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Background: Breast cancer subtypes are often used as therapeutic and prognostic measures; however, it is unclear whether there is an association between molecular subtypes and site-specific metastasis. Our study aimed to evaluate the relationship between molecular subtypes and developing metastasis in specific sites.

Methods: We selected 118 breast cancer patients with immunohistochemistry confirmed molecular subtype diagnosed in 2020 and 2021 at the Department of Surgery, University College Hospital, Ibadan. We classified the molecular subtypes into four categories, HR+/HER2-, HR-/HER2+, HR+/HER2+, and triple negative (HR-/HER2-). The different sites of metastasis of interest were lungs, liver, brain, and bone. We used the chi-square test to determine the proportions and significance of the subtypes based on the different sites assessed.

Results: According to our study, 45.50%, 18.20%, and 36.40% of patients presented with lungs, liver, and other (multiple organs and contralateral breast) metastasis respectively. Additionally, HR+/HER2- and TNBC patients developed metastasis at a higher rate and account for a combined 90.10% of all metastases (the site-specific distribution was even between both subtypes).

Conclusion: Overall, while there are limitations in our study based on sample size, our data shows that some molecular subtypes are associated with a higher risk of metastasis. Additionally, while not significant in our study, breast cancer subtypes are associated with different metastatic sites.