Objective: To evaluate the association between primary tumour-brain metastasis receptor status mismatch and outcomes in patients with breast cancer brain metastasis (BCBM). Methods: Patients who (1) had a histologically verified breast-to-brain metastasis, (2) were 18 years old or above on the day of surgical resection, and (3) had the ER (estrogen receptor), PR (progesterone receptor), and HER2 (human epidermal growth factor receptor 2) statuses of both the primary breast tumour and the secondary brain metastasis were retrospectively recruited. Univariate time-to-event analysis was performed using the Kaplan-Meier method. The exposures analysed were the various combinations of ER, PR, and HER2 statuses between the primary breast tumour and the secondary brain metastasis. The outcomes were overall mortality and recurrence. Results: Of the 158 patients who underwent surgical resection of brain metastases during the study period, 31 were included in the analysis. The mean (SD) age of the study population was 56.7 (12.2), and most patients were Chinese (54.8%). On univariate analysis of the association between the various receptor status combinations and overall mortality, ER (p=0.920) and PR (p=0.390) status conversion were both found to not be associated with overall mortality. However, HER2 status conversion was found to be significantly associated with overall mortality (p=0.026). Specifically, patients whose primary tumour was HER2+ but whose secondary brain metastasis was HER2- had the poorest outcome, with a median overall survival of 3.4 months. On the other hand, the median overall survival of the other HER2 receptor status combinations ranged from 10.9 to 16.6 months. There were no statistically significant associations between status conversion of any of the receptors and recurrence. Conclusions: Among patients who underwent surgical resection of BCBMs, patients with primary tumour HER2+ but secondary brain metastasis HER2- had a significantly higher risk of mortality. However, ER and PR status conversion was not significantly associated with outcomes.

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