Breast cancer (BC) is a highly common cancer among women of reproductive age.¹ Young survivors of BC face unique challenges, including reduced survival rates, fertility concerns, and psychosocial stressors.¹ While the safety of pregnancy after hormone receptor-positive (HR+) BC has been evaluated primarily in retrospective studies, limited data exist on the feasibility and safety of breastfeeding following BC treatment.¹ Dr. Fedro A. Peccatori, a medical oncologist from the European Institute of Oncology (IEO) in Milan, Italy, presented new findings on this topic from the POSITIVE trial at the ESMO Congress 2024.¹

The POSITIVE trial is a prospective, single-arm study involving 518 women aged 42 or younger with HR-positive, stage I to III BC who expressed a desire to conceive.¹ Participants had undergone 18 to 30 months of adjuvant endocrine therapy (ET) before temporarily halting treatment to attempt conception.¹ Following a 3-month washout period, participants were given up to two years for conception, delivery, and potentially breastfeeding.¹ After this period, participants were expected to resume ET and continue follow-up for up to 10 years.¹

The primary endpoint of the trial was breast cancer-free interval (BCFI), measured from the first live birth to the first invasive local, regional, or distant BC recurrence or contralateral BC.¹ With a median follow-up period of 41 months, 9 BCFI events were recorded, including 3 local recurrences.¹ During the study, 44 participants interrupted treatment due to a BC event.¹ The 3-year BC incidence rate was 8.9% among those who interrupted treatment, similar to the 9.2% rate observed in the external cohort enrolled in the SOFT/TEXT trial with similar clinical characteristics (HR=0.81; 95% CI: 0.57-1.15).¹ Among women who breastfed, the rate of first invasive recurrence within 12 months was 1.1% (95% CI: 0.3%-4.4%), compared to 1.9% (95% CI: 0.5%-7.5%) for those who did not breastfeed.¹ At 24 months, the BCFI rates were 3.6% (95% CI: 1.5%-8.8%) among those who breastfed and 3.1% (95% CI: 1.0%-9.5%) for those who did not.¹

A secondary endpoint of the study examined breastfeeding practices among women who had at least one live birth and had not undergone bilateral mastectomy (n=196).¹ Most of these participants were aged 35 or older (62.7%), were nulliparous prior to enrollment (83.6%), had no nodal involvement (pN0) (67.8%), and had received prior chemotherapy (57.6%).¹ Among those who breastfed, the majority had undergone breast-conserving surgery (66.3%) and breastfed only from the contralateral breast (69.2%).¹ Factors associated with a higher likelihood of breastfeeding included breast-conserving surgery (77.8% vs. 45.2% for unilateral mastectomy), age of 35 or older (67.6% vs. 55.7% for age less than 35), nulliparity prior to trial enrolment (66.4% vs. 48.5% for parity), and residence in the Asia/Pacific/Middle East regions (85.7% vs. Europe [62.7%] vs. North America [51.2%]).¹ The median breastfeeding duration for the first child was 4.4 months (95% CI: 4.0-5.3), with 37.1% of participants breastfeeding for at least 6 months, 12.8% for 12 months or more, and 1.5% for at least 24 months.¹ Per study protocol, participants resumed ET within2 years of stopping.¹

The POSITIVE trial represents the largest prospective evaluation of breastfeeding practices and their impact on BCFI in young women with HR+ early-stage BC.¹ In conclusion, breastfeeding was feasible for most participants who gave birth, and early follow-up results suggest its safety.¹ These findings underscore the importance of individualized support and pre-conception counseling for young BC survivors.¹