Source strength verification and quality assurance of sterile, pre-loaded iodine-125 seed trains used for prostate brachytehrapy

Like all brachytherapy sources, radioactive “seeds” that are supplied to the clinic in pre-loaded needles require quality assurance measures that include, if possible, verification of the individual strengths of all the seeds. Currently, there is no commercially available apparatus or established technique that allows such measurements to be made on seeds already loaded into needles. Source strength verification by the physicist receiving the order is generally limited to assaying seeds from a separate sample belonging to the same batch that was used to load the needles. However, this method does not assay the seeds that will actually be implanted into the patient, and there is potential for errors to occur between the time the needle loading service assays the seeds and the time that the loaded needles and verification seeds are packaged and sent to the clinic. We investigated a method to verify the positions and strengths of brachytherapy seeds loaded into implant needles. The system used consisted of a flat panel image detector, an autoclavable apparatus (“jig”) to hold the needles in a fixed geometry relative to the detector, and software to analyze the detected signals. In addition to defining the geometry of the detection system, the jig serves to maintain a sterile field around the needles as they are being assayed. The goal will be for this system to provide relative dosimetric readings that can be compared to readings from needles loaded with seeds of known strength, as established by measurement in a well chamber. Results showed a good correlation between detector reading and radioactive source strength of up to 6 individual seeds loaded into needles as trains with 1 cm or greater spacing between seeds. In addition, the detector has a sufficiently large image capture area to allow several needles (10-15) to be analyzed simultaneously. Good correlation between detector reading and exposure time, as well as between detector reading and source strength, demonstrated the linear characteristic of the detector. This study showed that a flat panel detector dosimetry system allowed simultaneous characterization in a sterile environment of 100% of seeds used in a brachytherapy treatment procedure.Science, Faculty ofPhysics and Astronomy, Department ofGraduat