The impact of seed localization uncertainties on predicting I/sup 125/ prostate brachytherapy dose distributions

The purpose of this work is to quantify the dosimetric effect of two types of uncertainty in permanent prostate brachytherapy implants: failure to (1) image or (2) precisely localize all the seeds implanted. Both of these aspects are due to limitations of current imaging technology. An idealized pre-plan as well as clinical post-implant data for I-125 were used in this analysis. Monte Carlo simulations were done to evaluate the impact of missing seeds and of seed localization uncertainty on predicting the dose distributions. The indices chosen to compare dose distributions were the dose received by 90% of the target volume (D90), and the tumour control probability (TCP). Initial results show that for both types of uncertainties considered in this work, the mean values for TCP and D90 decreased as the magnitude of the uncertainty (number of seeds missing or magnitude of displacement) increased. For both sources of uncertainty, the effects were larger for the pre-plan (with a fixed lattice of seeds) than for the clinical cases (in which seeds were less orderly). Although the trends seen in mean values for TCP and D90 allow generalizations on the expected effect of seed localization uncertainties for a series of patients, the variability in those mean values demonstrate that for a single treatment in an individual patient it is very difficult to predict the values of TCP and D90 in the presence of the imaging uncertainties considered here.