SU-G-JeP2-04: Comparison Between Fricke-Type 3D Radiochromic Dosimeters for Real-Time Dose Distribution Measurements in MR-Guided Radiation Therapy.

PURPOSE: To assess MR signal contrast for different ferrous ion compounds used in Fricke-type gel dosimeters for real-time dose measurements for MR-guided radiation therapy applications.

METHODS: Fricke-type gel dosimeters were prepared in 4% w/w gelatin prior to irradiation in an integrated 1.5 T MRI and 7 MV linear accelerator system (MR-Linac). 4 different ferrous ion (Fe2?) compounds (referred to as A, B, C, and D) were investigated for this study. Dosimeter D consisted of ferrous ammonium sulfate (FAS), which is conventionally used for Fricke dosimeters. Approximately half of each cylindrical dosimeter (45 mm diameter, 80 mm length) was irradiated to ∼17 Gy. MR imaging during irradiation was performed with the MR-Linac using a balanced-FFE sequence of TR/TE = 5/2.4 ms. An approximate uncertainty of 5% in our dose delivery was anticipated since the MR-Linac had not yet been fully commissioned.

RESULTS: The signal intensities (SI) increased between the un-irradiated and irradiated regions by approximately 8.6%, 4.4%, 3.2%, and 4.3% after delivery of ∼2.8 Gy for dosimeters A, B, C, and D, respectively. After delivery of ∼17 Gy, the SI had increased by 24.4%, 21.0%, 3.1%, and 22.2% compared to the un-irradiated regions. The increase in SI with respect to dose was linear for dosimeters A, B, and D with slopes of 0.0164, 0.0251, and 0.0236 Gy(-1) (R(2) = 0.92, 0.97, and 0.96), respectively. Visually, dosimeter A had the greatest optical contrast from yellow to purple in the irradiated region.

CONCLUSION: This study demonstrated the feasibility of using Fricke-type dosimeters for real-time dose measurements with the greatest optical and MR contrast for dosimeter A. We also demonstrated the need to investigate Fe(2+) compounds beyond the conventionally utilized FAS compound in order to improve the MR signal contrast in 3D dosimeters used for MR-guided radiation therapy. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. LH- 102SPS.