Room: Exhibit Hall | Forum 4
Purpose: A new bioabsorbable mesh containing a 1cm 2D array of Cs-131 brachytherapy seeds is being used in our clinic for intra-operative radiation therapy treatments. We have investigated the post-implant movement of the seeds and the impact of this motion on the dose distribution.
Methods: In the past year, 8 patients diagnosed with locally recurrent head and neck carcinoma received surgical resection and implantation of Cs-131 seed mesh into the surgical bed. 11 to 68 (median of 29) seeds were implanted per patient, for a prescribed dose of 60â€“70Gy (65.5Gy median) to 5mm depth from the plane of the mesh. 7 patients, implanted with a total of 229 seeds, had a post-implant CT acquired 1-2 days after the implant procedure, and at least one follow-up CT. Each of the follow-up CTs was rigidly registered to the initial post-operative CT using MIM software, and the DICOM coordinates of each seed obtained to determine its movement.
Results: The average observed seed movement consistently increased with every subsequent CT acquired: by 60 days, the average deviation was 4.3mm. The mean bulk displacement of the entire mesh implant was 2.5mm by the same time. Kaplan-Meier plots obtained for the probability of a seed not having been observed to move a given distance revealed that after 60 days, 98.8% of the studied seeds had moved <10mm, 65.8% by <5mm, and 21.7% by <2.5mm. The typical resulting change in volume of the prescription isodose line was ~3%.
Conclusion: Over the first 60 days following implantation (enough time for the Cs-131 seeds to deposit ~99% of the prescribed dose), most seeds in the mesh remained close to their original locations as visualized on the first post-implant CT. This provides assurance that the dose distributions are clinically acceptable despite the seed motion or mesh deformation and movement.