Organized Breast Cancer Screening Programs in Canada

1999 and 2000 Report

Cancer Detection

The cancer detection rate is a meaningful indicator for program evaluation when it is observed in relation to the abnormal recall rate, post-screen cancer detection 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 10). These rates also tend to be higher among women who do not return for screening within the recommended interval. Performance of CBE leads to small increases in cancer detection rates (Table 6). Gains are small, however, 4.6% and 5.9% of cancers detected on first versus subsequent screens being attributed to CBE alone¹⁴. 

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 screening at minimizing unnecessary follow-up. The factors that influence cancer detection rate and abnormal recall 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 (Figure 11). A greater prevalence of cancers also tends to increase PPV. Delayed intervals ³ 2.5 years) to rescreen tended to increase PPV for both these reasons (Figure 11). Even though abnormal recall rates did not differ substantially with age (Table 4), the PPV increased with age, reflecting the increased number of cancers with advancing age and the improved discriminating power of mammograms for less dense breasts. 

Table 6
Cancer detection rates^a per 1,000 screens by mode of detection and age group, 1999 and 2000 screen years 

^a    Includes invasive, in situ, and unclassified cancers. 

^b    Independent of CBE delivery or CBE findings. 

^c    Independent of mammography delivery or mammography findings. 

^d    All provinces provide mammography; Manitoba, Ontario, Nova Scotia, Prince Edward Island, and Newfoundland also provide clinical breast examinations (CBEs). 

Note: Only first screens, with one year of follow-up, are included for Quebec data. 

In 1999 and 2000, screening programs detected a total of 6,463 cancers, of which 79.8% were invasive and 20.2% were ductal carcinoma in situ (DCIS) (Table 7). The proportion of screen-detected cancers that were invasive increased with age. A performance measure has not been established for DCIS 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. 

The prevention of breast cancer death through mammographic screening depends on detecting cancers at an early stage, before they are large enough to be felt or to have spread. Patients with cancer detected at an early stage have more treatment options, reduced cancer recurrence and improved survival¹⁶. Of women with stage I breast cancers, 93.2% survive for at least five years; this stage accounted for 50.7% of screen-detected cancers (with complete staging information) in women aged 50 to 69 (Table 7). 

Table 7
Characteristics of cancers detected by age group, 1999 and 2000 screen years 

^a    Unclassified cancers are not included in this analysis. 

^b    Quebec data are not included in this category. 

^c    Includes missing values and cases in which dissection was not done. 

Note: Only first screens, with one year of follow-up, are included for Quebec data. 

Table 8
Screening outcome summary by program, women aged 50-69
at screening, 1999 and 2000 screening years

Survival decreases as the stage of the cancer increases, reflecting larger tumours, more lymph node involvement and increased probability of distant metastasis. During 1994 to 1997 five-year relative survival rates among women aged 50 to 69 in a Canadian setting were approximately 83.2% for stage II cancers, just over 41.2% for stage III and 15.3% for stage IV cancers*. As a key component of cancer staging, tumour size is a good prognostic indicator (Table 2). Among women aged 50 to 69 in 1999 and 2000, 38.0% of invasive cancers detected by program screens were # 10 mm in diameter (Table 8), and only 24.3% of cases were node positive. These results exceed the Canadian performance targets of 25% and 30% respectively. 

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