Clinical equipoise exists around the optimal time to start adjuvant endocrine therapy in
      patients who will receive post-operative radiotherapy for breast cancer. Patients receive
      either concurrent or sequential endocrine and radiation therapy, where concurrent therapy
      consists of endocrine therapy started before, with or during radiotherapy, while sequential
      treatment is defined as endocrine therapy starting after the completion of radiotherapy. A
      recent survey of Canadian oncologists showed that the main reason for prescribing sequential
      endocrine therapy was a concern that concurrent endocrine therapy and radiotherapy would
      worsen the toxicity of endocrine treatment. This is despite the absence of any clinical trial
      evidence to support this. Indeed, a recent systematic review by our group was unable to
      confirm or refute whether increased toxicities, related to the timing of endocrine therapy
      and radiotherapy actually exist in clinical practice. The investigators are therefore
      proposing a pragmatic randomised trial to assess whether or not concurrent endocrine therapy
      and radiotherapy worsens endocrine treatment-related symptoms in an era of modern endocrine
      and radiation therapy.