An observational study using γ-H2AX foci to investigate cardiac doses of radiation following adjuvant radiotherapy for breast cancer: Standard external beam radiotherapy to the breast versus intraoperative radiotherapy.

TPS129 Background: External beam radiotherapy is a gold standard adjuvant treatment after breast conserving surgery for localised breast cancer. A recent phase III trial has shown the non-inferiority of intraoperative radiotherapy compared with external beam radiotherapy (Vaidya J et al. Lancet 2010) with a reduction in radiotherapy toxicity in the intraoperative radiotherapy group (0.5% vs 2.1%; p = 0.002). Intraoperative radiotherapy also has advantages in cost saving and patient convenience, particularly for those living some distance from a cancer centre. There are data to suggest an increase in morbidity and mortality from stroke and heart disease in those exposed to radiation and this includes late effects from breast radiotherapy. Intraoperative radiotherapy uses low energy x-rays (50 KV) and is therefore likely to reduce the radiation exposure of the cardiovascular system compared with external beam radiotherapy. We are using γ-H2AX foci formation in peripheral blood lymphocytes as a surrogate marker of radiation dose to the heart and great vessels. The phosphorylated histone H2AX protein (γ-H2AX) is a very sensitive assay for the induction of double strand breaks caused by ionising radiation, which will be created as the lymphocytes pass through, and adjacent to, the irradiated field. We will study the induced foci number after patient radiotherapy treatment for localised breast cancer, as a surrogate for dose to the heart and great vessels using the different radiotherapy techniques. Methods: 36 patients are being recruited, having either external beam radiotherapy or intraoperative radiotherapy as part of their planned treatment program. We are collecting data including demographics, disease status and radiotherapy delivery variables, and γ-H2AX foci number with 12 patients having intraoperative radiotherapy and 24 having external beam radiotherapy. We will have equal numbers or right and left sided treatments and are comparing foci number between right and left external beam radiotherapy groups as well as intraoperative radiotherapy versus external beam radiotherapy.