Maywood, IL—Far fewer endometrial abnormalities than expected were found in breast cancer patients using tamoxifen as breast cancer treatment, according to a new multi-center study.

A presentation at the 2016 annual meeting of the American Society of Clinical Oncology in Chicago should help reassure patients who worry the breast cancer treatment could increase their risk of uterine cancer, suggested researchers from Loyola University Chicago Stritch School of Medicine.

For the research, the study team enrolled 296 eligible postmenopausal breast patients, average age 59.5, with estrogen receptor-positive breast cancer, randomizing them to take tamoxifen alone or tamoxifen plus the hormone progestin. Participants were followed up at Years 2 and 5, as researchers sought to determine if progestin would decrease the risk of abnormalities that can develop in the endometrium.

At Year 2, five endometrial abnormalities were found in the tamoxifen-alone group and one abnormality in the tamoxifen-plus-progestin group. All abnormalities were benign, and the difference had no statistical significance, according to the report. Only one additional benign abnormality was found at 5 years, and no cancers were detected at 2 years or 5 years, study authors point out.

At the same time, only 6% of patients in the tamoxifen-alone group, and 3.6% in both groups combined had benign endometrial abnormalities after 2 years. Based on previous studies, researchers had projected that 30% of the women in the tamoxifen-alone group would show endometrial abnormalities.

First author Kathy S. Albain, MD, said screening requirements might have been responsible for the lower abnormality rates; all participants underwent uterine ultrasound exams before taking tamoxifen. If the ultrasounds showed a thickening of the inner lining of the uterus greater than 5 mm, a biopsy was performed, and, if the biopsy showed an endometrial abnormality, the woman was excluded from the study.

“For women who are concerned about taking tamoxifen to treat or prevent breast cancer, our study suggests that a normal ultrasound before treatment may provide additional reassurance,” Albain explained.

The study explains that tamoxifen is a highly effective drug that is used both to treat breast cancer and help prevent breast cancer in healthy women who are at high risk of developing the disease.

“However, many women who would benefit from taking tamoxifen fail to do so because they fear getting endometrial cancer,” co-principal investigator is Ronald K. Potkul, MD, added in a Loyola press release. “Our study found that for women who did not have endometrial abnormalities when they began taking tamoxifen, there was a very low rate of developing pre-malignant conditions.”

Study authors conclude, “The event rate in both arms was much lower than projected, making treatment arm comparisons less informative. A normal endometrium prior to tamoxifen may provide reassurance regarding future endometrial events. However, validation in a larger trial is needed before changing practice in asymptomatic postmenopausal women.”

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