Patient selection

We retrieved electronic medical records of 1185 consecutive patients with newly diagnosed MBC referred to the Division of Medical Oncology of National Cancer Centre Singapore (NCCS) from 1^st January 2011 to 31^st December 2017 from the Joint Breast Cancer Registry. The study was reviewed and approved by the SingHealth Centralised Institutional Review Board Reference: 2018/2400.

Patients with histologically-proven breast cancer, radiological evidence of metastatic disease, and HER2 positivity on immunohistochemistry (IHC) or fluorescence in situ hybridization were selected. HER2 positivity was defined as a score of 3+ on IHC or an IHC score of 2+ and a HER2/CEP17 ratio ≥ 2.0 for samples after 1 January 2014 and HER2/CEP17 ratio ≥ 2.2 for samples before 1 January 2014 on fluorescence in situ hybridization testing.

After exclusion of 855 patients with HER2 negative and unknown HER2 status, clinical and treatment data were collected for 329 HER2 positive MBC patients. Patients with unknown Pertuzumab usage were then excluded, and data analysis was conducted for all 304 HER2 positive MBC patients. For further cost analyses, patients with incomplete billing data were excluded.