RESUMEN
The aim of study was to evaluate the frequency of nonproliferative epithelial alteration and expression of Ki67 and estrogen receptors (ER) in patients using tamoxifen. Forty-four women were selected who had been taking 20mg of tamoxifen daily for at least 12 months for adjuvant treatment of breast cancer. The women underwent core biopsy in the contralateral breast into an area of highest fibroglandular mammographic density. Fragments were analyzed by immunohistochemistry for monoclonal antibody Ki67 and ER, and histopathologic analysis. It was verified that 82% of the patients presented nonproliferative epithelial alteration, 70% were ER-negative, and all had low Ki67 expression. There was no association between duration of tamoxifen therapy, patient age, mammographic density, and presence of nonproliferative alteration (P>0.05). In conclusion, tamoxifen for more than a year showed a high frequency of nonproliferative epithelial alteration and low expression of Ki67 and ER in the normal breast tissue, consistent with low cell proliferation.