Background: Projecting individualized probabilities of developing breast cancer is needed for counseling and chemoprevention for Japanese women, in whom breast cancer incidence has been rapidly increasing.
Methods: We calculated individualized probabilities of developing breast cancer within 10-20 years and until life expectancy for Japanese women by multiplying the relative risk for each risk factor combination by the cumulative risk for the reference group. The risk factors used were age at menarche, age at first delivery, family history of breast cancer, and body mass index (BMI) (in post-menopausal women). The relative risk by menopausal status for each risk factor combination was estimated from a case control study conducted at Osaka Medical Center for Cancer and Cardiovascular Diseases (OMCC), Japan. The cumulative risk of breast cancer for the reference group within 10-20 years and until life expectancy was estimated to divide the corresponding cumulative risk for Japanese women by the weighted average of the relative risk. The weight is an expected proportion of those who have each risk factor combination among the general population. The cumulative risk for Japanese women was estimated using a data file from the Osaka Cancer Registry (OCR).
Results: We obtained cumulative risks for any age women within a certain range according to various risk factor combinations by menopausal state. For example, the highest risk group had about a 5 times higher risk probability of developing breast cancer than the general population at initial age 40, within 10-20 years, and until life expectancy.
Conclusion: The cumulative risk of breast cancer varied according to individuals' risk factors among Japanese women. The availability of concrete individualized risk estimation figures will be of use to health care providers in encouraging Japanese women to seek counseling and to adopt self-control of body weight as a primary preventive measure, as well as to have breast cancer screening.