LONDON (Reuters) - Young women under 45 years old diagnosed with early breast cancer have a higher risk of dying from the disease than older patients, scientists said on Thursday.

"The younger the woman, the poorer the chance of survival," said Dr Vincent Vinh-Hung, of University Hospital in Brussels.

Breast is the most common cancer in women, with more than a million new cases detected worldwide each year. Most are in women over 50 -- the disease is rare in young women.

But Vinh-Hung told Reuters that although only a small fraction of early breast cancers occur in young women, they account for a disproportionate percentage of deaths.

In a study of 45,000 breast cancer patients presented at the 5th European Breast Cancer Conference in Nice, France, Vinh-Hung and his colleagues found that the odds of dying from breast cancer rose by 5 percent for every year that a woman was under 45 when diagnosed.

They focused on women with early breast cancer which had not spread beyond the breast and whose tumors were less than 2 cm (0.8 inches) in size.

Doctors had previously suspected that the poorer prognosis in young women was due to being diagnosed later with a more advanced cancer. Breast tissue in younger women is usually denser which can make detecting a tumor more difficult.

Younger women may also ignore a suspicious lump in their breast thinking it is harmless or that they are too young to have breast cancer.

But Vinh-Hung said the findings suggest that in young women, age, more than other factors, affects the chance of survival.

He and his team suspect there may some type of unknown genetic damage that may increase the chances of developing the disease early and contribute to the poor prognosis in younger patients.

A family history of breast cancer, early puberty, late menopause, not having children or having them late and genetic mutations are risk factors for breast cancer.