Inter-Fraction Monitoring of Brain Metastases Resection Cavities during Fractionated Stereotactic Radiosurgery on the 0.35 T MRI -Linac

Document Type

Conference Proceeding

Publication Date

9-30-2025

Publication Title

Med Phys

Keywords

adult, brain metastasis, brain migration, brain size, brain tissue, clinical article, cone beam computed tomography, conference abstract, excision, female, human, male, MRI guided linear accelerator, MRI scanner, nuclear magnetic resonance imaging, planning target volume, radiation dose, radiotherapy, retrospective study, steady state, stereotactic radiosurgery, surgery

Abstract

Purpose: Resection cavity volumes shrink gradually over time after surgical resection of brain metastases. Fractionated stereotactic radiosurgery (fSRS) is often delivered to the cavity to prevent recurrences. Therefore, we sought to use 0.35T MRI-guided fSRS to quantify brain metastases resection cavity changes during the fSRS course, as these cavities are typically not visualized during cone-beam CT. Methods: A retrospective analysis was conducted on five patients treated with MRI-guided fSRS at three centers. Each patient underwent 3-5 treatment fractions. Pre-treatment and fractional MRIs were analyzed on the balanced Steady State Free Precession (bSSFP: TR=3.84ms, TE=1.92ms, Voxel size = 1.5x1.5x1.5mm) images to assess relative planning target volume change (PTV index), PTV Hausdorff distances, and brain migration volume (the brain volume pulled into the PTV by shrinking cavity). Comparisons were made between initial and each fraction MRIs, focusing on changes in target coverage and healthy brain tissue sparing. Results: Four of five patients exhibited resection cavity shrinkage during treatment. The PTV index ranged from 0.59 to 1.13, with a median value of 0.85±0.14. Brain migration volume varied between -1.57 and 16.53ml, with a median of 2.59±4.78ml. PTV Hausdorff distances (maximum linear change) ranged from 2.34 to 9.84mm, with a median value of 4.30±1.96mm. Conclusion: Per-fraction MRI imaging with a 0.35T MRI-Linac during fSRS identifies shrinking cavities in most postoperative brain metastasis patients. As cavities shrink, normal appearing brain moves into the high dose PTV. Since fSRS is correlated with the volume of brain receiving high dose radiation, this study identifies possible benefit of realtime adaptive PTV reduction to reduce treatment toxicity.

Volume

52

Issue

10

First Page

292

Last Page

293

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