Differences in distant metastasis risk of breast cancer originating from various primary sites: a retrospective analysis based on the SEER database

Breast cancer's primary site diversity may influence distant metastasis, yet research is scant. This study explores primary site impacts on metastasis and patient survival. We included 240,943 female breast cancer patients in the Surveillance, Epidemiology, and End Results Program (SEER) data between 2010 and 2019. The primary sites were classified as upper-inner quadrant (UIQ, C50.2), lower-inner quadrant (LIQ, C50.3), upper-outer quadrant (UOQ, C50.4), lower-outer quadrant (LOQ, C50.5), axillary tail (C50.6), overlapping region (C50.8), and the central and nipple portion (TCNP, C50.0-C50.1). The distant metastatic organs include bone, brain, liver, and lung. Chi-square tests were used to compare patient characteristics, logistic regression for identifying predictive factors. Cox regression was utilized to compare the survival among patients with different primary sites and different metastatic sites. Breast cancer's primary site affects distant metastasis patterns and survival. Compared to the UOQ, tumors located in the axillary tail are associated with a higher risk of bone metastasis (P = 0.003) and lung metastasis (P = 0.006); tumors in the UIQ are linked to a lower risk of bone metastasis (P = 0.028), while TCNP presents a higher risk of both bone metastasis (P < 0.001) and liver metastasis (P = 0.001). Moreover, metastatic patterns affect patient survival, with brain metastasis generally indicating a poorer prognosis compared to bone metastasis (P < 0.001). When metastasis occurs in the same organ, the impact of primary site on survival prognosis becomes less significant (P > 0.05). Differences in the primary site of breast cancer shows variations in the patterns of metastasis to distant organs, which may assist in focusing attention and implementing preventive measures on organs more susceptible to distant metastasis at the early stage of breast cancer diagnosis.