DESIGN: A pragmatic multi-centre randomised clinical trial to test whether TARGeted Intraoperative radioTherapy as a tumour bed Boost (TARGIT-B) is superior in terms of local relapse within the treated breast compared with standard post-operative external beam radiotherapy boost in women undergoing breast conserving therapy who have a higher risk of local recurrence. Patients can be entered before the primary surgery or in a smaller proportion of cases, post-pathology. SETTING: Specialist breast units in UK, USA, Canada, Australia and Europe; 31 centres currently recruiting in the TARGIT-A trial and several are ready to join. TARGET POPULATION: Breast cancer patients suitable for breast conserving surgery, but with a high risk of local recurrence. Details of inclusion and exclusion are given in part 2. Briefly the patients should be either younger than 45 or if older, need to have certain pathological features that confer a high risk of local recurrence of breast cancer. HEALTH TECHNOLOGIES BEING ASSESSED. The TARGIT Technique: The Intrabeam® (Carl Zeiss, FDA approved and CE marked) is a miniature electron beam-driven source which provides a point source of low energy X-rays (50kV maximum) at the tip of a 3.2mm diameter tube. The radiation source is inserted into the tumour bed immediately after excision of the tumour and switched on for 20-35 minutes to provide intra-operative radiotherapy accurately targeted to the tissues that are at highest risk of local recurrence. The physics, dosimetry and early clinical applications of this soft x-ray device have been well studied. For use in the breast, the technique was first developed and piloted at University College London. The radiation source is surrounded by a spherical applicator, specially designed (and available in various sizes) to produce a uniform field of radiation at its surface, enabling delivery of an accurately calculated dose to a prescribed depth. It is inserted in the tumour bed and apposed to it with surgical sutures and/or other means. As the x-rays rapidly attenuate the dose to more distant tissues is reduced; this also allows it to be used in standard operating theatres. 20 Gy is delivered to the tumour bed surface in 20-35 minutes, after which the radiation is switched off, the applicator removed, and the wound closed in the normal way. This simple technique has potentially several advantages over convential external beam radiotherapy, interstitial implantation of radioactive wires or conformal external beam radiotherapy. The first pilot of twenty-five cases was at performed at UCL using TARGIT technique as a replacement for the boost dose of radiotherapy; full dose external beam treatment was subsequently given. The phase II study of 300 patients was published and recently updated with long term data along with favourable toxicity and cosmetic outcome results of individual cohorts. A mathematical model of TARGIT developed recently (funded by Cancer Research UK) suggests that it could be superior to conventional radiotherapy. Translational research has found that TARGIT impairs the surgical-trauma-stimulated proliferation and invasiveness of breast cancer cells. This effect of radiotherapy may act synergistically with its tumouricidal effect yielding a superior result. MEASUREMENT OF COST AND OUTCOME: Patient assessments will be clinical examination (6 monthly x 3 years then yearly x 10 years) and mammography (yearly). with ulstrasound (if needed) . Primary outcome: histologically/cytologically proven local recurrence. Secondary: site of relapse in the breast, overall survival, local toxicity (RTOG and LENT SOMA criteria), cosmesis, quality of life, patient satisfaction and health economics. The cost and cost-effectiveness of TARGIT versus EBRT, both as boost, will be calculated from a NHS and personal social services (PSS) perspective. Costs directly incurred by patients will also be assesed, since EBRT as a boost is likely to impose additional time and travel expense to patients and families.