This retrospective study aims to estimate the prevalence of different HER2 expression levels (HER2+, HER2-low, HER2 Ø) in approximately 200 breast cancer patients at FUSCC through the year of 2015 and 3000 breast cancer patients from about 10 medical centres in China between July 2021 and July 2022

This is a multicenter, retrospective study to estimate the prevalence of different HER2 expression levels (HER2+, HER2-low, HER2 Ø) in approximately 200 breast cancer patients at FUSCC through the year of 2015 and 3000 breast cancer patients from about 10 medical centres in China between July 2021 and July 2022.

This study will be divided into two parts： The first part will include 200 subjects who were pathologically diagnosed with breast cancer at FUSCC between Jan 2015 and Dec 2015 (PART 1).This part will collect rescored HER2 expression level results based on the archived HER2 IHC slides and re-staining and re-scoring HER-2 IHC while relative demographic, treatment, and clinicopathologic data will be extracted from the FUSCC Breast Cancer Single Disease Database.

The second part will include 3000 patients from all sites in China who were pathologically diagnosed with breast cancer between July 2021 and July 2022 (PART 2). This part will collect re-assessed HER2 expression level results based on the archived HER2 IHC slides. The general information of patients, diagnosis, clinicopathological information were collected from medical records, Hospital Information System (HIS) and Laboratory Information Management System (LIS).

In PART 2, among 3000 patients, 270 from all participating sites except FUSCC should have enough tumor tissue for re-staining and re-scoring HER-2 IHC at FUSCC to analyse the concordance between leading site and other centers.

In PART 2, among 3000 patients, 800 HER-2 IHC samples will be selected for the validation through AI-assisted HER2 assessment system

Inclusion Criteria:

Patients fulfilling all of the following criteria will be eligible for this study:

• Male and female patients must have a histological confirmed diagnosis of BC
• The patients must be ≥18 years old at the diagnosis.
• In PART 1, all patients should be diagnosed at FUSCC between Jan 2015 and Dec 2015.
• In PART 2, all patients should be diagnosed at all participating sites between July 2021 and July 2022.
• Provision of at least 1 archived HER2 IHC slides associated with confirmed diagnosis of BC, which are in good condition for rescoring.
• FISH result is available for HER2 IHC2+ in primary scoring.

Exclusion Criteria:

Patients who meet any of the following criteria will be disqualified from entering the study:

• Absence of all demographic, histopathologic, clinicopathologic information .
• Have a history of other malignancies, other than basal cell carcinoma of the skin and squamous cell carcinoma of the skin.

Approximately 3000 eligible breast cancer patients from about 10 medical sites in China from July 2021 to July 2022 and 200 patients admitted at FUSCC from January 1st 2015 to December 31st 2015. Eligible patients will be those who had qualified archived HER2 IHC slides.