The goal of the experiment is surgical de-escalation. Patients with newly diagnosed cT1-3, cN1, cM0 and HR+, HER2- breast cancer and low clinical risk usually first receive breast cancer surgery, after which chemotherapy can be added based on the final anatomopathological results or additional Gene Expression Profiling (GEP) testing.

Chemotherapy helps reduce the cancer recurrence or metastasis risk. Adjuvant chemotherapy (AC) can be given for large tumours, aggressive tumours with high grade tumour cells, or if axillary lymph node invasion is discovered during surgery. If the investigators were able to identify patients requiring chemotherapy prior to surgery, they could treat them with Neo-Adjuvant Chemotherapy (NAC) instead. By using MammaPrint® GEP, the investigators would be able to stratify patients into either a low- or high-risk category. Since the high-risk group is known to benefit from chemotherapy to improve overall survival, they would be treated with NAC, which could also potentially lead to surgical de-escalation. Breast conserving procedures, such as a lumpectomy instead of a mastectomy, could allow more aesthetically pleasing results and increase patient quality of life. Using NAC for patients with axillary lymph node invasion could lead to nodal complete pathological response (cN1 -> ypN0), allowing patients that are willing to enter the TADANAC trial a lymph node sparing procedure instead of a full axillary lymph node dissection.

Inclusion Criteria:

• Tumour characteristics: HR+, HER-
• Histopathological tumour characteristics: cT1-3, cN1, cM0, all histological types included
• Age: 18-85 years

Exclusion Criteria:

• Tumour characteristics: cT4, cN2-3, cM1
• Age: <18 or >85 years old
• Received chemotherapy in the last 5 years
• Pregnancy

Patients of the Breast Clinic