Performance Analysis of a Prototype MR-Visible Injectable Gel Dosimeter Under Different Conditions
Abstract
Purpose
In vivo dosimetry is almost exclusively limited to surface or intracavitary sites and is generally unable to measure dose directly in deeper tumors or nearby critical structures. An injectable Fricke gel dosimeter has been developed to address this limitation and has been shown in vitro to be able to measure clinically relevant doses across multiple radiation sessions. Given an intended application as an injectable in vivo dosimeter, the purpose of this study is to determine how temperature affects dosimeter response.
Methods
Imaging and irradiation were performed on a 1.5 T MR Linac using a thermally insulated phantom. A single batch of gel was divided into four cuvettes. Two cuvettes were kept at room temperature and two at 37 °C. The cuvettes were irradiated over five sessions. In each session, both temperature groups were irradiated with a single open field in 150 MU steps up to 900 MU. After each step, two T1-weighted scans at different flip angles were acquired to generate T1 maps. The average R1 was determined in each cuvette at the depth at which 1 MU approximately equaled 1 cGy. The linear fit slopes of the R1 vs dose plots were determined for each temperature group and compared.
Results
The gel dosimeter showed a reduced dose response at 37 °C compared to room temperature with an average slope of 1.78e-4 ΔR1(s-1)/cGy and 2.77e-4 ΔR1(s-1)/cGy respectively.
Conclusion
The MR-visible injectable Fricke gel dosimeter exhibits temperature dependent dose response with reduced sensitivity at body temperature. These results indicate that the dosimeter must either be calibrated at body temperature or corrected for temperature effects for accurate dose measurement in future applications.