2.1. Population and Data Collection
cancer, death, EOD, AJCC
CANCER, METASTASIS, BREAST CANCER
Utilizing the SEER*Stat 8.4.1.2 software, we conducted a comprehensive search within the SEER database (The information gathered from the SEER database include age at diagnosis, race (white, black, or others), marital status at diagnosis (married or unmarried), laterality (left or right), histologic type (ICD-O-3 8010/3, 8013/3, 8140/3, 8500/3, 8507/3, 8520/3, 8522/3, 8523/3, 8560/3, 8575/3, 8211/3, 8246/3, 8401/3, 8480/3, 8541/3), grade (well, moderate, poor, or undifferentiated), breast subtype, derived AJCC stage 7th edition (2010–2015), derived SEER combined stage group (2016–2017), derived EOD 2018 stage group (2018), surgery of primary site (code with 30, 43–49, 53–59, 63–69, 73–75), radiotherapy and chemotherapy recode, metastasis at distant site, SEER cause-specific death classification (alive, dead of other cause, or death attributable to this cancer), survival time (month), and vital status (alive or dead). X-tile software (version 3.6.1; Yale, New Haven, CT, USA) was employed to determine the optimal age cutoff value. Based on the calculated optimal cutoff value, patients were categorized into two age groups: ≤50 years and >50 years. The primary endpoint of this study was breast cancer-specific survival (BCSS), which was defined as the duration from the diagnosis of dnMBC to the time of death specifically attributable to breast cancer.