Tamoxifen interruption for pregnancy feasible in young breast cancer survivors

A real-world Asian study presented at ESMO Asia 2025 supports temporary tamoxifen interruption for young breast cancer survivors who are considering pregnancy.

Using data from the Korean National Health Insurance Service – National Health Information Database, researchers retrospectively evaluated the effect of tamoxifen interruption for pregnancy and resumption after childbirth on recurrence and mortality in breast cancer patients. The analysis included 984 women aged 19–45 years who were diagnosed with invasive breast cancer and underwent curative surgery from 2009 to 2014.

The participants were categorized based on tamoxifen interruption and pregnancy status: interruption and resumption (n=135; group 1), interruption without resumption (n=316; group 2), tamoxifen initiation only after childbirth (n=41; group 3), and no interruption or pregnancy (n=492; control group). [ESMO Asia 2025, abstract 2MO]

Group 3 had the highest proportion of women aged 40–45 years (24.4 percent) and the highest chemotherapy rate (75.6 percent). Group 2 had the highest proportion of women aged 19–34 years (78.2 percent) and those who received radiation therapy (84.2 percent). Approximately 83 percent of the overall population had not received anti-HER2 therapy.

A majority of participants in groups 1 and 2 had ≥2 years of tamoxifen use (65.2 percent and 76.9 percent, respectively), noted study investigator Dr Min Sung Chung from the Hanyang University College of Medicine, Seoul, Republic of Korea, who presented the results at ESMO Asia 2025.

After adjusting for multiple* covariates, group 1 had a significantly lower risk of recurrence compared with the control group after a median follow-up of 11.3 years (hazard ratio [HR], 0.39; p=0.011). A more pronounced effect was observed in group 2 vs the control group (HR, 0.11; p<0.001).

“The HRs would suggest that this strategy of interrupting tamoxifen did not appear to have any adverse effect on prognosis and, in fact, it almost appears like it might have been protective,” noted discussant Dr Prudence Francis from the Peter MacCallum Cancer Centre, Melbourne, Australia, at ESMO Asia 2025.

Group 1 also demonstrated better overall survival than the control group (HR, 0.37; p=0.007), more so in group 2 (HR, 0.12; p<0.001).

Regarding pregnancy outcomes, compared with group 3, groups 1 and 2 had markedly higher incidences of full-term pregnancies (76.3 percent and 74.4 percent vs 39 percent) and lower abortion rates (23 percent and 24.7 percent vs 56.1 percent).

Child-bearing important in this cohort

“Young breast cancer survivors often wish to become pregnant after treatment,” said Chung.

“Child-bearing is a very important issue for young women with breast cancer,” echoed Francis. “Moreover, it is particularly relevant in Asia, given the higher proportion of premenopausal breast cancer diagnosis.”

“However, for hormone receptor-positive (HR+) breast cancer, endocrine therapy (ET) typically lasts 5–10 years, creating concerns about whether interrupting tamoxifen for pregnancy may increase recurrence or mortality,” Chung highlighted.

Along with the results from the landmark POSITIVE** trial, the current findings show that ET interruption for conception does not appear to significantly increase recurrence or mortality risks, Chung noted.

In POSITIVE, there was no clear worsening of breast cancer outcomes after at least 18 months of temporary interruption of tamoxifen to allow for pregnancy in select women with a history of HR+, stage I–III breast cancer. [N Engl J Med 2023;388:1645-1656]

“For HR+ breast cancer, interrupting prior tamoxifen therapy for pregnancy shows better survival than starting tamoxifen after childbirth,” Chung said.

“This long-term, real-world evidence provides reassurance for counselling young breast cancer survivors who wish to pursue pregnancy,” he concluded. Chung called for further investigation to explore the long-term impact of tamoxifen interruption on breast cancer prognosis.