Estimation of the Prevalence of HER2 Low and Describe the SoC, Treatment Patterns, and Outcome in Real-world Practice Among Unresectable and/or Metastatic Breast Cancer Patients With HER2 Low Status

This is a worldwide, multicenter, non-interventional, retrospective study. The study will consist of 2 components. The first component involves local lab rescoring of qualified historical HER2 fixed tissue immunohistochemistry (IHC) stained slides (preferably using Ventana 4B5 assay) at sites (post-training) for mBC patients previously identified as HER2-neg, and independent central retesting of HER2 status using Ventana 4B5 assay for any enrolled patients with available archived tissue samples at designated central laboratories. Local lab rescoring and independent central retesting/local lab retesting will be conducted blinded of historical HER2 IHC scores.

The second component involves linking the rescored IHC status to the patient medical record either through registry databases or patient chart review. Such information will be used to describe the patient demographics, histopathological features, clinical presentation, and treatment patterns following mBC diagnosis, and clinical outcomes in real-world settings for all patients with HER2 scores of 0, >0 and < 1+, and 1+2+/ISH- (HER2 low). The clinicopathological and other relevant BC biomarker information will also be examined based on historical biomarker testing results and/or new testing conducted as part of this study.

Inclusion criteria

• Men or women:

• ≥ 18 years of age when consent provided for future sample and clinical data use - applicable for all countries participating in the study except Japan
• ≥ 20 years of age when consent provided for future sample and clinical data use - applicable for Japan only
• Must have a histological or cytological confirmed diagnosis of unresectable or/and mBC between 01 January 2015 and 31 December 2017
• Must have provided written consent allowing for data and samples to be used in the future and this study would be covered by the consent for future use. If the patient is deceased, a waiver may be accepted
• Diagnosed as HER2-neg (HER2 IHC 0, 1+, 2+/ISH-), regardless of hormone status
• Progressed on any systemic anti-cancer therapy (eg, endocrine therapy, chemotherapy, CDK4/6i, targeted therapies other than anti-HER2, or immunotherapy) in the metastatic setting
• Must have historical HER2 fixed tissue IHC stained slides (preferably stained using Ventana 4B5 assay) in acceptable quality for accurate rescoring.

Exclusion criteria

• Have a history of other malignancies, other than basal cell carcinoma of the skin and squamous cell carcinoma of the skin
• Patients with historical HER2 status of IHC 2+/ISH+ or 3+, or HER2 amplified.