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Dosimetric Impact of Radioisotope Type On Permanent Brain Seed Implants

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D Pinnaduwage

D Pinnaduwage*, E Youssef , S Sorensen , S Srivastava , X Yan , D Brachman , Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, AZ


TU-L-GePD-T-6 (Tuesday, August 1, 2017) 1:15 PM - 1:45 PM Room: Therapy ePoster Lounge

Purpose: The utility of seed based brain brachytherapy is hampered by frequent brain necrosis. We modeled dosimetric differences between Cs-131, Pd-103 and I-125 isotopes in permanent brain brachytherapy using an intraoperative seed carrier/spacer designed to avoid necrosis from direct seed/tissue contact.

Methods: We retrospectively identified thirteen patients who previously underwent intraoperative brachytherapy with Cs-131 seeds imbedded in a biocompatible collagen carrier/spacer on an IRB approved trial. For each patient Cs-131 dosimetric plans were compared to plans for I-125 and Pd-103 seeds, respectively. Substituting these isotopes, we used the exact same seed locations and modified the activities to match the previously achieved 60Gy Cs-131 dose volumes. The collagen seed carrier/spacers provide approximately 3 mm offset between the long axis of the seeds and tissue.

Results: To create 60Gy volumes, average seed strengths of 3.68, 0.69 and 4.1U, for Cs-131, I-125 and Pd-103 were necessary and the average 60 Gy volume was 32cm³. On average, Pd resulted in an increase of 4.5cm³ at 150Gy and 4.3cm³ at 200Gy over Cs; with I-125 the increase was 1.35cm³ at 150Gy and 1.26cm³ at 200Gy over Cs. Average 30 Gy volumes were higher for Cs by 16.7cm³ vs. Pd, and by 5.1 cm³ vs I-125. Pd exhibited the steepest dose fall-off; the Gradient Index (GI) was 1.88 vs. 2.23 with I-125, and 2.40 with Cs. Compared to Cs, the mean implant dose was higher by 1.3x and 1.1x with Pd and I-125, respectively. Dose/volume differences between radioisotopes increased with increasing volume.

Conclusion: Dose fall off was fastest with Pd followed by I-125, then Cs. However, Pd and I-125 resulted in higher intra-target doses and higher inhomogeneity compared to Cs. The larger high-dose volume with Pd and I-125 potentially increase the risk for radiation necrosis, and the inhomogeneity becomes more pronounced with increasing target volume.

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