Incidence, treatment, and survival of synchronous peritoneal metastases in breast cancer

This nationwide population-based study assessed the incidence, treatment, and survival of patients with synchronous peritoneal metastases (PM) from breast cancer. All patients diagnosed with synchronous metastatic breast cancer between 2009 and 2020 were included from the Netherlands Cancer Registry (NCR). Treatments were categorized into tumor-directed treatment (surgery, systemic chemotherapy, or endocrine therapy), or best supportive care (BSC). Overall survival (OS) was analyzed using multivariable Cox regression. Among 9,353 patients with synchronous metastatic breast cancer, 358 (4%) presented with synchronous PM. In patients with PM, 257 (72%) patients received tumor-directed treatment, predominantly endocrine therapy ( n = 146, 57%), chemotherapy ( n = 61, 24%), or both ( n = 35, 14%), while 101 (28%) patients received BSC. Median OS was higher in patients without peritoneal involvement, 31.1 months, compared to 14.1 months in patients with synchronous PM. The median OS of patients who received BSC was 1.3 months versus 23.6 months for patients who received tumor-directed treatment. Age above 70 years (hazard ratio 1.62 [95% confidence interval 1.18–2.21]) was associated with worse prognosis. Synchronous PM was present in 4% of all metastatic breast cancer patients. Although PM negatively impacted OS compared to metastatic breast cancer patients without PM, the majority (> 70%) received tumor-directed treatment, significantly prolonging OS.