689 poster I-125 SEED CALIBRATION USING THE SEEDSELECTRON AFTERLOADER. PRACTICAL SOLUTION TO FULFIL AAPM RE-COMENDATIONS

Purpose: SeedSelectron is an afterloader system used in prostate insterstitial permanent brachytherapy with I-125 seeds. The system is composed of an array of diodes to assay all implanted seeds. Because seeds are on preesterilized cartridges, just 1–2 seeds can be assayed to check manufacturer SK and to calibrate the diode array. This is done during the intraoperative procedure once the interactive planning is completed assuming the manufacturer SK value. To fulfil AAPM Recommendations two are the critical points: high practical difficulties to assay the required seed quantity and the low accuracy of the diode system. The purpose of this work is to present the practical solution adopted to fulfil AAPM Recommendations. Materials and Methods: An agreement has been done with Nucletron to obtain a separate container with 10 seeds from the same batch for all users under request. These seeds are measured prior the implant. Seeds SK are evaluated using an ionization chamber (SourceCheck PTW Germany) calibrated at PTB (Germany) for the SelectSeed 125I source model. To optimize the measuring procedure, in this work, an insert has been developed to be used leaned to the wall chamber plate. It allows measuring SK of the overall 10 seeds set. A corresponding factor to be operated by the camber calibration factor was obtained for the developed insert from the calibration chamber factor using a NIST standard source. Chamber profile response along 10 cm length was evaluated. Typical results are presented for 10 patients: comparison with manufacturer SK from both the average of 10 non-sterile seeds and the isolated seed extracted during the intraoperative procedure. Results: Corresponding factor obtained has been 1.88 ±0.04. Chamber profile response resulted within ±1.5%. For the 10 patients assayed, the values of the differences with manufacturer value for the 10 seeds had an average, standard deviation and range of -1.81%, 0.02%, and -4.6% to 3%, respectively; and isolated seeds: -2.9%, 0.03%, -7.8% to 2%, respectively. Conclusions: This new solution allows evaluating seeds SK prior the intraoperative procedure, avoiding the diode limitations and the deviations because dispersion/uncertainty involved in just one seed assay. The developed insert allows to perform efficiently the mean SK evaluation. This procedure has been adopted by Nucletron to be extended to all SeedSelectron users.