Three-Dimensional Dose Verification Using an Optimized Fricke Xylenol Orange Gel with Optical Computed Tomography and Magnetic Resonance Imaging
Abstract
Purpose
To optimize a Fricke xylenol orange (FXO) gel formulation for high-sensitivity readout using optical computed tomography (optical CT) and magnetic resonance imaging (MRI), and to quantitatively demonstrate that MRI-based readout provides three-dimensional (3D) dose verification equivalent to optical CT.
Methods
An FXO gel formulation was optimized to maximize dose sensitivity for optical CT (Vista 16 Optical CT Scanner) and MRI (Siemens Magnetom Vida 3T) while minimizing auto-oxidation and diffusion. Ferrous ammonium sulfate (FAS) and xylenol orange (XO) concentrations were varied over a range of 0.15–1.0 mM and 0.01–0.10 mM, respectively. The optimized formulation was 0.42 mM FAS, 0.05 mM XO, 65 mM sulfuric acid, and 6.3 wt% gelatin. The gel was irradiated in a cylindrical phantom with a stereotactic radiosurgery plan with a scaled down prescription dose of 4.5 Gy to avoid saturating the optical CT readout.
Results
Measured 3D dose distributions from optical CT and MRI showed good agreement with each other and with the Eclipse treatment planning system (TPS). Line profile comparisons across the calibrated gel volume demonstrated good geometric agreement between optical CT, MRI, and TPS. The mean measured dose within the TPS generated 80% isodose structure was 3.98 Gy for optical CT and 3.93 Gy for MRI, corresponding to a mean dose difference of 5% relative to TPS.
Conclusion
This work provides a direct quantitative comparison of optical CT and MRI readout using FXO gel dosimetry and demonstrates that an optimized FXO gel formulation enables reliable dual-modality 3D dose verification. Our results indicate that MRI-based readout can serve as a robust primary modality for quantitative 3D FXO gel dosimetry without compromising performance relative to optical CT. Further studies are underway to improve SNR for MRI readout and to investigate the observed 5% dose difference between the measured gel volumes and TPS.