Biologically effective dose for permanent prostate brachytherapy taking into account postimplant edema

To study the influence of radiobiologic and physical parameters and parameters related to edema on the biologically effective dose (BED) for permanent prostate implants and to determine the optimal timing of seed reconstruction for BED calculation.

On the basis of the linear-quadratic model, an expression for the BED was derived, including the edema parameters. A set of parameter values was defined, and these parameter values were varied one at a time to examine the effect on the BED and the theoretically effective treatment time (t(eff)). A ratio epsilon was defined to investigate the optimal timing of seed reconstruction.

The maximal BED decreases when the extent of lethal damage is smaller, the potential tumor doubling time is smaller, the half-life time of the seeds is shorter, and the magnitude of prostate volume increase is larger. For 125I, the optimal timing of seed reconstruction is 25 days after implantation. Seed reconstruction 1 day after the implantation results in an underestimation of the BED of at most 43%, depending on the magnitude and half-life of edema. An overestimation of the BED of at most 22% is calculated when seed reconstruction took place at the effective treatment time.

The maximal BED depends strongly on the value of alpha, the potential tumor doubling time, and the choice of isotope. If prostate volume increase due to edema is not taken into account, the BED will be underestimated shortly after the implantation and overestimated if the calculations are based on images taken several months after implantation. The optimal timing of BED evaluation for 125I seed implants and typical prostate edema values is 25 days after implantation.