Dose fractionation and biological optimization in breast cancer

Standard radiotherapy in breast cancer is performed at the dose of 1.8-2 Gy daily 5 fractions a week for a total dose between 45 and 60 Gy. However research is addressed to different fractionations. For total time reduction, the interest was focused on conventional brachytherapy which radiobiologically represents "continuous" accelerated hyperfractionation, as well as on conventional external beam radiotherapy with accelerated hyperfractionation. A phase I study was conducted to define and validate a radiotherapy schedule with non conventional fractionation. Nine patients with metastatic breast cancer were enrolled in the study. None of them had undergone breast surgery or lymph node dissection. They were sequentially divided into three different, progressively increasing dose levels administered with double daily fractionation. Each schedule of accelerated fractionation (AF) included the administration of 1.8 Gy in two daily fractions, at least six hours apart for 10, 11 and 12 days and a total dose of 36, 39.6 and 43.2 Gy, respectively. Results of dose escalation, acute toxicity and mathematical calculation of radiobiological equivalence led to consider the dose of 36 Gy in 20 fractions during 10 days the most suitable for cost/benefit ratio within a non conventional fractionation.