By Michelle Roberts
BBC News, Health reporter in Lyon
A therapy that temporarily shuts down the ovaries could protect the fertility of women with cancer, say researchers.
Chemotherapy can cause infertility
Australian scientists used a drug to turn off a hormonal "switch" in the brain that triggers ovulation.
Once the woman's toxic chemotherapy has ended, the drug is stopped and the ovaries switch back on with their store of eggs in tact.
Cancer experts praised the preliminary study, presented at a fertility conference in Lyon.
The pilot study involved 18 women, but the researchers plan a larger randomised trial on 400 women being treated for cancer.
Women who need chemotherapy but want to preserve their fertility can opt first to have eggs removed and frozen for future use.
But not all women will want or be able to delay having chemotherapy to undergo ovarian stimulation.
Certain tumours, including some breast cancers, can grow if the woman takes drugs to stimulate the ovaries, for example.
Dr Kate Stern and colleagues at the Royal Women's Hospital in Melbourne said the treatment, cetrorelix, could offer these women another option.
The 18 women they studied, aged 18-35, were given three doses of the drug, which blocks the action of a brain chemical called gonadotrophin-releasing hormone (GnRH), to cover the course of their chemotherapy.
Test results showed the treatment suppressed ovarian function as desired with few side effects.
Afterwards 94% of the women resumed spontaneous ovulation and monthly cycles within a year.
Dr Stern told the annual meeting of the European Society of Human Reproduction and Embryology: "We don't completely understand the mechanism, but we think that by suppressing ovarian function this is somehow protecting the ovaries.
"We see it as a complementary treatment that is one of a variety of options."
Dr Joanna Owens, of Cancer Research UK said: "These are early but encouraging results in an important area of research.
"We are currently funding a large-scale trial testing whether using hormone therapy to stop a woman's ovaries working during chemotherapy could protect a woman's fertility.
"We hope research such as this could ultimately improve the quality of life of cancer patients, enabling many to have families of their own."