A study of risk factors for breast cancer in young women suggests risk assessment and prevention, using techniques that avoid radiation, such as MRI, should start much earlier in life. The study shows that breast tissue composition in young women could be linked to the risk of breast cancer in middle age and older. The findings are reported in the June edition of The Lancet Oncology, written by Dr Norman Boyd, Campbell Family Institute for Breast Cancer Research, Toronto, Canada, and Dr Mike Bronskill at Sunnybrook Health Sciences Centre, Toronto, Canada,and colleagues.

The amount of dense breast tissue, which is calculated from mammograms and termed mammographic density (MD), varies considerably throughout the female population. It is a significant risk factor for breast cancer in middle-aged and older women, with risk increasing as MD increases. However, little is known about the development of MD in early life, and how MD of young women is related to their height, weight and age, and the MD of their mothers.

This study, the first of its kind, recruited 400 young women, aged 15—39 years, and their mothers. Instead of measuring MD with mammograms, MRI was used to measure breast water concentration in the young women, as this gives a similar indication of MD to that of a mammogram, without exposing the women to radiation. Blood was obtained from each woman on the same day of the menstrual cycle (within 10 days of the start of her most recent period). Mothers (total 365) underwent mammography and a random sample of 100 also consented to have a breast MRI scan.

The researchers found that in mothers, per cent breast water as measured by MRI was strongly linked with per cent MD measured by mammogram. Per cent breast water in daughters (median 45%) was significantly higher than in mothers (28%), and decreased as their age and weight increased, but increased with increasing height. It was also positively associated with increasing MD in their mothers. Weight, height, and the mother’s MD are known risk factors for breast cancer.

The Pike model of breast-cancer incidence predicts that the breast will be most susceptible to carcinogens (cancer causing agents) at early ages. This is supported by evidence from World War II, since Japanese women aged 20 years or younger at the time of exposure to radiation from the atomic bombs had the greatest risk of breast cancer in later life. The current study shows that per cent breast water variation is higher in 15—19 year olds than in 20—30 year olds, and decreases with age, as backed by analysis of the 100 mother—daughter pairs who both had MRI. The authors say: “A high degree of mammographic density in middle age, when it is a strong risk factor for breast cancer, may arise from the subset of the population with the greatest amount of fibro-glandular tissue in early life, when susceptibility to potential carcinogens is greatest. Interventions directed at the prevention of breast cancer may therefore be more effective if they are started in early life rather than adult life.” Currently in the UK, breast cancer screening begins at age 50 years.

Higher blood growth hormone concentrations were also linked to higher per cent breast water. Growth hormone regulates general growth as well as that of the breast, suggesting that taller women could be more at risk of breast cancer, with a Danish study backing this. The current study showed that each 5cm difference in height in daughters was associated with a 3% increase in per cent breast water, which suggests a mechanism by which growth might affect the risk of cancer.

The authors conclude: “It is known that the breast is most susceptible to the effects of carcinogens at early ages. Our findings suggest that differences in breast tissue composition in early life may be a potential mechanism for this increased susceptibility. By identifying the environmental and genetic factors that influence breast tissue composition in early life we may be able to develop safe and effective methods of prevention.”