NEW YORK (Reuters Health) - Reproductive factors, such as breastfeeding and the age at which menstrual periods begin, have different effects on the types of breast cancer seen in postmenopausal women, according to a report in the journal Cancer.

In the study, the researchers looked at three types of breast cancer: luminal, HER-2 overexpressing, and triple negative.

With luminal breast cancer, cell receptors for estrogen are present and the prognosis is generally favorable. HER-2-overexpressing cancers generate excess amounts of HER-2, a growth factor receptor, and have a worse prognosis. Triple negative breast cancers lack estrogen, progesterone, and HER2 receptors, which makes them difficult to treat and they grow fairly rapidly.

The current results suggest that a reproductive factor that greatly increases the risk of HER-2 overexpressing disease, for example, may have little effect on the risk of luminal disease.

Hormone receptor status and HER-2 expression can be used to define various breast cancer subtypes that are important from a prognostic standpoint, lead author Dr. Amanda I. Phipps, from the Fred Hutchinson Cancer Research Center in Seattle, and colleagues note. The reproductive factors that correlate with the subtypes, however, have not been established.

Because of the effect that reproductive factors have on sex hormone levels, the researchers hypothesized that they could influence the risk of the various breast cancer subtypes.

To investigate, the team analyzed data from studies that included 1023 women with luminal breast cancer, 39 with HER-2-overexpressing breast cancer, 78 with triple-negative breast cancer, and 1476 subjects without cancer.

Early age at first menstrual period increased the odds of HER-2-overexpressing disease by 2.7-fold, but minimally affected the risk of the other cancer types. Late age at menopause as well as use of hormone therapy increased the risk of luminal disease by 1.6- and 1.7-fold, respectively.

Some reproductive factors, however, had a protective effect. For instance, breastfeeding for 6 months or longer cut the risk of luminal disease by 20 percent and that of triple-negative disease by 50 percent.

"Although definitive conclusions regarding which risk factors are more strongly related to certain...subtypes of breast cancer cannot be made based on the current study, our data do support the premise that risk factor profiles vary by breast cancer subtype," the authors conclude.

SOURCE: Cancer, October 1, 2008.

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