Author: Daniel Ball, Alex Dresner, Geoffrey S. Ibbott, Leonard H. Kim, Christopher Tyerech, Sebastián Vega 👨🔬
Affiliation: MD Anderson Cancer Center at Cooper, Cooper Medical School of Rowan University, The American Board of Radiology, Department of Radiation Oncology, University of Pennsylvania, Rowan University, Philips Healthcare MR Oncology 🌍
Purpose:
In vivo dosimetry is valuable to radiation therapy for its ability to report the actual dose received by patients but is currently limited to surface and intracavitary measurements, leaving most tumor locations and organs-at-risk inaccessible to dosimetric measurement. Combining work in self-forming patient-injectable hydrogels, MRI readout of Fricke gel dosimeters, and MR-linacs, we have developed an injectable gel dosimeter that could potentially be used in adaptive MR-guided radiation therapy. We report on the dosimeter's stability, reproducibility, re-usability, and dose response in vitro.
Methods:
Separate batches of dosimeter components were manufactured 84 days apart to assess inter-batch reproducibility. Each batch was injected into cuvettes to form multiple 1-mL gel dosimeter samples to assess intra-batch reproducibility. Samples were irradiated and imaged on a 1.5-T MR-linac using a 2-minute variable flip angle technique to determine R1 and the dosimeter's response and linearity. Samples were imaged multiple times within a session to assess measurement uncertainty in R1 and dose. Samples were irradiated over multiple sessions 6 to 51 days post-injection to assess stability and re-usability.
Results:
The studied combination of imaging technique and gel formulation resulted in a dose uncertainty of 1.5 Gy within a 5x5x3-mm3 volume. The intra- and inter-batch dose response difference were 3.2 and 3.6% respectively. Pre-irradiation imaging was required to correct inter-session R1 drift in order to obtain cumulative dose. Samples were stable and responsive through the entire experimental time period.
Conclusion:
A novel, MR-readable injectable dosimeter was developed that, in phantom, appears capable of quickly measuring clinically relevant dose levels for multiple irradiations over several weeks. With further development, this technology has potential as a cumulative in vivo dosimeter for adaptive MRgRT.