Radiobiological Principles of Fractionated Radiotherapy and the Potential for Hypofractionation
Colin G. Orton, Wayne State University, Detroit, MITU-B-137-1 Tuesday 9:00AM - 9:55AM Room: 137
Firstly, we will review why we fractionate at all, looking first at physical reasons then at the radiobiological aspects. Mechanisms by which radiation kills cells and ways cell damage can be repaired will be reviewed. Using the linear-quadratic model we will show the effect of radiobiological parameters which we can control for radiotherapy including dose/fraction, number of fractions, time between fractions, overall treatment time, and dose rate. We will show that there is a “Window of Opportunity” at low doses/fraction within which we can control cancers without exceeding normal tissue tolerance. We will then show how the use of highly conformal radiotherapy widens this “Window of Opportunity” so that we can use higher doses/fraction and hence hypofractionation. Finally, we will review some potential caveats that might reduce the effectiveness of hypofractionation and, possibly, make it inappropriate for some cancers. These are the presence of significant fractions of hypoxic cancer cells, the possibility that low-dose hyper-radiosensitivity in normal surrounding tissue cells might reduce the effectiveness of conformal therapies, and the lengthening of treatment times when using high doses/fraction thus allowing intra-fraction repair of cancer cells.
1. To understand why we fractionate and how the linear-quadratic model can be used to demonstrate the effect of radiobiological parameters which we can control for radiotherapy.
2. To understand why hypofractionation might be possible with highly conformal radiotherapy.
3. To understand the circumstances when hypofractionation might not be appropriate.