows little variation by age group. Since 2002, the rate has decreased for both initial and subsequent screening mammograms suggesting a shift away from the use of open surgical biopsy (Tables 6-8. pg29-35 ).

Benign to Malignant Open Surgical Biopsy Ratio

This indicator has been removed due to the small number of open biopsies performed on a provincial basis. This has occurred because there has been a shift to the use of core biopsy as a means to achieve definitive diagnosis resulting in less stable ratios that are sensitive to small changes and have become difficult to interpret.

Benign Core Biopsy Rate

The rate of benign core biopsy can provide an indication of the quality of pre-surgical assessment but no target has been set for this indicator.

In 2005 and 2006, the benign core biopsy rate was 13.0 and 4.8 per 1,000 screens (initial and subsequent screens respectively). The biopsy rate is lowest among older women (70+ years) undergoing subsequent screens. Since 2002, the rate has increased for both initial and subsequent screening mammograms suggesting a shift toward the use of core biopsy (Tables 6-8. pg29-35 ).

Benign to Malignant Core Biopsy Ratio

The ratio of benign to malignant core biopsies, can provide an indication of the quality of pre-surgical assessment but no target as yet has been set for this indicator. As with the open surgical biopsy rate this indicator has been updated to close the diagnostic interval after 6 months of benign test results.

In 2005 and 2006, the benign to malignant core biopsy ratio was 2.9:1 for initial screens and 1.4:1 for subsequent screens, and is lowest in older women (70+ years). For women, 50 to 69 years, undergoing subsequent screens the ratio has remained stable at approximately 1.4:1 since 2002. For women undergoing their first screen, the value has been relatively stable since 2002 at approximately 2.8:1 (Tables 6-8. pg29-35 ).

Cancer Detection by Organized Breast Cancer Screening Programs

In total, organized screening programs detected 7,872 cancers (invasive, in situ and unclassified types combined) among women aged 50 to 69 during 2005 and 2006 (Table 6. pg29 ). In order to ensure consistency between provinces this report identifies screen-detected cancers as those diagnosed within 6 months from the screen date. Other breast cancers among Canadian women were detected by opportunistic screening (outside of an organized program) or when a woman became symptomatic of disease.

Among all women diagnosed with cancer through an organized screening program (≥40 years) 80.4% (7,719 women) were diagnosed with invasive and 19.6% (1,886 women) were diagnosed with in situ cancers. The proportion of cancers considered invasive increased with age; 71.0% of women aged 40 to 49 were diagnosed with invasive cancers compared to 85.5% of women 70 or more years. Women aged 50 to 59 and 60 to 69 were diagnosed with 77.1% and 83.0% invasive respectively (Table 5. above ).

In Situ Cancer Detection Rate

Ductal carcinoma in situ (DCIS) is a form of cancer detected through mammography screening, but, there is limited evidence supporting the transition of all forms of DCIS to invasive cancer. Because of this, no target has been set for in situ cancer detection rates in Canada. Despite this, it is important to monitor rates of detection until appropriate targets can be set.

In Canada, women (50 to 69 years) undergoing their first screen had a DCIS detection rate of 1.2 cases per 1,000 screens. Women undergoing subsequent screens had a DCIS detection rate of 0.9 case per 1,000 screens ^{[Note 7]} (Table 6. pg29 ).

Among women aged ≥40 years, diagnosed with breast cancer in 2005 and 2006, 46.9% of tumours were classified as stage I and 3.9% were classified as stage III/IV (Table 5. pg25 ). Among women aged 50 to 69, the proportion of invasive tumours less than 10 mm was 34.1% and almost 75% of women had negative lymph nodes at diagnosis (Table 6. pg29 ). A larger proportion of older women had tumours smaller than 10 mm at diagnosis (range: 30.7% to 37.2%) and negative lymph nodes at diagnosis (range: 72.0% to 80.8%) compared to younger women (Table 7. pg32 ).

Invasive Cancer Detection Rate

The targets for invasive cancer detection rates established in Canada are >5 per 1,000 first screens and >3 per 1,000 subsequent timely screens.

In Canada, women undergoing their first screen had an invasive cancer detection rate of 4.6 cases per 1,000 screens. Women undergoing subsequent screens had an invasive cancer detection rate of 3.7 cases per 1,000 screens ^{[Note 7]} (Table 6. pg29 ). As anticipated, the invasive cancer detection rates were highest among initial screens and also increased in older women and when subsequent screening was not timely (Figure 9. pg27 ).

Invasive Tumour Size and Negative Node Rate

Cancer detected at earlier stages has more treatment options, less recurrence, and improved survival. Research in Canada has shown that among women diagnosed with breast cancer, participants of organized breast cancer screening programs have more breast conserving surgery and less chemotherapy compared to non-participants.⁽⁹⁾ In addition, 97.9% of women with stage I breast cancer survive at least five years while only 27.9% of women diagnosed in stage IV survive for five years.⁽¹⁰⁾ Early stage cancer has smaller tumours and no lymph node involvement. The Canadian target is for greater than 25% of invasive tumours to be ≤10mm and greater than 50% of invasive tumours to be ≤15mm. The second target is for >70% of women with invasive cancer to have no lymph node involvement.

Post-Screen Invasive Cancers

Post-screen invasive cancers are those cancers that develop after a normal screening mammogram but before the next screen, and the post-screen invasive cancer rate serves as an indicator of the sensitivity of the screening program. Post-screen invasive cancers include two types of cancers: those that occur after the recommended 12 or 24 months among women who do not return for their regular annual or biennial screen respectively (non-compliant cancers), or among women who become symptomatic before their next regular screen (interval cancers). This methodology differs from that reported in the Evaluation Indicators Report and has been updated include cases referred for diagnostic follow-up with a benign result that later developed breast cancer.⁽⁴⁾ Post-screen invasive cancer rates were calculated based on all women screened from 2002 to 2003 who developed an interval cancer during 2004 to 2005.^{[Note 7]} In order to ensure consistency between provinces this report also considers interval cancers to include those detected by a screening mammogram that have taken longer than 6 months to diagnosis. Due to the changes in the method of calculation, it is anticipated that the rate reported in this report should be higher than in previous reports assuming stability in the true incidence.

The target is for less than 6 women per 10,000 person years to be diagnosed with a post screen cancer within 12 months of screening and less than 12 women per 10,000 person years within 24 months.

Nationally, the post-screen invasive cancer rate was 6.5 per 10,000 person years within 12 months and 8.0 per 10,000 person years within 24 months (Table 6. pg29 ).