Cell Survival

The measurement of the clonogenic survival is a first step, to determinate the influence of a radiation on cells. Photon irradiation leads in most cases to a shouldered dose-effect curve that can be described by the linear-quadratic equation

The shoulder that is characterised be the ratio α/β is a measure for the repair capacity of the cell.

Particle irradiation leads to a reduction in the shoulder with increasing LET up to pure exponential curves (Fig. 1). This is caused by the higher local ionisation density in the ion track. The resulting higher efficiency of the ions is described by the relation D_photon/D_particle leading to the same biological effect and is called Relative Biological Effectivenes (RBE).

The knowledge of this enhanced efficiency of particle irradiation and its dependencies from dose, energy, ion type, oxygen status and repair capacity of the cell is used in radiotherapy as well as in radioprotection.

In a complex irradiation field like an extended tumor volume RBE may change over a broad range. An example is shown in figure 2 where CHO-K1 chinese hamster cells where irradiated in a head phantom and compared to predictions of the TRiP98 treatment planning system.

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