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Background: The prognostic relevance of multifocal and multicentric breast cancer remains unclear and current staging systems do not consider focality. The aim of this study was to explore whether women with multifocal breast cancer have less favourable tumour characteristics and worse survival compared with women with unifocal breast cancer.

Methods: Patient and tumour characteristics were obtained from Breast Cancer Database Sweden 3.0, which includes data for all Swedish women diagnosed with invasive breast cancer between 2008 and 2019 and who underwent surgery. Overall and breast cancer-specific survival rates were calculated using the Kaplan–Meier method and multivariable analysis was used to identify independent predictors of survival using the Cox proportional hazard model.

Results: A total of 71 419 women were included in the study: 59 445 (83.2%) had unifocal breast cancer, 7286 (10.2%) had multifocal breast cancer with two invasive foci, and 4688 (6.6%) had multifocal breast cancer with three or more invasive foci. Multifocal breast cancer was associated with higher clinical T and N categories compared with unifocal breast cancer. The median follow-up time was 5.96 (interquartile range 3.078.80) years. The breast cancer-specific 10-year survival rates were 86.1% for women with multifocal breast cancer with three or more foci, 86.5% for women with multifocal breast cancer with two foci, and 88.4% for women with unifocal breast cancer. In a multivariable analysis adjusted for patient and tumour characteristics, the HR for breast cancer-specific death was 1.17 (95% c.i. 1.03 to 1.32) for women with multifocal breast cancer with three or more foci compared with women with unifocal breast cancer. There was no statistically significant difference in overall survival between the three groups.

Conclusion: The present study suggests that focality provides prognostic information that is additional to that provided by traditional tumour characteristics.