DESCARTES: De-ESCAlation of RadioTherapy in Patients With Pathologic Complete rESponse to Neoadjuvant Systemic Therapy

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