Author: Brankica Andelic, Young-Bin Cho, Tara M. Gray, Bingqi Guo, Andrew Robert Hadfield, Hakan Nordstrom, Jacob Scott, John Suh, Nara Yoon 👨🔬
Affiliation: Adelphi University, Elekta, Cleveland Clinic 🌍
Purpose: To investigate the biological effect of Spatially Fractionated Radiation Therapy (SFRT) within Gamma-Knife (GK) using retrospective planning for brain metastases patients. This study presents Equivalent Uniform Dose (EUD) and Therapeutic Ratio (TR) of GK SFRT plans as well as dosimetric characterization as a function of target size and choice of shot.
Methods: CT images and target contours were obtained from 20 patients over the past 10 years with largest brain metastases enrolled in IRB-approved study. SFRT vertices (spheres) volumes were generated by in-house scripts and imported to GK planning system. Shots were manually placed to cover sphere volumes inside of tumor with 18Gy to 70% isodose line for single fraction. Sphere arrangement was face-centered-cubic (FCC) structure for both 4mm and 8mm collimator. Shot placement and weights were tuned to deliver the adequate coverage of V18Gy > 50%.
Results: The average tumor target size is 72 cc (ranges from 51 to 120cc), the number of spheres is 29.2 (17-46), 4.4 (3-7), mean dose to sphere is 18.6 (17-19), 20.5 (19-22) Gy, and valley to peak ratio (D90/D10) is 0.324 (0.2-0.4), 0.2 (0.15-0.25) for 4mm and 8mm plans respectively. This evaluation shows that for radioresistant versus radiosensitive tumors, EUD is 0.9, 0.5 Gy higher and TR increases by 182%, 142%. As well, this method shows significant inconsistency in EUD within GK depending on the size of shot used (p << 0.001).
Conclusion: In this study, an SFRT planning method for brain metastases was presented using FCC sphere arrangement seamlessly integrated in already established GK workflow. Dosimetric characteristics and biologic impact were evaluated for both 4mm and 8mm sphere targets. 8mm plans demonstrated hotter and highly modulated dose distribution compared to 4mm plans. Such inconsistent performance between 4mm and 8mm GK SFRT plans warrants further optimization of sphere target generation.