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

HER2-low Breast cancer (BC) has emerged as a new subtype of BC with distinct clinical, pathological, and prognostic features. Little is known about the prevalence of the HER2-low subtype in HER2-negative patients, and previous reports showed variations in the criteria used to define the HER2-low subtype. Besides, data on the clinical features and prognosis of HER2-low patients are limited, and it is still unclear whether HER2-low BC has a prognostic value. Identifying the prevalence and clinical features of HER2-low BC can help establish a more accurate and reproducible definition of HER2-low BC. In the Gulf Cooperation Council (GCC) region, BC is the most common malignancy in women and still poses a significant burden on healthcare resource utilization, moreover, there is only one record for reimbursed HER2 IHC status, categorized as HER2-positive and HER2-negative. It is important to understand the prevalence, clinical features, and outcomes of HER2-low in BC patients from the GCC In this retrospective, non-interventional, multicenter study, the aim to describe the prevalence of HER2-low BC among the current HER2-negative BC population using rescored HER2 IHC samples. The local treatment patterns and the outcomes will be analyzed using the information abstracted from the corresponding medical chart review. The study will cover the GCC region countries (United Arab Emirates \[UAE\], Saudi Arabia, Qatar, Kuwait, and Oman)

Sex

Female

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.

Exclusion Criteria:

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