Prognosis of small screen-detected invasive breast cancers

The emphasis of mammographic breast screening is to detect small invasive breast cancers at a time in their natural history when early detection and treatment will reduce significantly the risk of death. However, breast screening cannot be absolutely specific in its approach and detects a wide spectrum of breast cancer, ranging from microfocal low-grade ductal carcinoma in situ to large high-grade invasive cancer. It is well recognized that many of the low-grade, special invasive cancers identified at screening have an excellent prognosis but may be so indolent that they would never have presented clinically or have threatened the life of the patients. It has been proposed alternatively that a proportion of these low-grade invasive tumours might, if not detected, de-differentiate over time into more aggressive, less well-differentiated tumours. Identification and removal of such cancers when they are at a low grade would avoid such progression. Detection of high-grade invasive cancers when they are small is clearly a means by which screening could reduce breast cancer mortality; for example, the Two-County Trial in Sweden has shown that histological grade 3 invasive cancers detected when <10 mm have an excellent prognosis, while it is widely recognized that large high-grade invasive cancers have a poor prognosis. In addition, the presence of vascular invasion and lymph node metastasis, which are associated with development of metastatic disease, are rare in grade 3 tumours <10 mm, grade 2 tumours <10 mm and grade 1 tumours <20 mm, indicating that detecting tumours under a certain size should be beneficial.