Breast Cancers Arising Between Mammograms Have Aggressive Features

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Breast cancers that are discovered in the period between regular screening mammograms—known as interval cancers—are more likely to have features associated with aggressive behavior and a poor prognosis than cancers found via screening mammograms. These high-risk features include higher stage and grade, larger size, and lack of estrogen receptors (ER) and progesterone receptors (PR), researchers reported online May 3, 2011 in the Journal of the National Cancer Institute.
 
The results highlight the need for more sensitive screening methods and for women to monitor their breast health between mammograms, concluded Dr. Victoria Kirsh of Cancer Care Ontario and her colleagues.

The researchers compared the traits of breast cancers in women who had been screened in the Ontario Breast Screening Program between 1994 and 2002. The analysis included 87 women who had "missed" interval cancers, 288 women who had true interval cancers, and 450 women whose cancers were detected by screening mammography, from a population of over 400,000 screened women. Missed interval cancers were cancers that could have been detected by mammography but were overlooked due to error or difficulty in reading the x-ray films. True interval cancers were not detectible at the time of last screening, even on review.

Some clear differences between the tumors were found. Compared with screen-detected cancers, true interval cancers were almost four times as likely to be larger than 2 cm in diameter, more than four times as likely to be stage III or IV instead of stage I, more than three times as likely to be poorly differentiated (to have very abnormal-looking cells), about three times as likely to have a high proliferative rate (to be growing quickly), and about twice as likely to be estrogen receptor negative and progesterone receptor negative. Missed interval cancers were also more likely to be large, to be poorly differentiated, and to have invaded the lymph nodes than screen-detected cancers.

Women in the study were older than age 50 and predominantly white. It is not clear whether these results would be similar in younger women or in a more diverse population.

"A small number of cancers will not be detected by mammography," explained co-author Dr. Anna Chiarelli in an e-mail. This study, she continued, serves as a general reminder that "women should be 'breast aware' and see their health care provider if breast symptoms arise between mammograms."