Background: Interval breast cancers are defined as carcinomas occurring within 2 years after a negative screening Distinction has to made between cancers existent at the time of screening but missed for some reason, and fast-growing incident cancers. This is important because the natural history and the implications for the treatment of the patient might differ.
Methods: Radiologic and histopathologic characteristics were assembled for 104 interval cancers diagnosed within the DOM project, the Utrecht program for the early detection of breast cancer. At a mammographic review for 27 cases, signs of malignant or benign tumor were found (missed cases). For 77 cases no radiologic signs were present on review. Twenty of these cases had a mitotic rate of > 3 and a high tumor growth rate (mean doubling time: 51 days). This combination seemed implausible, therefore, it was thought hypothesized that these tumors were most likely present, although radiologically invisible (masked), at the time of screening. The remaining cases (n = 57) were classified as true interval cancers and further divided into 14 fast-growing cases (mitotic rate > or = 3/high-power field [HPF]) and 43 cases with an intermediate growth rate (mitotic rate < 3/HPF).
Results: Factors associated with the masking of tumors were the histologic tumor type, absence of microscopic calcifications, and presence of dense breast tissue. Fast-growing tumors were characterized by a young patient age, absence of microscopic calcifications, and a high percentage of regional lymph node positive tumors. The 5-year survival probability was 100% for missed cases, 70% for masked cases, 80% for cases with an intermediate growth rate, and 54% for fast-growing cases.
Conclusions: It is possible to separate interval breast cancers in true interval cases and cases (most likely) existent at the time of screening. Part of this last group is invisible by mammography (masked).