Radiobiological aspects of Intra operative radiotherapy

Radiation is delivered to the tumor bed during intraoperative radiotherapy (IORT) via direct surgery vision. IORT uses the advantages of irradiating the target with a quite high single dose, minimizing the dose at the surrounding organs, and removing them from the treatment field through geometrical skill. The possibility to deliver adjuvant radiotherapy in a single fraction, in comparison to the 6–7 weeks requested for a conventional adjuvant treatment, would imply a remarkable improvement in the patient quality of life and in the radiotherapy department work load. Since IORT is delivered in one session, it is important to consider its radiobiological aspects in comparison to the conventional radiotherapy regimes. Recent clinical experiences have shown that IORT may improve local control and disease-free survival, especially when used in adjuvant setting, combined with external beam irradiation in the stomach, pancreas, colon-rectum cancer and soft tissue sarcoma. Several radiobiological models have been used over the years to understand and quantify the radiation tolerances of tumor and normal tissues. Linear-quadratic (LQ) surviving predicts that survival of clonogenic units follows the shape of a parabola. Using LQ model, the shape of the survival curve is determined by the a/b ratio which is nominated as a tissue radiosensitivity. In this presentation we tried to explore some radiobiological features of single fraction irradiation including normal tissues reaction, vascular changes, dose rate effect and tumor response to irradiation. The present model may contribute to an understanding of the normal tissue reaction observed studies on IORT with in breast cancer patients treated with breast-conserving surgery.