Results of a dummy run of postimplant dosimetry between multi-institutional centers in prostate brachytherapy with 125I seeds

The purpose of this study was to determine the reproducibility and precision of postimplant dosimetry following 125I prostate brachytherapy (PB) and to evaluate the effects of learning and experience in CT-based postimplant dosimetry. One-month postimplant CT data from two patients who underwent PB alone or combined therapy (PB+external beam radiation therapy (EBRT)) were sent to 28 institutions for postimplant dosimetry and analyzed in 2006 (study 1). Similarly, 1-month postimplant CT data from two other patients were also analyzed in 2008 (study 2; 23 institutions). For both modalities in studies 1 and 2, the variance of the difference between CT-based D90 at each institution and CT/MRI fusion-based D90 was estimated. In monotherapy, F test and Mann-Whitney U test revealed no significant difference in the variance in studies 1 and 2 (P=0.72, 0.46). In combined therapy, the variance significantly converged in study 2 compared with study 1 (P<0.05). Even in the two studies, however, the difference between the median CT-based D90 and fusion-based D90 was at least 20-30 Gy. Marked interobserver variability was seen in the prostate volume and D90 with CT alone. The precision of postimplant dosimetry based on CT alone was revealed to be limited. (author)