CORRESPONDENCE Re: Biologic Characteristics of Interval and Screen-Detected Breast Cancers

The goal of mammography is to detect breast cancer before it spreads beyond the breast. Two recent studies (1,2) have compared histologic and other prognostic features of mammographically detected breast cancers with cancers that were detected clinically in the interval following a negative screen. These studies find that interval cancers are of higher grade and are more likely to have adverse prognostic features (such as being p53 positive and ER negative) than are mammogram-detected cancers. The tumors that are missed by mammography may, therefore, be more likely to metastasize and to lead to early fatality. Young age and a positive family history of breast cancer have also been associated with screening failure (3). These findings may be relevant for young women from high-risk groups who are enrolled in breast screening programs; for example, the majority of breast cancers associated with BRCA1 mutations appear in young women and are of high grade, are p53 positive and are ER negative (4). The authors conclude from these comparisons that interval cancers are rapidly growing tumors and that the tumors were likely too small to have been detected on the previous mammogram. Another interpretation is that mammographic detectability per se is associated with favorable clinicopathologic features. To address this question, we reviewed the records of the Ontario Breast Cancer Screening Program for women diagnosed with breast cancer in the province from 1991 through 1998. The women in this program are screened by both clinical breast examination (CBE) and by mammography, usually on the same day. We compared the 84 tumors that were detected by CBE alone with 643 tumors detected by mammography alone. We restricted our study to women whose tumors were at least 1 cm in size. There was no difference in the mean age of the two groups (62.6 years for CBE-detected tumors versus 63.4 years for mammogram-detected tumors). Tumors detected by CBE alone were, on average, larger than those detected by mammography (2.1 versus 1.6 cm; P<.001), were more likely to be of lobular or mixed lobular histology (31% versus 11%; P<.001), and were more likely to be lymph-node positive (34% versus 20%; P .01). Lobular histology is also an adverse prognostic feature and has been reported previously to be overrepresented in interval-detected tumors (2,5). Our data are consistent with the hypothesis that mammography preferentially identifies cancers with favorable prognostic features. However, our study differs from previous studies in that the CBE-detected cancers were detected on the same day as the negative mammogram; therefore, we cannot claim that the mammography would have performed better if the screening interval were shortened.