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Purpose. – To study the number of migrating seeds, the anatomical site of migration and possible predictive parameters of migration, after prostate cancer brachytherapy using a loose-seed (I125) implantation technique.Patients and methods. – The charts of the 170 patients consecutively treated by the Institut Curie/Hospital Cochin/Hospital Necker Group between September 1, 2001 and August 31, 2002, were analysed. All seeds having migrated to the lungs and seen on the chest X-ray systematically performed at 2 months, have been recorded, as well as the seeds lost by the urines (after sieving) or in the sperm (condom).Results. – Among 12,179 implanted seeds, 44 were found to have migrated (0.36%). Most of the migrating seeds (32/44; 73%), were found in the lungs. Overall, one or several seed migrations were observed in 35 patients (21% of the total number of patients in this series). In the majority of cases (77%), only one seed migrated. A significant relationship (P = 0.04) was found between the number of migrating seeds and the number of implanted ones (or with the prostate volume, but those two parameters were closely linked in our series). More specifically, a significant relationship (P = 0.02) could be demonstrated between the number of seeds implanted at the periphery of the prostate and the number of seeds migrating to the lungs.Conclusion. – The percentage of migrating seeds observed in this series is low, actually one of the lowest found in the literature when using the loose-seed technique. There was no clinical consequences and the loss of–usually–only one seed is very unlikely to alter the quality of the dose distribution. However, the predominance of pulmonary migrations in our series led us to slightly modify our implantation technique. We now try to avoid too “peripheral” seed implantations, due to the risk of migration towards the periprostatic veins, and subsequently to the lungs. [Copyright &y& Elsevier]