Author: Brianna Baek, David J. Carlson, Zhe (Jay) Chen, Dae Yup Han, Bon Tack Koo, Hyojun Park 👨🔬
Affiliation: Columbia University, Canyon Crest Academy, Department of Therapeutic Radiology, Yale University School of Medicine, Department of Radiation Oncology, Chung-Ang University Gwang Myeong Hospital 🌍
Purpose: To find an optimized treatment technique and shielding design for minimizing the dose to a fetus, a Monte-Carlo simulation was performed to evaluate the dosimetric contribution.
Methods: Geant4 simulation toolkit was used to model 6X and 6X FFF beams (Varian Trilogy) and a box-shaped water phantom (930×30×30cm³) with 1×1×1mm³ voxels to simulate a simplified previously treated head and neck (H&N) cancer patient in our center. The dose was calculated from the energy deposited to each voxel. The beam center was positioned 30 cm from the phantom top, with a source-to-isocenter distance of 100cm. The field size was chosen as 25×25cm², and gantry and collimator angles were chosen as 0 degrees. To find the dose contribution to the fetus, three shielding configurations were modeled: no-shielding, internal shielding block embedded within the phantom, and the shielding placed above the phantom. Shielding was placed 10cm away from the beam field to the bottom side of the phantom.
Results: The dose consistently decreased with increasing distance from the treatment area. Beyond 20 cm from the beam edge until 42 cm, the average dose for the 6X FFF beam was 48.8% lower than that of the 6X beam. At 25cm from beam edge, dose with internal shielding was 28% lower than the dose with top shielding. At 30cm, dose with top shielding was 54% lower than the dose with internal shielding. These differences varied with distance, measured at 48.4%, 37.8%, and 41.9% at 32cm, 37cm, and 42cm, respectively.
Conclusion: This study highlights using a 6FFF beam with external shielding can minimize fetus dose. The findings support the understanding of beam and shielding selection strategies to minimize unintended exposure to a fetus during H&N cancer radiotherapy. Future work will validate these results with patient-specific treatment data and explore advanced shielding designs to improve treatment safety.