Comparing multifocality to unifocal breast cancer and the relation to survival: a national cohort study

Abstract Background The prognostic relevance of multifocal and multicentric breast cancer remains unclear and current staging systems do not consider focality. The aim was to explore whether women with multifocal breast cancer have less favourable tumour characteristics and worse survival compared to women with unifocal breast cancer. Methods Patient and tumour characteristics were obtained from Breast Cancer Data Base Sweden 3.0 which include all Swedish women operated for invasive breast cancer between 2008 and 2019. 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 In 71,607 included women, 59,445 (83.2%) had unifocal breast cancer, 7,286 (10.2%) had multifocal breast cancer with 2 invasive foci, and 4,688 (6.6%) had ≥3 invasive foci. Multifocal breast cancer was associated with higher T- and N-categories, compared to unifocal breast cancer. Median follow-up time was 5.96 years (interquartile range 3.07-8.80 years). The breast cancer-specific 10-year survival probabilities were 86.1%, 86.5%, and 88.5% for multifocal breast cancer with ≥3 foci, multifocal breast cancer with 2 foci, and unifocal breast cancer, respectively. In a multivariable analysis adjusted for patient and tumour characteristics, the hazard ratio for breast cancer-specific death was 1.17 (95% confidence interval, 1.03–1.32) in cases of multifocal breast cancer with ≥3 foci compared to those with unifocal breast cancer. There was no statistically significant difference in overall survival between the three groups. Conclusion The current study suggests that multifocality when compared to unifocal breast cancer carries additional prognostic information over traditional tumour characteristics.