Estrogen receptor and endocrine responsiveness in Japanese versus American breast cancer patients.

We have compared the incidence of estrogen receptor (ER) in breast tumors and its clinical correlation with responses to endocrine therapies in Japanese and American patients. There was no correlation between tumor histopathology and the presence of ER, and the ER values in primary and metastatic lesions from the same patients were similar in most Papanese cases. Japanese patients with low and high plasma estradiol levels had identical incidences of ER-positive tumors. The correlation between tumor ER and response to endocrine therapy is similar between Japanese and American patients. The incidence of ER-positive tumors is higher in postmenopausal American patients in both primary and metastatic lesions. It is possible that the reported increase in tumor lymphocyte infiltration in Japanese patients may explain this difference. The reported 5-year survival advantage of Japanese breast cancer patients cannot be explained by differences between the two populations in the response to endocrine therapy for advanced disease.