Inclusion Criteria:

- Histologically or cytologically confirmed infiltrating breast cancer

- Oligometastatic disease defined as one to three metastatic lesions, with or without
primary tumor, local recurrence, or locoregional lymph node metastases, including the
axillary, parasternal, and ipsilateral periclavicular regions. All lesions must be
amenable to resection or radiotherapy with curative intent. Staging examinations must
have included a PET-scan plus diagnostic CT-scan of the chest and abdomen, and an
isotope bone scan. When the isotope bone scan is doubtful and plain radiographs do
not explain the abnormality, MRI or CT-scan of the affected skeletal region must be
performed.

- The tumor must be HER2-negative (either score 0 or 1 at immunohistochemistry or
negative at in situ hybridization in case of score 2 or 3 at immunohistochemistry).

- The tumor must be ER positive (≥ 10% nuclear staining at IHC) and poorly
differentiated (grade 3).

or ER negative; the rare tumors that are ER-negative and PgR-positive will be eligible, if
this pattern of hormone receptor expression can be verified in the NKI-AVL reference
pathology lab.

- Known BRCA1 or BRCA2 mutation carriers are eligible regardless of the estrogen
receptor status of their tumor.

- Age ≥ 18 years

- World Health Organisation (WHO) performance status 0 or 1

- Adequate bone marrow function (ANC ≥ 1.0 x 109/l, platelets ≥ 100 x 109/l)

- Adequate hepatic function (ALAT, ASAT and bilirubin ≤ 2.5 times upper limit of
normal)

- Adequate renal function (creatinine clearance ≥ 60 ml/min)

- LVEF ≥ 50% measured by echocardiography or MUGA

- Absence of any psychological, familial, sociological, or geographical condition
potentially hampering compliance with the study protocol and follow-up schedule

- Signed written informed consent

- Able to comply with the protocol

Exclusion Criteria:

- No malignancy other than breast cancer, unless treated with curative intent without
the use of chemotherapy or radiation therapy

- No current pregnancy or breastfeeding. Women of childbearing potential must use
adequate contraceptive protection.

- No concurrent anti-cancer treatment or investigational drugs