Quantitative verification of ¹⁹² Ir PDR and HDR source structure by pin-hole autoradiography

For precise localization of the center and determination of the dimensions of the radioactive material within the capsule of brachytherapy sources, we have developed a method based on simultaneous pin-hole autoradiography of two sources. We constructed a variable magnification pin-hole camera consisting of two telescopically fitted Plexiglas cylinders which can accommodate two radioactive sources on the plate covering the top cylinder. The ¹⁹² Ir pulsed and high dose-rate sources were studied and an ¹⁹² Ir seed was used as a reference source. The magnification factor was determined from the dimensions of the ¹⁹² Ir seed image, which was geometrically well defined by a separate transmission radiography experiment. The observed position for the center of radioactivity in the PDR and the HDR source capsules are in agreement with the 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 closer to the tip as specified by the vendor. Quantitative verification of the internal source structure using this method enhances the accuracy with which the dose distribution near brachytherapy sources can be predicted by three-dimensional Monte Carlo calculations.