Radiotherapy to the breast and lymph nodes is routinely planned using CT scans. The introduction of magnetic resonance imaging (MRI) has the potential to better visualise the lymph nodes and so define a smaller treatment area or 'target'. This means that the dose delivered to the target could be increased without increasing the dose to normal tissues. The MIRABELLE study is designed to test if this is possible by recruiting patients diagnosed with breast cancer including lymph node disease. The investigators will ask participants to have a CT scan and an MRI scan before they have radiotherapy. The investigators will then plan radiotherapy using both these scans and compare the possible dose delivered to the lymph nodes using the MRI and CT defined lymph nodes. This will not affect the patient's future treatment.

MR imaging has the potential to improve both the localisation of pan-LN treatments and the subsequent treatment delivery using MR-linac technology. The MIRABELLE study will apply the MR sequences developed in LN-negative volunteers/patients to the context of node-positive breast cancer patients with the aim of reducing the delineated nodal volume thereby facilitating dose-escalation to involved LNs. The benefit of using MRI for LN localisation as compared to conventional CT imaging will be investigated by determining whether a higher dose can be delivered using MRI- as compared to CT-based delineation. Any increase in dose will be deemed a success as this has potential to improve tumour control. In addition, inter-observer errors in outlining will be explored to investigate whether these are reduced using MRI. Reduced inter-observer errors would result in smaller margins for error such that less normal tissue could be irradiated.

Inclusion Criteria:

• Age ≥18 years
• Female or male
• Invasive carcinoma of the breast (left or right-sided)
• cT1-T4,N3,M0-1 disease
• Due to proceed to primary treatment (chemo, surgery, radiotherapy and/or endocrine therapy)
• Histopathological involvement of axillary lymph nodes confirmed on FNA or Bx
• Likely to undergo locoregional radiotherapy as part of their breast cancer management

Exclusion Criteria:

• Implanted pacemakers and/or pacing wires
• Cochlear implants
• Programmable hydrocephalus shunts
• Implanted neurostimulation systems
• Implanted drug infusion pumps
• Ferromagnetic implants
• Claustrophobia