Inclusion Criteria:

          -  Histologically confirmed breast adenocarcinoma

          -  Postmenopausal women

          -  Core biopsy confirmation of invasive carcinoma on core biopsy, ≥T1c, either clinical
             NX or N0-N3

          -  ER positive (Allred≥3) and HER2 negative

          -  2 groups of patients are potentially eligible:

               -  Cohort A: Patients whose cancers have been deemed to be operable by the
                  Multi-Disciplinary Team (MDT), with surgery planned for the next 2-6 weeks

               -  Cohort B: Patients with early or locoregionally advanced breast cancer planned
                  for primary endocrine therapy, either in lieu of surgery or as neoadjuvant
                  therapy prior to surgery- such patients must begin PIONEER trial therapy prior to
                  starting any other endocrine therapy.

          -  ECOG performance status of 0, 1 or 2

          -  Adequate Liver, Renal and Bone marrow function, defined as:

               -  Adequate liver function where bilirubin is ≤1.5 x ULN

               -  Adequate renal function with serum creatinine ≤ 1.5 x ULN

               -  Adequate bone marrow function with ANC ≥1.0 x 10*9/L and Platelet count ≥100 x
                  10*9/L

          -  Written informed consent to participate in the trial and to donation of tissue

        Exclusion Criteria:

          -  History of hormone replacement therapy in the last 6 months

          -  Previous treatment with Tamoxifen or an aromatase inhibitor in the last six months

          -  Known hypersensitivity or contraindications to aromatase inhibitors or Megestrol
             acetate

          -  Known allergy to lactose

          -  Known to have a progestogen-containing intrauterine system in situ, unless removed
             prior to randomisation

          -  Known metastatic disease on presentation

          -  Recurrent breast cancer (patients with a new primary invasive breast cancer will be
             eligible to participate)

          -  Serious concomitant disorders that would compromise the safety of the patient or
             compromise the patient's ability to complete the trial, at the discretion of the
             investigator

          -  Treatment with an investigational drug within 4 weeks before randomisation

          -  Inability to swallow orally administered medication and patients with gastrointestinal
             disorders likely to interfere with absorption of the trial medication

          -  Inability to give informed consent