UK researchers say they have found a way to forecast how long women will be fertile after being treated for cancer.
The calculation gives a time range of likely fertility
A simple formula, using the radiation dose and the age at which the woman received it, reveals her remaining "window of fertility".
Drs Tom Kelsey at St Andrews and Hamish Wallace at Edinburgh University say this will help doctors make reliable predictions when counselling women.
The American Society for Therapeutic Radiology & Oncology put out the work.
A woman is born with all the potential eggs that she will ever have and loses them from her ovaries as she ages until she reaches the menopause and is no longer fertile.
Radiotherapy treatment can damage the ovaries and lead to premature menopause.
Until now, doctors were only able to tell women and young girls with cancer undergoing radiotherapy that there was a risk and were unable to accurately quantify it.
By using the new formula, they should be able to predict what dose would cause sterility in the vast majority of girls or women at any given age.
For example, a dose of 12 Gray - the typical dose used for total body irradiation when someone has leukaemia and needs a bone marrow transplant - at 15 years of age would mean most would reach menopause at the age of 19, said Dr Wallace.
A few would be left sterile immediately after the treatment and a few would be fertile until they were 23, he said.
As the dose goes up, the damage is worse. Similarly, older women will be damaged at lower doses than younger girls because by that age they have fewer eggs.
Dr Wallace said: "This is the first time we have been able to predict with accuracy what their window of opportunity is.
"The dose and the age at which you receive it are the important things.
"What we can do is give women choice and more understanding about their treatment."
He said women with a partner may want to have her eggs fertilised and stored for later use before undergoing radiotherapy.
"If they haven't got a partner, they may want some of their eggs to be frozen.
"If we have a very young patient who has not gone through puberty yet, then the options are limited. It might be worth taking a biopsy of the ovary and freezing it."
Scientists recently reported a pregnancy in a woman who had strips of ovarian tissue implanted. However, there is still controversy over whether this treatment actually restores fertility or not.
He said that about 7,500 young girls were diagnosed with childhood cancers each year. About 10-20% of these are treated with radiotherapy.
In most cases the treatment does not leave them sterile, but it does reduce the number of years that they will be fertile for in the future.
He said knowing when a woman's fertility would likely run out would also allow doctors to prescribe drugs to protect against menopause-related diseases like osteoporosis.
Dr Richard Fleming, an expert in reproductive endocrinology at Glasgow Royal Infirmary and member of the British Fertility Society, said: "It's not precise but it will mean doctors can give women an age range for fertility. It's very helpful.
"To have a degree of certainty, although it is a very unfortunate series of events that person is going through, makes it much easier for people to handle."
Dr Lesley Walker of Cancer Research UK said: "The real test of this model will be in its application in the clinic."