Breast cancers detected by mammographic screening have a better prognosis than those found outside of screening, even when other factors that influence disease outcome are the same, new research suggests. The reason for this association, however, is unclear.

In the study, breast tumors that were not detected by mammography were nearly twice as likely to recur at another site than cancers detected by mammographic screening. Because the choice of chemotherapy is based on the estimated risk of recurrence, the new findings could have treatment implications.

The results, published in the Journal of the American Medical Association, are based on a study of 2,842 women who were diagnosed with breast cancer in the early 1990s and logged in the Finnish Cancer Registry. The study focused on 1983 women with invasive cancer in one breast; 443 of these women had tumors detected by mammographic screening.

For all tumors, except those larger than 30 mm, detection by screening was linked to significantly better 10-year distant disease-free survival than detection outside of screening, lead author Dr. Heikki Joensuu, from Helsinki University Central Hospital, and colleagues note.

Specifically, tumors detected outside of mammography were 90 percent more likely to recur at another site compared with those detected by mammography.

Detection of breast cancers by mammography screening was a “favorable prognostic variable” the researcher conclude. This held true regardless of the number of positive lymph nodes, tumor size, patient age at diagnosis and the aggressiveness of the tumor, they add.

However, for tumors larger than 30 mm, survival did not vary based on detection method.

In a related editorial, Dr. Ruth M. O’Regan, from Emory University in Atlanta, comments that “it is not clear why patients whose tumors were detected by mammography screening had a better outcome, and it would be interesting to examine these tumors molecularly to determine if the differences between tumor detection modes can be delineated at a genetic level.”

SOURCE: Journal of the American Medical Association, September 1, 2004.