Asymptomatic cancers were smaller than symptomatic cancers, and early-stage cancers were more common in asymptomatic women (58 percent) than in symptomatic women (23 percent). Fewer women with asymptomatic than symptomatic contralateral cancer had node metastases, an indication the cancer may have spread.

The study was published online March 18 in the Annals of Oncology.

"Our study provides new evidence on several aspects of early detection of second breast cancers," study leader Nehmat Houssami, a breast physician in the School of Public Health at the University of Sydney, Australia, said in a news release from the journal. "We set out to estimate the effect of early, asymptomatic detection while adjusting for the two main biases known to be associated with non-randomized studies of the impact of early detection -- lead time and length bias -- so we believe that the estimates we report are more valid than previously reported estimates, while acknowledging the limitation that the evidence is not from a randomized, controlled trial."

"In addition, we have estimated this for early detection of either ipsilateral or contralateral breast cancer, while other studies have focused on one or the other," Houssami said. "So our estimates may be more useful for clinicians discussing this aspect of breast cancer follow-up with their patient."

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Editor:Yang Jie