S Ng, Y Yue, K Shiue, Y Le*, Indiana University School of Medicine, Indianapolis, Indiana

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  Y Le

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PO-GePV-T-42 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To evaluate dosimetric changes due to Cs-131 seed displacement cause by cavity shrinkage in brain brachytherapy.

Methods: Eight glioma patients had 4-11 GammaTile placed along the resection bed after maximal safe resection during craniotomy. Each GammaTile composed of four Cs-131 seeds imbedded in a biodegradable collagen sponge to minimize seed movement and maintain seed to cavity surface distance. Cs-131 seed positions were identified using VariSeed on day1 and day30 post-implant CT images and dosimetry parameters were calculated. Iterative closest point (ICP) algorithm was used to compute rigid transformation between day1 and day30 seeds clouds. Seeds displacement was calculated after registration. Volume covered by 100% isodose line (V100), PTV coverage (D90_PTV, V100_PTV) and dose to OARs were calculated for both CT datasets to determine dosimetric changes due to seed displacement.

Results: Average seed displacement of 1.9±1.0 mm for all patients was observed between day1 and day30. Maximum seed displacement for each patient ranges from 2.3mm to 7.3mm. Average V100 difference between day1 and day30 was 2.7cc (range: 0.5cc – 6.5cc). Average D90_PTV are 100.5% (range: 70.0% - 131.0%) and 95.7% (range: 19.9% - 149.0%) on day1 and d30, respectively. Average V100_PTV are 89.6% (81.3% - 99.1%) and 87% (range: 47.0% - 99.7%) on day1 and d30, respectively. Our results showed that brainstem dose changed from 63.5Gy (day1) to 55.8Gy (day30) for one patient and from 52.0Gy to 45.3Gy for another patient. Meanwhile, optic nerve dose changed from 34.1Gy to 35.9Gy for one patient.

Conclusion: We observed up to 7.3mm maximum seed displacement and 6.5cc V100 volume change due to seed displacement caused by cavity shrinkage. Cavity shrinkage also affected PTV coverage due to change in PTV volume/shape and seed displacement. Reduced dose to OARs was observed in cases where cavity shrinkage caused Cs-131 seeds to shift away from OARs.

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