TOPLINE:

An increased use of adjuvant systemic therapy over three decades was associated with improved survival in patients with synchronous or metachronous bilateral breast cancer (SBBC or MBBC), according to a longitudinal cohort study.

METHODOLOGY:

• Researchers conducted a prospective longitudinal population-based study including 4878 women with early-stage BC treated with breast-conserving therapy between 1984 and 2015.
• Overall, 10.7% of women had invasive BBC: 1.2% with SBBC and 9.5% with MBBC. SBBC was defined as BC diagnosed in both breasts simultaneously or within 3 months of the first tumour diagnosis, and MBBC was defined as contralateral BC diagnosed after 3 months of the first tumour diagnosis.
• The median follow-up duration was 171 months for the entire cohort, 132 months for women with SBBC, 224.5 months for those with MBBC, and 166 months for those with unilateral BC (UBC).
• Outcomes of SBBC and MBBC were compared to those of UBC, with specific focus on the impact of the use of adjuvant systemic therapy over time.

TAKEAWAY:

• The average use of adjuvant systemic therapy increased from 26.8% in 1984-1998 to 49.1% in 1999-2015.
• MBBC was an independent predictor of local failure (adjusted hazard ratio [aHR], 1.7; 95% CI, 1.4-2.2) compared to UBC. SBBC was an independent predictor of worse distant metastasis-free survival (aHR, 2.3; 95% CI, 1.4-3.7) and worse disease-specific survival (aHR, 2.4; 95% CI, 1.5-4.0) compared to UBC.
• The 20-year MBBC-free survival improved significantly during the second period (1999-2015) for those who received adjuvant systemic therapy vs those who did not (aHR, 0.5; 95% CI, 0.4-0.6).
• For SBBC, the 20-year distant metastasis-free survival improved from 41.8% in 1984-1998 to 87.8% in 1999-2015 (aHR, 0.2; 95% CI, 0.07-0.7), and the 20-year disease-specific survival improved from 38.9% to 86.6% (aHR, 0.2; 95% CI, 0.08-0.8).

IN PRACTICE:

"The increased use of AST [adjuvant systemic therapy] over time had a positive effect on incidence of MBBC and on survival for SBBC, MBBC and UBC," the authors wrote.

SOURCE:

This study was led by Jan J. Jobsen, MD, PhD, Department of Epidemiology, Medisch Spectrum Twente, Enschede, Netherlands. It was published online on March 06, 2026, in Clinical Oncology.

LIMITATIONS:

The study had a single-centre and observational design.

DISCLOSURES:

This study did not receive any funding. The authors reported having no conflicts of interest.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.