Planned study population consists of approximately 240 adult patients with HER2-positive mBC receiving anti-HER2 therapy in 12 oncological centers (in each center it is expected to recruit about 20 patients) in different regions in order to provide representative study sample. Patients will be included consecutively from the least recent diagnosis (within defined time period).
Planned retrospective follow-up period for 1 patient is a period starting from the date of mBC diagnosis until end study or until patient's death, whichever occurs first. End of study will be at least 12 months after the latest date of mBC diagnosis to ensure all patients have the opportunity to contribute at least 12 months of data.