Assessment of the strength of individual Ir-192 seeds in ribbons

Assessing the strength of individual seed-type sources in ribbon assembles remains a challenge in brachytherapy quality assurance. Geometries to measu re a single source in the ribbon usually fail because of low signals if usi ng very thick shielding to block the radiation from the other sources, or c ontributions from ail the other sources if they are not shielded well. A no rmal well-type chamber with partial lead shielding forming a small slot pro vides a differential response along the chamber axis that, through a deconv olution/simultaneous-equations technique, sorts the contributions from each source, allowing the derivation of each source's strength. (C) 1999 Americ an Association of Physicists in Medicine. [S0094-2405(99)03311-8].