Organized Breast Cancer Screening Programs in Canada - Report on Program Performance in 2001 and 2002

2001 and 2002 Results

This report presents selected statistics for the 2001 and 2002 calendar years using data submitted to the CBCSD up to January 2005. Data submissions from the programs are staggered across several months. This may impact the completeness of cancer-related data for certain programs. Unless otherwise noted, the summary statistics for all programs include data from all 10 provinces. Tables 6, 7 and 8 summarize the performance measures by program, age group and screen year.

Participation and Retention in Screening Programs

Organized breast cancer screening programs promote participation through a variety of recruitment methods. Although currently no program meets the national performance target of at least 70% participation in biennial screening, participation of women aged 50 to 69 in organized breast cancer screening programs increased slightly to reach 33.9% nationally in 2001 and 2002 (Figure 3).

Although currently no program meets the national target of .70% participation in biennial screening, participation of women aged 50 to 69 in organized breast screening programs increased slightly to reach 33.9% nationally.

Delivery of screening services through organized programs contributes to increased effectiveness and efficiency of screening⁸. In 2001 and 2002, fewer than half of women were screened through longstanding programs in Alberta, Nova Scotia, Ontario, and Newfoundland (Figure 3, Table 6). Self-reported screening rates in the target age group were not dramatically lower in these provinces relative to others (Figure 4). This indicates that in these provinces, much screening occurs in the fee-for-service sector, where programmatic elements such as identification and recruitment of the target group, an effective referral system once an abnormality has been detected, and procedures for evaluating and monitoring the programs, are often absent. In future years, participation rates in organized screening in Alberta are expected to rise as the program expands province-wide. More recently implemented programs in Quebec and Prince Edward Island are fast approaching 50% participation. Close to or greater than half of the eligible population participated in biennial screening through organized screening programs in British Columbia, Manitoba, Saskatchewan, and New Brunswick. Greater participation in organized screening in these programs can be attributed, in part, to lower rates of nonprogrammatic screening, successful recruitment campaigns, and the ability of these programs to access rural women through mobile mammography.

Between 1998 and 2002, the number of target aged women receiving mammography in the ten provincial organized programs nearly doubled from 328,674 to 608,967.

Over half the women screened in 1998 and 1999 had been previously screened within the program, and the retention rate of 75.2% exceeded the national target of at least 75% (Table 6). For all age groups, the probability of returning for a rescreen stabilizes at 30 months following the proceeding screen (Figure 5), although women aged 40-49 are more likely to return at a 1-year interval.

Overall, participation and retention rates in most longstanding programs have stabilized or declined, reflecting, in part, limited program capacity to provide screening to a growing target population. However, some programs direct a third or more of their program capacity to screening women aged 40-49 (Figure 6), and in some programs annual recall is more common. In 2001 and 2002, the proportion of total screens that were delivered to women aged 50 to 69 ranged between programs from 51.1% to 100.0% (Figure 6). Nevertheless, even programs that apply a strict biennial screening interval and target only women aged 50 to 69 are reaching the limits of their capacity. Between 1998 and 2002 the number of target-aged women receiving mammography in the 10 provincial organized programs nearly doubled, from 328,674 to 608,967 (Table 8).

Results of Screening

Organized programs aim to ensure that they identify breast cancers in asymptomatic women while minimizing the number of healthy women who receive an abnormal screening result and require followup procedures. The proportion of screened women who receive an abnormal screening result (abnormal call rate) is one measure of the degree to which programs minimize the potential harms of screening among participants.

The benefits of breast screening depend on regular participation every two years. 75.2% of women returned for screening within 30 months of their previous screen, exceeding the national target of . 75%.

In 2001 and 2002, for women in the target age range, the observed overall abnormal call rates of 13.1 and 7.4 for first and subsequent screens, respectively, were higher than in previous years, and exceed the national targets (Table 6 and Table 8), which specify that less than 10% of first screens and less than 5% of women returning for screening should have abnormalities detected on their screening exam. The proportion of screened women who receive an abnormal screening result on first screens is normally higher, reflecting prevalent cancers among previously unscreened women (Figure 7). With delays to rescreen, abnormal call rates begin to rise, showing the benefits of returning for a subsequent screen in a timely fashion. Although the use of clinical breast exam in combination with mammography results in higher abnormal call rates, programs that provide screening using both modalities did not consistently demonstrate higher abnormal call

The abnormal call rate, a measure of the degree to which programs minimize the potential harms of screening, was higher than in previous years and did not meet the national targets.

rates than those providing mammography only⁹. Several factors and practices have been shown to increase the proportion of screened women who receive an abnormal screening result, among them, radiologist inexperience and low reading volumes. Although formal accreditation programs in Canada require a reading volume of 480 mammograms per year, research from Canadian organized programs suggest that optimal reading volumes are much higher^{10, 11}. The practice of double reading mammograms has also been shown to reduce abnormal call rates while maintaining high cancer detection rates, though few programs in Canada incorporate double reading. Incorporation of such evidence into accreditation standards and continued promotion of accreditation of screening program facilities can help optimize the delivery of mammography by programs. The setting of minimum standards of training and performance for radiologists and mammography technologists as well as guidelines for quality assurance programs are further outlined in the document of the Quality Determinants Working Group of the National Committee for the CBCSI¹². Programs undergoing expansion must pay particular attention to the additional training needs of personnel new to organized screening programs. Continued monitoring of abnormal call rates will be critical as will ongoing efforts to reduce abnormal call rates while maintaining optimum cancer detection rates.

Diagnostic Investigations

Programs have established methods to streamline scheduling, track follow-up procedures and results, and provide additional support to women during the diagnostic follow-up process.

The vast majority of women who receive an abnormal screening result do not have breast cancer. When a lump or lesion is detected through CBE or mammography screening, additional assessment is normally required to establish or exclude the presence of cancer. The fear and anxiety associated with subsequent testing should be minimized by providing timely, well-coordinated follow-up with only the appropriate number of interventions. For this reason, a number of programs have established methods to streamline scheduling, track follow-up procedures and results, and provide additional support to women during the process. Progress towards reducing waiting times in the diagnostic follow-up process is detailed in the Special Topic portion of this report.

Diagnostic investigations may include a clinical evaluation, radiologic work-up including diagnostic mammography with additional views (spot compression or magnification views), a comparison with previous mammograms and/or ultrasonography. Figure 8 shows the proportion of women who received each diagnostic procedure after an abnormalscreen. Compared with previous years, more women in 2001 and 2002 underwent breast imaging alone (77.2%), indicating that although abnormal call rates have risen, most abnormalities are resolved without having to resort to invasive follow-up procedures.

Most abnormalities are resolved without having to resort to invasive follow-up procedures. Compared with previous years, more women underwent breast imaging alone and the proportion of women undergoing open surgical biopsy has declined.

In order that a final diagnosis can be obtained, a small number of women may undergo a surgical consultation, fine-needle aspiration, core biopsy and/or surgical biopsy where appropriate13. The growing trend towards using the less invasive procedures before resorting to open surgical biopsy continues. Compared with 1999 and 2000 data, the proportion of women undergoing open surgical biopsy in 2001 and 2002 declined from 8.3% to 7.2% (Table 4). Of the women aged 50 to 69 who did require a surgical biopsy, the benign to malignant open biopsy ratio was 0.9:1, which is well within the Canadian target of £2:1 and reflects a steady improvement over the past five years (Table 8). Keeping the ratio of benign to malignant biopsies appropriately low is necessary to avoid inducing unnecessary morbidity in healthy women.

Cancer Detection

The cancer detection rate is a meaningful indicator for program evaluation when it is observed in relation to the abnormal call rate, post-screen detected cancer rate and the underlying breast cancer incidence rate. The cancer detection rate in an organized screening program should generally exceed the cancer incidence rate that existed in the population before screening implementation, because screening detects asymptomatic cancers. Consequently, cancer detection rates will generally be higher for first screens (when prevalent cancers would be detected) than for rescreens (Figure 9). These rates also tend to be higher among women who do not return for screening within the recommended interval. Target-aged women who are rescreened within the recommended interval have similar cancer detection rates at 9-18 months and 18-30 months.

The positive predictive value (PPV) is determined by the proportion of women who had an abnormal screen and who subsequently received a diagnosis of cancer. A high PPV reflects the effectiveness of the screening program at minimizing unnecessary follow-up. The national picture indicates that the PPV of an abnormal mammogram meets targets of ³5% for initial screens and ³6% for rescreens. However, provision of CBE lowers the PPV by raising the abnormal call rates but only increasing cancer detection rates slightly. The factors that influence cancer detection rate and abnormal call rate must be taken into consideration when evaluating a program’s PPV. The PPV tends to improve with rescreening because the initial screen establishes a normal baseline for comparison. A greater prevalence of cancers also tends to increase PPV. Even though abnormal call rates did not differ substantially with age (Figure 7), the PPV increased with age (Table 7), reflecting the increased number of cancers with advancing age and the improved discriminating power of mammograms for less dense breasts.

The prevention of breast cancer death through mammography screening depends on detecting cancers at an early stage where treatment is most effective. In 2001 and 2002, screening programs detected a total of 6125 cancers (Table 6), of which 81% were invasive and 19% were ductal carcinoma in situ (DCIS) (Table 5). Nationally, the cancer detection rates of 5.0 invasive cancers detected per 1000 screens and 3.9 invasive cancers detected per 1000 screens on first and rescreen, respectively, were within the targets set. The proportion of screendetected cancers that were invasive increased with age, and the lowest proportion of DCIS detected was among women aged 70-79. A performance measure has not been established for in situ cancer detection rates, given the lack of scientific consensus surrounding the interpretation of these rates. They are included in this report for monitoring purposes only. In situ cancer detection rates remained stable in the 5-year period from 1998 to 2002.

Preventing breast cancer deaths through screening depends on detecting cancers early, before they are large enough to be felt or to have spread. In 2001 and 2002, 36.4% of invasive cancers detected were £10 mm in diameter and 75.3% were node negative, both well within Canadian performance targets.

Patients with cancer detected at an early stage have more treatment options, reduced cancer recurrence and improved survival. Nearly 97.9% of women with stage I breast cancers survive for at least five years¹⁴. This stage accounted for 52.9% of screen-detected cancers (with complete staging information) in women aged 50 to 69 (Table 5). Among women aged 50 to 69 in 2001 and 2002, 36.4% of invasive cancers detected by program screens were £10 mm in diameter (Table 6), and 24.7% of cases were node positive, both well within the Canadian performance targets of ³25% and ³30% respectively.

Post-Screen Cancers

As an element of the quality control process, when post-screen cancers are detected, program radiologists (and, in some cases, technologists) review the previous screening film to arrive at a final decision regarding whether the cancers were newly developed in the interval between screens, were missed at screening or were missed at diagnosis. In cases of disagreement, resolutions are made either through consensus or by a majority decision by readers.

According to the Canadian performance targets, fewer than six post-screen detected invasive cancers per 10,000 person-years should be detected within 12 months from screening, and fewer than 12 per 10,000 person-years should be detected within 24 months from screening. While these targets were met or nearly met (Table 6), with overall rates per 10,000 person-years of 6.5 and 9.4 at 12 and 24-months, respectively, the figures must be interpreted cautiously for a number of reasons. Comparisons of post-screen cancer rates among programs require complete and up-to-date breast cancer registration and the assurance that post-screen cancers are counted in the same way. Good linkages with cancer registries will result in higher post-screen cancer rates because of higher levels of case ascertainment. In Canada, postscreen cancer rates may also be affected by the amount of screening delivered outside of screening programs, the performance of CBE and BSE between screening episodes, and differences in the classification of the end of a screening episode in the event of a screening abnormality.

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