Background: The availability of breast carcinoma data from trials of mammographic screening provides an opportunity to study the natural history of breast carcinoma.
Methods: The Swedish Two-County study is a randomized, controlled trial of mammographic screening for breast carcinoma in which 77,080 women were randomized to receive an invitation to mammographic screening and 55,985 were randomized to receive no invitation. During the trial, a total of 2468 breast carcinoma cases were diagnosed. The authors examined the effect of screening on the pathologic attributes of the tumors diagnosed, mortality and survival from breast carcinoma, and the consequences of arresting tumor development by screening.
Results: Screening reduces mortality from breast carcinoma largely through its effect in detecting tumors at a smaller size, decreasing the probability of lymph node metastases, and reducing the opportunity for worsening of the grade of malignancy of the tumor.
Conclusions: Breast carcinoma is not a systemic disease at its inception, but is a progressive disease and its development can be arrested by screening. The point at which the tumor's progression is arrested is crucial. Detection of small (<15 mm) and lymph node negative invasive tumors will save lives and confer an opportunity for less radical treatment. Tumor progression in the preclinical phase occurs more rapidly in women age <50 years, suggesting the need for a shorter screening interval for this group.