This study evaluates whether radiotherapy can safely be omitted in breast cancer patients with T1-2N0 tumors who achieve a pathologic complete response after neoadjuvant systemic therapy and breast-conserving surgery

The proportion of women diagnosed with breast cancer who are treated with neoadjuvant systemic therapy is increasing. Depending on the subtype, 10-75% of these patients will have a pathologic complete response. Currently breast conserving surgery with pathologic complete response is followed by radiotherapy. In this patient group risk of local recurrence is low, but radiotherapy may cause considerable morbidity.

The aim of this study is to investigate whether omitting radiotherapy is safe for patients with a node-negative breast tumor <5cm treated with neoadjuvant systemic therapy and breast conserving surgery who achieve a pathologic complete response

Inclusion Criteria:

• Women, aged ≥ 18 years
• Invasive HR positive/Her2 negative, Her2+ (ER/PR +/-) or TN breast cancer
• Concurrent DCIS in pre-NST biopsy is allowed if there is no suspicion of extensive component i.e. absence of non-mass enhancement on pre-NST MRI, contrast-enhanced mammography or breast-specific gamma imaging and/or absence of calcifications on pre-NST mammography
• Primary tumour (T) clinical stage cT1-2
• Unifocal disease; confirmed by pre-NST MRI, contrast-enhanced mammography or breast-specific gamma imaging
• Clinical nodal stage 0; absence of lymph node metastases should be confirmed by ultrasound or FDG-PET/CT
• Neoadjuvant systemic treatment (NST)
• Marker placed in breast tumour prior to NST
• Breast conserving surgery performed, i.e. no mastectomy
• Sentinel node biopsy performed before or after NST
• Pathologic complete response in breast and lymph nodes, i.e. no residual tumour cells or DCIS detected
• Written informed consent

Exclusion Criteria:

• Primary tumour (T) clinical stage cT3-4
• Pre- or post-NST diagnosis of nodal disease including isolated tumour cells
• Patients without axillary ultrasound or FDG-PET/CT pre-NST
• History of breast cancer or DCIS
• Synchronous contralateral breast cancer or DCIS
• Synchronous M1 disease
• Carrier of gene mutation associated with increased risk of breast cancer, i.e. BRCA1, BRCA2, CHEK2, TP53 or PALB-2