TH-CD-BRA-08: Novel Iron-Based Radiation Reporting Systems as 4D Dosimeters for MR-Guided Radiation Therapy.

PURPOSE: To compare novel radiation reporting systems utilizing ferric ion (Fe(3+) ) reduction versus ferrous ion (Fe(2+) ) oxidation in gelatin matrixes for 3D and 4D (3D+time) MR-guided radiation therapy dosimetry.

METHODS: Dosimeters were irradiated using an integrated 1.5T MRI and 7MV linear accelerator (MR-Linac). Dosimeters were read-out with both a spectrophotometer and the MRI component of the MR-Linac immediately after irradiation. Changes in optical density (OD) were measured using a spectrophotometer; changes in MR signal intensity due to the paramagnetic differences in the iron ions were measured using the MR-Linac in real-time during irradiation (balanced-FFE sequences) and immediately after irradiation (T1 -weighted and inversion recovery sequences).

RESULTS: Irradiation of Fe(3+) reduction dosimeters resulted in a stable red color with an absorbance peak at 512 nm. The change in OD relative to dose exhibited a linear response up to 100 Gy (R(2) =1.00). T1 -weighted-MR signal intensity (SI) changed minimally after irradiation with increases of 8.0% for 17 Gy and 9.7% after escalation to 35 Gy compared to the un-irradiated region. Irradiation of Fe(2+) oxidation dosimeters resulted in a stable purple color with absorbance peaks at 440 and 585 nm. The changes in OD, T1 -weighted-MR SI, and R1 relative to dose exhibited a linear response up to at least 8 Gy (R(2) =1.00, 0.98, and 0.99) with OD saturation above 40 Gy. The T1 -weighted-MR SI increased 50.3% for 17 Gy compared to the un-irradiated region. The change in SI was observed in both 2D+time and 4D (3D+time) acquisitions post-irradiation and in real-time during irradiation with a linear increase with respect to dose (R(2) >0.93).

CONCLUSION: The Fe(2+) oxidation-based system was superior as 4D dosimeters for MR-guided radiation therapy due to its higher sensitivity in both optical and MR signal readout and feasibility for real-time 4D dose readout. The Fe(3+) reduction system is recommended for high dose applications. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. LH-102SPS.