The damage caused to a woman's fertility by radiotherapy treatment for cancer may be twice as bad as previously feared, say doctors.

A study at the Universities of Edinburgh and St Andrew's in Scotland suggests that a woman's eggs may be far more vulnerable to the effects of radiation than thought.

The research, published in the journal Human Reproduction, found that a comparatively low dose of radiation, described as two "grays", can destroy half of the remaining eggs which should normally keep a woman fertile until the menopause.

Previously the dose needed to do this was estimated at four "grays".

A normal radiotherapy treatment might aim as much as 12 to 30 "grays" at the affected area - although not all of this will be aimed directly at the ovaries.

However, the new measures means that any dose of this magnitude aimed at the pelvic or abdominal regions, or across the whole body, would be much more likely to wipe out fertility.

Egg store

A woman is born with many thousands of "oocytes" in her ovaries.

These are immature eggs, and will mature and be released in small numbers on a monthly basis from puberty until the end of her fertile life.

Radiotherapy gives a dose of radiation designed to destroy or fatally weaken cancerous cells, but oocytes are equally vulnerable.

The treatment may completely remove fertility, or younger women with a bigger reserve of oocytes may find that their stock comes to an end earlier in life, with menopause starting earlier as well.

Professor Hamish Wallace, who led the study, said that pre-treatment counselling might be improved, and perhaps in future women given a more accurate idea of the impact of radiotherapy on their fertility.

Freezing option

In the journal paper, the researchers wrote: "Calculation of the dose of radiation received by each ovary, combined with a more accurate estimate of the radiosensitivity of the human oocyte, will facilitate our ability to provide more scientific fertility counselling to young women at risk of a premature menopause."

Women facing cancer treatment are likely to have greater options in future to preserve their fertility.

Scientists are working on ways to take out ovarian tissue containing oocytes prior to treatment, freeze it, then either reimplant it later, or extract and artificially mature the oocytes it contains so they can be used in IVF treatments.