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Breast cancer is the most common cancer worldwide for females. A frequent complication following breast cancer surgery is surgical site infection (SSI). Complications can result in diminished quality of life, increased morbidity, elevated healthcare costs, delayed initiation of adjuvant therapy, loss of reconstruction, and potentially adverse oncological outcomes. In Paper I, the primary objective was to investigate the purported association between SSI and breast cancer recurrence. In addition, the study aimed to explore a potential link between any postoperative infection and breast cancer recurrence. This population-based, retrospective cohort study found that neither SSI nor other postoperative infections were associated with worse oncologic outcomes. Paper II investigated risk factors for SSI following breast cancer surgery, as well as risk factors for other wound complications. This research was conducted through a prospectively registered cohort study. Multivariable analysis identified BMI 25-30 and BMI >30 as the only significant risk factors for SSI. Additionally, significant risk factors for any wound complication included mastectomy with or without reconstruction, as well as BMI 25-30 and BMI >30. In Paper III, the primary aim was to evaluate whether SSI increases the risk of systemic breast cancer recurrence. Secondary objectives included assessing the impact of SSI on the risk of locoregional recurrence (LRR), breast cancer-specific survival (BCSS), and overall survival (OS). This analysis utilized high-quality data from national population-based registers, checking for confounding variables such as patient and tumour characteristics. In conclusion, SSI following breast cancer surgery does not significantly increase the risk of systemic recurrence, LRR, overall death, or breast cancer-specific death. Paper IV evaluated the risk of systemic breast cancer recurrence following major systemic postoperative infection or other major event. It also assessed the impact of these exposures on LRR, OS, and BCSS. Utilizing the same cohort as in Paper III, the findings indicated that postoperative major systemic infection was associated with an increased risk of systemic recurrence, overall death and breast cancer-specific death, but not with LRR.

In conclusion, this thesis does not support the hypothesis that SSI is associated with poorer oncological outcomes. However, it demonstrates an association between major systemic infections and worse oncological outcomes.