Screening activity in 1996 resulted in the detection of 1,443 breast cancers, 82.7% of which were invasive (Table 9). The remaining 17.3% of cancers were ductal carcinoma in situ (DCIS). DCIS occurs when the cancer cells are located within a milk duct and there is no invasion of the surrounding fatty breast tissue. The proportion of screen-detected cancers that were DCIS declined with increasing age. The overall proportion of in situ cancers (17.3%) lies within the standards developed by the Australian program (10%-20%). It is important that most of the cancers detected are small invasive ones rather than in situ cancers, because the detection of a large number of in situ cancers may not result in a reduction in breast cancer mortality.⁹

The primary objective of screening is to detect invasive cancers before they have had a chance to spread in the body. This involves detecting breast cancers at a small size (< 15 mm diameter) and without axillary (armpit) lymph node involvement at diagnosis. Stage, tumour size and nodal involvement are three outcomes that allow programs to measure the extent of invasive cancer at the time of diagnosis. In terms of these outcomes, Canadian breast screening programs compare favourably with previously established standards and guidelines. Tabar et al⁹ state that to achieve a substantial reduction in mortality, 50% of screen-detected invasive cancers should be less than 15 mm in diameter, and at least 70% of screen-detected tumours should not have lymph node metastases. Within Canadian breast screening programs, of invasive cancers detected, 50.8% were less than 15 mm and 77.7% did not have lymph node metastases.

The guidelines developed by Tabar et al⁹ and national programs such as those in the U.K.¹¹ and Australia¹² were developed for women in target age groups (i.e. 50-64 or 50-69). As Table 9 shows, outcomes from Canadian provinces are particularly favourable for women between the ages of 50 and 69 years.