[A case-control study of risk factors of breast cancer detected by mass screening].

A case-control study has been performed to elucidate the risk factors of women with a breast cancer that was detected by mass screening. Studied were the cases of fifty-two women with a primary breast cancer that had been detected by mass screening from 1978 to 1989. Age-matched mass screening controls and hospital controls were randomly selected at the rate of two controls for each case. Risk factors investigated were the age at menarche, the age at marriage, the number of pregnancies, the number of full-term pregnancies, the age at first delivery, the status of lactation, the age at menopause, the family history of breast cancer, benign breast disease, exogenous hormonal agents, and obesity. In comparing the mass screening subjects with the controls, a significant odds ratio (OR) was shown in no lactation (OR = 2.67, p = 0.02) and the history of a previous benign disease (OR = 2.56, p = 0.03). No lactation (OR = 2.29, p = 0.02) was significant when compared with the hospital controls. Furthermore, early menopause and an early age at first delivery seemed to act as a protective factor against breast cancer.