MR-guided RAdiotherapy to Breast With Dose Escalation to regionaL Lymph NodEs

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.