Women with hormone receptor-positive (HR-positive) early breast cancer who temporarily interrupted endocrine therapy to pursue pregnancy can safely breastfeed without increased risk of cancer recurrence, according to new findings from the international POSITIVE trial published in the Journal of Clinical Oncology.
The prospective study provides the first comprehensive data on breastfeeding outcomes in breast cancer survivors, addressing a critical knowledge gap for young women facing treatment decisions. At a median follow-up of 41 months, researchers found no significant difference in breast cancer events between women who breastfed and those who did not.
Trial Design and Patient Population
The POSITIVE trial enrolled women aged 42 years or younger with stage I-II HR-positive breast cancer who had received 18 to 30 months of endocrine therapy. The protocol allowed for temporary treatment interruption of up to two years to enable pregnancy, delivery, and breastfeeding, with patients encouraged to complete their planned 5-10 years of endocrine therapy afterward.
Of 497 women evaluable for pregnancy outcomes, 317 participants had at least one live birth. After excluding patients with bilateral mastectomy, 313 women were eligible for breastfeeding analysis, with 196 women (62.6%) choosing to breastfeed a total of 232 babies.
Breastfeeding Patterns and Duration
The study revealed distinct breastfeeding patterns based on surgical history. Among women who underwent breast-conserving surgery, 77.8% chose to breastfeed, with 69% nursing exclusively from the contralateral (unaffected) breast. In contrast, 45.2% of women who had unilateral mastectomy breastfed, while 38 patients (12%) nursed from both breasts.
Breastfeeding rates were higher among women older than 35 years (67.6% versus 55.7% in younger women) and those without previous children (66.4% versus 48.5%). Geographic variations were also observed, with women from Asian/Pacific/Middle East regions showing the highest breastfeeding frequency at 85.7%.
The median breastfeeding duration was 4.4 months, with over half of nursing mothers (52.6%) continuing for more than four months. Notably, 37.1% breastfed for at least six months, 12.8% for one year, and 1.5% for two years.
Safety Outcomes and Recurrence Risk
The primary safety analysis showed reassuring results regarding cancer recurrence. At two years from first live birth, the cumulative incidence of breast cancer events was 3.6% in the breastfeeding group compared to 3.1% in the non-breastfeeding group, representing an absolute difference of just 0.5%.
During follow-up, nine breast cancer events occurred across both groups, including three local recurrences. The 12-month breast cancer-free interval estimates were 1.1% for breastfeeding women versus 1.9% for non-breastfeeding women, with 24-month rates of 3.6% versus 3.1% respectively.
"After a median follow-up of 41 months from enrollment (22 months from first live birth), breastfeeding was not associated with a higher incidence of breast cancer-free interval events," wrote lead author Dr. Fedro A. Peccatori from the European Institute of Oncology in Milan, Italy.
Clinical Implications
These findings address a significant evidence gap in breast cancer survivorship care. Previously, limited prospective data existed to guide counseling for young breast cancer survivors considering breastfeeding after treatment.
The study's co-authors noted that "nearly two-thirds of women who gave birth after breast cancer diagnosis breastfed, many for four months or longer. In early follow-up, no impact on breast cancer events was observed."
For women who underwent breast-conserving surgery, the predominant pattern of breastfeeding from the contralateral breast suggests both feasibility and patient preference for this approach. Among the 40 patients who breastfed from the affected breast, 85% had previously received radiation therapy, indicating that prior radiotherapy did not preclude successful breastfeeding.
Study Limitations and Future Directions
The researchers acknowledged that the trial involved a highly motivated patient and healthcare provider population, which may limit generalizability. Additionally, longer follow-up data will be needed to confirm the long-term safety of this approach.
The investigators concluded that "these results are key for women who wish to pursue pregnancy and breastfeeding after breast cancer," providing evidence-based guidance for reproductive counseling in this population.
As noted by Dr. Anna H. Partridge from Dana-Farber Cancer Institute in previous commentary on the trial, "POSITIVE will open the door for more young women to say, 'I can take a break and try to become pregnant'. At least in the short term, it appears safe."
