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Prediction of Dose Increments by Brain Metastases Resection Cavity Shrinkage Model with I-125 and Cs-131 LDR Seed Implantations

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D Han

D Han*, S Braunstein , P Sneed , M McDermott , L Ma , University of California San Francisco, San Francisco, CA


SU-F-T-43 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

This work aims to determine dose variability via a brain metastases resection cavity shrinkage model (RC-SM) with I-125 or Cs-131 LDR seed implantations.

Materials and Methods:
The RC-SM was developed to represent sequential volume changes of 95 consecutive brain metastases patients. All patients underwent serial surveillance MR and change in cavity volume was recorded for each patient.
For the initial resection cavity, a prolate-ellipsoid cavity model was suggested and applied volume shrinkage rates to correspond to 1.7, 3.6, 5.9, 11.7, and 20.5 months after craniotomy. Extra-ring structure (6mm) was added on a surface of the resection volume and the same shrinkage rates were applied. Total 31 LDR seeds were evenly distributed on the surface of the resection cavity. The Amersham 6711 I-125 seed model (Oncura, Arlington Heights, IL) and the Model Cs-1 Rev2 Cs-131 seed model (IsoRay, Richland, WA) were used for TG-43U1 dose calculation and in-house-programed 3D-volumetric dose calculation system was used for resection cavity rigid model (RC-RM) and the RC-SM dose calculation.

The initial resection cavity volume shrunk to 25±6%, 35±6.8%, 42±7.7%, 47±9.5%, and 60±11.6%, with respect to sequential MR images post craniotomy, and the shrinkage rate (SR) was calculated as SR=56.41Χexp(-0.2024Χt)+33.99 and R-square value was 0.98.
The normal brain dose as assessed via the dose to the ring structure with the RC-SM showed 29.34% and 27.95% higher than the RC-RM, I-125 and Cs-131, respectively. The dose differences between I-125 and Cs-131 seeds within the same models, I-125 cases were 9.17% and 10.35% higher than Cs-131 cases, the RC-RM and the RC-SM, respectively.

A realistic RC-SM should be considered during LDR brain seed implementation and post-implement planning to prevent potential overdose. The RC-SM calculation shows that Cs-131 is more advantageous in sparing normal brain as the resection cavity volume changes with the LDR seeds implementation.

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