*2.3. Histopathology*

According to standard of care, the biopsy of the primary tumor was used to evaluate the histological subtype, grade (according to the Bloom–Richardson grading system), ER, PR, HER2 expression and mitotic activity. Compliant with Dutch guidelines ER-/PRpositivity on immunohistochemistry (IHC) was established if ≥ 10% of cell nuclei were immunoreactive, and HER2 was classified positive with 3+ or 2+ and amplified [3]. Mitotic activity was defined as the number of mitoses per 2 mm2. Suspect locoregional or distant lesions visible on conventional imaging and/or FDG PET that were decisive for therapy choices were verified by core needle biopsy and/or fine-needle aspiration cytology.

The pathological reports of lymph node resection were classified as follows: in the case of the presence of malignant cells, pathologically verified malignant lymph node; in the case of fibrosis compatible with complete response after neo-adjuvant therapy, pathologically verified malignant lymph node before neo-adjuvant treatment; and in the case of no malignant cells or fibrosis by cytology and/or histology, benign lymph node.
