Peripheral blood monocytes could be associated with brain metastasis and affect patient prognosis in breast cancer: a retrospective study

Objective The aim of this study was to test the hypothesis that changes in peripheral blood monocyte-related ratios, particularly the lymphocyte-to-monocyte ratio (LMR), at the time of brain metastases (BM) diagnosis are associated with survival outcomes in patients with breast cancer. Methods This study included 117 patients with breast cancer who developed BM from 2008 to 2023. We analyzed peripheral blood counts at primary diagnosis and at BM diagnosis to examine changes in monocyte count. The impact on survival was assessed using the monocyte-related ratios. Results The absolute monocyte count (AMC) increased 2.11-fold at BM diagnosis compared with initial breast cancer diagnosis (p = 0.038). The high LMR group had a median survival of 20.0 months (95% CI 15.2–28.5) compared with 5.13 months (95% CI 3.9–7.5) in the low group (log-rank p < 0.001). Similarly, the high platelet-to-monocyte ratio (PMR) group had a median survival of 14.17 months (95% CI 7.0–18.7) versus 5.73 months (95% CI 3.6–9.9) in the low group (log-rank p = 0.028). Prognostic factors significantly associated with poorer survival included triple-negative breast cancer subtype, low LMR, and number of BMs, while age and extracranial metastasis exerted minimal impact. Conclusion Changes in AMC and monocyte-related ratios at the time of BM diagnosis are associated with the development of BM and can impact survival outcomes in patients with breast cancer. These findings suggest that monocyte dynamics may serve as exploratory prognostic markers, which require validation in prospective studies.