Doctors may soon be able to better predict whether breast cancer patients will respond well to chemotherapy, the results of a recent study conducted in part by Harvard Medical School affiliates at the Dana-Farber Cancer Institute suggest.

The study, which appeared in the Journal of the American Medical Association last week, “add[s] to a growing body of evidence that breast cancer is not one homogeneous disease, but rather a disease with many subtypes and requires a variety of new treatment approaches,” lead author Eric P. Winer, who is associate professor at HMS, said in a press release.

Winer and his colleagues from institutions around the country reexamined three past breast cancer studies, each of which spanned a separate five-year period, beginning in 1985. The researchers compared the responses to chemotherapy of women whose tumors were sensitive to the hormone estrogen (ER-positive) and women whose tumors were not sensitive to estrogen (ER-negative).

The researchers examined how evolving treatment regimens have affected the relapse and death rates of study participants. The researchers found that lower relapse and death rates correlated to improvements in chemotherapy for ER-negative patients. Chemotherapy increased the five-year survival rate of women with ER-negative tumors by 23 percent. But it increased the five-year survival rate of patients with ER-positive cancer by a statistically-insignificant 7 percent, the study found.

Judy E. Garber, a Dana-Farber researcher and associate professor of medicine at HMS, who was not affiliated with the study, explained that the use of hormone therapy on women with ER-sensitive tumors may account for the discrepancy in chemotherapy’s relative effectiveness.

“In tumors that depend on hormonal mechanisms to drive them, therapy that targets the hormones has a huge effect. As a result, there is not a lot of room for the added effect of chemo,” Garber said.

Because hormone therapy drugs like tamoxifen reduce an ER-positive patient’s death rate by 30 percent, women with ER-positive tumors receive less benefit from chemotherapy than women with ER-negative tumors that do not respond to hormone therapy. The study results present doctors with a new challenge—deciding which patients with ER-positive tumors should receive chemotherapy in addition to hormone treatments, Garber said.

“We do have some sense of which of the patients with ER+ patients benefit from chemotherapy.  We need to do more research to define better this subset,” Winer wrote in an e-mail.

Garber said that the study will not change treatment in the short-term.

“This paper will not change treatment today, but it should make us challenge everything we thought we knew,” she said. “It’s now our responsibility to go back and look again to make sure we’re doing the right thing.”