Author: Eyub Y. Akdemir, Gregory A Azzam, Rupesh Kotecha, Gregory J. Kubicek, Natalia Lutsik, Eric Mellon, Siamak P. Nejad-Davarani, Parag Parikh, Karen C. Snyder 👨🔬
Affiliation: Miami Cancer Institute, Baptist Health South Florida, Department of Radiation Oncology, University of Miami, Henry Ford Health 🌍
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 post-operative 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 real-time adaptive PTV reduction to reduce treatment toxicity.