Introduction: The incidence of breast cancer in the unaffected breast of women with previous breast malignancy remains constant after the first diagnosis. We investigated whether there is a similar pattern in the breast cancer incidence in first-degree relatives of breast cancer patients. We studied the risk for breast cancer in mothers at ages older than their daughter's age at diagnosis.
Methods: We analyzed a Swedish population-based cohort with complete family links and calculated incidence rates of breast cancer in mothers of 48,259 daughters diagnosed with breast cancer.
Results: The risk for breast cancer in mothers of breast cancer patients is elevated relative to the background population at all ages. Mothers have an overall incidence of 0.34%/year at ages older than a daughter's age at diagnosis. This rate is not affected to any large extent by the daughter's age at diagnosis. A constant incidence rate of 0.40%/year from age 35 years onward is seen in mothers of breast cancer patients diagnosed before 35 years of age. For mothers of daughters diagnosed at age 35 to 44 years the incidence pattern is less clear, with the rate being stable for approximately 20 years after the daughter's age at diagnosis and rising thereafter. Older age at a daughter's diagnosis (> or = 45 years) appears to confer an age-dependent increase in incidence in the mother.
Conclusions: Incidence of familial breast cancer in first-degree relatives may increase to a high and constant level by a predetermined age that is specific to each family. This phenomenon appears inconsistent with accepted theories of malignant transformation.