A biological marker, strongly associated with early oral contraceptive use, for the selection of a high risk group for premenopausal breast cancer.

In a population-based group of women, consecutively diagnosed, with premenopausal breast cancer there was a significant correlation between tumour size and plasma prolactin (r = 0.30; P less than 0.004). The concentration of estrogen receptor was negatively correlated to tumour size (r = 0.17; P less than 0.09). There were no substantial correlations between tumour size and progesterone receptor, plasma progesterone or estradiol. Adjustments for menstrual cycle day and age did not alter the above findings. The ratio of plasma prolactin and estrogen receptor was significantly greater (P less than 0.037) for the group of the patients that had started using oral contraceptives before the age of 20 as compared with the other patients. Consequently, the tumour size was significantly greater in the group of early users (P less than 0.003). The findings indicate that breast tumours developing in previous early users of oral contraceptives have a low estrogen receptor concentration, while these patients have higher plasma prolactin. The tumour size is greater in early users indicating a poorer prognosis than other women with breast cancer. As early use of oral contraceptives increases breast cancer risk and a high ratio of plasma prolactin and estrogen receptor concentration of the primary tumour characterize early oral contraceptive users the ratio may be a valuable marker for the breast cancer risk.