HER2 TREAT Study: Retrospective Study to Estimate the Prevalence of HER2-low in Unresectable and/or Metastatic Breast Cancer Patients Who Progress on Anti-Cancer Therapy Identified as HER2-Negative From Patient Medical Records in The Gulf Cooperation Council

Inclusion Criteria

• •women ≥ 18 years of age at date of diagnosis
• •Patients must have a histological or cytological confirmed diagnosis of Unresectable or/and mBC between 01st January 2018 and 31st December 2022.
• •Patients with at least 12 months of follow-up data (from the index date) in the medical records at the participating site unless patient died.
• •Patients must have been diagnosed as HER2-negative (HER2 IHC 0, 1+, 2+/ISH-), regardless of hormone status.
• •Patients who are diagnosed and/or progressed on at least one prior systemic anticancer therapy (e.g., ET, chemotherapy, CDK4/6i, targeted therapies other than anti- HER2, or immunotherapy) in the metastatic setting.
• •Patients must have historical HER2 fixed tissue IHC stained slides in acceptable quality to allow retesting of HER2 expression only in case of the absence of accurate scoring of IHC/ISH in pathology report.