QUANTITATIVE VERIFICATION OF IR-192 PDR AND HDR SOURCE STRUCTURE BY PINHOLE AUTORADIOGRAPHY

For precise localization of the center and determination of the dimens ions of the radioactive material within the capsule of brachytherapy s ources, we have developed a method based on simultaneous pin-hole auto radiography of two sources. We constructed a variable magnification pi n-hole camera consisting of two telescopically fitted Plexiglas cylind ers which can accommodate two radioactive sources on the plate coverin g the top cylinder. The Ir-192 pulsed and high dose-rate sources were studied and an Ir-192 seed was used as a reference source. The magnifi cation factor was determined from the dimensions of the Ir-192 seed im age, which was geometrically well defined by a separate transmission r adiography experiment. The observed position for the center of radioac tivity in the PDR and the HDR source capsules are in agreement with th e vendor specifications. The distance from the tip of the PDR capsule to its center of radioactivity was found in this way to be 0.79+/-0.21 mm, which agrees with the position (0.85 mm) of the pellet situated c loser to the tip as specified by the vendor. Quantitative verification of the internal source structure using this method enhances the accur acy with which the dose distribution near brachytherapy sources can be predicted by three-dimensional Monte Carlo calculations.