A very low dose of estrogen might help women whose breast cancer has come back after treatment, researchers reported on Tuesday.

Even though most treatments are aimed at stopping estrogen from fueling tumors, the researchers said after years of this therapy the body may need some of the hormone to fight them off.

Their findings, published in the Journal of the American Medical Association, suggest a cheap way to help some patients with advanced breast cancer.

Dr. Matthew Ellis of Washington University School of Medicine in St. Louis and colleagues studied 66 women with advanced breast cancer who had been treated with newer drugs called aromatase inhibitors.

They include Pfizer's Aromasin, Novartis's Femara, and AstraZeneca Plc's Arimidex.

The women in the study had all experienced a relapse while on estrogen-lowering drugs, and their disease was progressing, Ellis said in a statement.

So they were faced with undergoing chemotherapy. We found that estrogen treatment stopped disease progression in many patients and was much better tolerated than chemotherapy would have been.

They gave the women a form of estrogen called estradiol, in both high and very low doses. Both doses helped 30 percent of the women, Ellis and colleagues found.

We demonstrated clearly that the low dose was better tolerated than the high dose and was just as effective for controlling metastatic disease, Ellis said.

The treatment was not always permanent. In 30 percent of the women helped by the estrogen, the tumors started growing again. But going back on the aromatase inhibitors -- a daily pill that is far less toxic than chemotherapy -- helped a third of these women.

More than 400,000 women die from breast cancer globally every year. About 75 percent of breast cancers are estrogen-receptor-positive, meaning they are fed by estrogen, and treatment with drugs such as tamoxifen and the aromatase inhibitors cuts off this supply of hormones.