Radiobiological principles: their application to γ knife therapy.

Gamma Knife treatments are regarded as single dose exposures, however, in reality the total dose delivered is the addition of a variable number of individual smaller doses from the variable number of iso-centres or shots, selected to cover a lesion. The dose prescription, in terms of dose and dose rate, to different points on a given physical iso-surface, will vary according to location. In radiobiological terms this treatment pattern does not represent a single exposure, but a schedule with a variable number of different sized dose fractions given at different dose rates with multiple incompletes repair intervals (the time between shots). Using the concept of biologically effective dose (BED), incorporating a fast and a slow component of repair, the biological effectiveness of a 12-shot protocol was found to vary with the decay in the activity of the (60)Co sources and the time interval between shots. However, the largest effect was found when this standard protocol was compared with one involving only 2 shots. It should be recognised in individual Gamma Knife treatments that there are many variables which have the potential to influence the biological effective of the treatment and thus the importance of a single variable may be difficult to determine in isolation. Reports in the literature support the results of these simulated calculations into the factors likely to change the biologically effective dose with the use of the Gamma Knife.