Basic Properties of a New Polymer Gel for 3D-Dosimetry at High Dose-Rates Typical for FFF Irradiation Based on Dithiothreitol and Methacrylic Acid (MAGADIT): Sensitivity, Range, Reproducibility, Accuracy, Dose Rate Effect and Impact of Oxygen Scavenger

Abstract

The photon induced radical-initiated polymerization in polymer gels can be used for high-resolution tissue equivalent dosimeters in quality control of radiation therapy. The dose (D) distribution in radiation therapy can be measured as a change of the physical measurement parameter T2 using T2-weighted magnetic resonance imaging. The detection by T2 is relying on the local change of the molecular mobility due to local polymerization initiated by radicals generated by the ionizing radiation. The dosimetric signals R2 = 1/T2 of many of the current polymer gels are dose-rate dependent, which reduces the reliability of the gel for clinical use. A novel gel dosimeter, based on methacrylic acid, gelatin and the newly added dithiothreitol (MAGADIT) as an oxygen-scavenger was analyzed for basic properties, such as sensitivity, reproducibility, accuracy and dose-rate dependence. Dithiothreitol features no toxic classification with a difference to THPC and offers a stronger negative redox-potential than ascorbic acid. Polymer gels with three different concentration levels of dithiothreitol were irradiated with a preclinical research X-ray unit and MR-scanned (T2) for quantitative dosimetry after calibration. The polymer gel with the lowest concentration of the oxygen scavenger was about factor 3 more sensitive to dose as compared to the gel with the highest concentration. The dose sensitivity (α = ∆R2/∆D) of MAGADIT gels was significantly dependent on the applied dose rate trueD˙ (≈48% reduction between trueD˙ = 0.6 Gy/min and trueD˙ = 4 Gy/min). However, this undesirable dose-rate effect reduced between 4–8 Gy/min (≈23%) and almost disappeared in the high dose-rate range (8 ≤ D˙≤ 12 Gy/min) used in flattening-filter-free (FFF) irradiations. The dose response varied for different samples within one manufacturing batch within 3%–6% (reproducibility). The accuracy ranged between 3.5% and 7.9%. The impact of the dose rate on the spatial integrity is demonstrated in the example of a linear accelerator (LINAC) small sized 5 × 10 mm2 10 MV photon field. For MAGADIT the maximum shift in the flanks in this field is limited to about 0.8 mm at a FFF dose rate of 15 Gy/min. Dose rate sensitive polymer gels likely perform better at high dose rates; MAGADIT exhibits a slightly improved performance compared to the reference normoxic polymer gel methacrylic and ascorbic acid in gelatin initiated by copper (MAGIC) using ascorbic acid.

Hydrogels for Three-Dimensional Ionizing-Radiation Dosimetry

“…An example of the response curve obtained through MRI of a polyacrylamide gel (Dithiothreitol and Methacrylic Acid, MAGADIT) exposed to linear accelerator (LINAC) photons is shown in Figure 8 [ 93 ]. The effect of the Dithiothreitol oxygen scavenger was also investigated: the sensitivity reduction seen in Figure 8 with increasing dithio concentration can be attributed to interactions of this oxygen scavengers with radicals (with following reduction of the number of polymerization initiating radicals).…”

“…A dependence on the dose rate would lead to inaccurate dose maps. Some gels such as BANG-2, NIPAM, PAGAT (PolyAcrylamide Gelatin gel fabricated at ATmospheric conditions) and PASSAG (Poly 2-Acrylamido-2-MethylPropane Sulfonic acid-Sodium Salt and Gelatin) gels show response independent of dose rate, whereas some others such as MAGAT (which comprises methacrylic acid, hydroquinone, gelatin and tetrakis hydroxymethyl phosponium chloride, THPC) have a dose rate dependent response and should be used only for doses below 2 Gy [ 55 , 76 , 94 , 95 , 96 , 97 ] or for dose rates above 4 Gy/min as MAGADIT [ 93 ]. An example of the dependence of MR sensitivity on dose rate is shown in Figure 9 .…”

“… Dependence of the sensitivity of Dithiothreitol and Methacrylic Acid (MAGADIT) on the dose rate. Reproduced from [ 93 ]. …”

“…Figure 8. Dosimetric response of methacrylic acid gels produced with different concentrations of dithiothreitol oxygen scavenger (DIT1: 2 mmol/kg, DIT2: 10 mmol/kg, and DIT3: 50 mmol/kg).Reproduced from[93].…”

Functional Polymer Solutions and Gels—Physics and Novel Applications

“…Khan et al [25] showed one example of the use of polymer gels as dosimeters, providing a high-resolution equivalent of tissue in quality control of radiation therapy. Warman et al [26] reported the development of a device for the tomographic imaging of radio-fluorogenic gels (RFGs), which allows for scanning the 3D distribution of radiation in a gel.…”