Netherlands Cancer Institute Histopathology

Formalin-fixed paraffin-embedded (FFPE) tumors were sectioned at 4μm and stained with hematoxylin and eosin for histological evaluation according to standard procedures. For immunohistochemistry, sections were stained with antibodies against ER (rabbit monoclonal SP1, Ventana, Tuscon, Arizona), PR (rabbit monoclonal 1E2, Ventana), and HER2/neu (rabbit monoclonal 4B5, Ventana), using the Ventana Benchmark Ultra Automated stainer according to manufacturers' instructions. Estrogen receptor (ER) and progesterone receptor (PR) positivity was defined when at least 10% of the tumor cells stained positive for ER and PR, respectively.