Basket Study for Oligo-metastatic Breast Cancer

Inclusion Criteria: * Histologic proof of infiltrating HER2-positive breast cancer (as determined by IHC 3+ and/or amplification by ISH)\[8\] * Histologic or cytologic proof of breast cancer metastases (at least one lesion) * Histologic determination of level of ER-expression * Oligo-metastatic disease as determined by standard of care diagnostics. The number of total individual distant metastases is limited to five, either in one organ or in 2-5 organ systems. Clustered lymph nodes that can be irradiated with curative intent in a single field are defined as single lesion. Pleuritis carcinomatosa, miliary spread of metastases (even within one organ), or peritoneal spread of metastases rules out oligo-metastatic disease and is not allowed. Initial staging by PET-CT (whole body) and MRI of breast and brain are mandatory, as is MRI liver or spine and pelvis in case of liver or bone metastases respectively. * In case of recurrent disease, a disease-free interval of 24 months. * Measurable disease according to RECIST1.1 * Patients must be at least 18 years of age and be able to give written informed consent and comply with study procedures. * World Health Organization (WHO) performance status 0 or 1 Exclusion Criteria: * prior line of therapy for metastatic disease. Exceptions are endocrine therapy or radiation considered to be part of the curative treatment, within 3 months before enrolment * leptomeningeal disease or central nervous metastases * clinically relevant obstruction or compression of spinal cord, central nervous, gastro-intestinal or cardiovascular system, that cannot be alleviated before start of treatment. * other malignancy, unless treated with curative intention and a long-term survival probability of \>95%, including in-situ or pre-malignant lesions.