Central Nervous System

PV QA 2 - Poster Viewing Q&A 2

MO_11_2727 - Dosimetric Impact of Cesium 131 Seed Migration in Collagen Carrier Brain Brachytherapy Implants

Monday, October 22
10:45 AM - 12:15 PM
Location: Innovation Hub, Exhibit Hall 3

Dosimetric Impact of Cesium 131 Seed Migration in Collagen Carrier Brain Brachytherapy Implants
D. Pinnaduwage1, S. P. Sorensen1, E. F. Youssef2, S. P. Srivastava1, X. Yan1, and S. Jani1; 1Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, AZ, 2Barrow Neurological Institute, Phoenix, AZ

Purpose/Objective(s): Brain brachytherapy utilizing intraoperative permanently implanted Cesium 131 (Cs-131) seeds imbedded in a novel biocompatible 3-dimensional carrier/spacer that prevents seed -to- brain contact and stabilizes the initial seed-to-seed distance has shown promise at our institution. However, there are no studies on seed migration following collagen carrier implants, or if migration results in clinically significant dosimetric changes over the relatively short half-life (9.69d) of Cs-131. The purpose of this study is to quantify seed migration and possible dosimetric impact in Cs-131 collagen carrier/spacer brain implants.

Materials/Methods: We identified 7 patients who underwent Cs-131 brain implantation and planned subsequent IGRT on an ongoing prospective, single arm IRB approved trial. CT’s obtained for EBRT planning (1.5- 3mm) and subsequent kV CBCT images (2mm) were identified and co-registered to the post-seed implant CT (1.3-1.5mm). Seed positions on each image set acquired between 7-57 days after implant were manually identified by a single user taking care to ensure one-to-one correspondence of seeds across image sets. 47 image sets were analyzed and tracked a total of 124 individual seeds (avg. 18/implant, range 10-28). Seed migration relative to the post-implant CT seed position was quantified for each patient. Additionally, using seed positions at each time point, and accounting for decay, dosimetry for each implant was recalculated for 2 hypothetical scenarios: 1) assuming the seed positions based on a given image set were unchanged until the subsequent image set is acquired, and 2) assuming the seed positions changed to the position of the subsequent image set the day following the acquisition of the prior images. Dosimetry for each scenario was then compared to the original post implant plan, and dose differences to the post implant brachytherapy prescription was quantified. Dose calculation was carried out per TG-43 using a point source approximation; tissue heterogeneity corrections were not performed.

Results: Across all seeds average seed migration was 1.7 mm (range 0.7-3.1 mm). Average maximum migration was 2.7 mm (range 1.2-4.3mm) and the minimum 1.0 mm (0.3-2.0 mm). Maximum migration observed for any seed was 5.5 mm. The error in manually identifying the seed center was determined to be 0.4 mm on average (range 0.1-0.8 mm). Overall, no specific pattern for seed migration over time was observed; we could not determine if migration was due to cavity deformation, carrier movement, or a combination. The average absolute value dose difference to the prescribed 60 Gy brachytherapy volume was 0. 4 Gy (0.6%, range 0.1-1.0Gy) and 0.9 Gy (1.5 % range 0.2-1.7Gy) for scenarios 1 and 2, respectively. The differences in V150 and V200 were < 0.3cc and <0.7cc.

Conclusion: The resulting dosimetric impact of this small average migration appears not to be clinically significant, especially in view of the short t1/2 of Cs-131.

Author Disclosure: D. Pinnaduwage: Employed Physicist; Dignity Health. S.P. Sorensen: Employed Physicist; Dignity Health. E.F. Youssef: Employed M.D.; Dignity Health. Stock; GT Medical Technologies. S.P. Srivastava: Employed Physicist; Dignity Health. X. Yan: Employed Physicist; Dignity Health. S. Jani: Employed Physicist; Dignity Health.

Dilini Pinnaduwage, PhD

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