Severe dose inaccuracies caused by an oxygen-antioxidant imbalance in normoxic polymer gel dosimeters

Abstract

Two oxygen scavengers have been successfully tested to produce normoxic polymer gel dosimeters under normal atmospheric conditions. The first is ascorbic acid and the second is a chloride (also sulfate) salt of tetrakis (hydroxymethyl) phosphonium. These antioxidants, added to the dosimeter during gel preparation, chemically remove dissolved oxygen that otherwise inhibits propagation of the polymerization reaction during irradiation of the dosimeter. These gel dosimeters are radiosensitive after manufacture under normoxic conditions. However, we show herein that the accuracy of the dosimetric measurement is compromised due to chemical reactions of the antioxidant with radicals. In addition, we provide evidence that both antioxidant and oxygen act as radical scavengers that affect the amount of polymer formed in the gel dosimeter. This can result in important dose inaccuracies in both methacrylic acid-based and acrylamide-based normoxic dosimeter gels.

Mathematical Modeling of Depth‐Dose Response of Polymer‐Gel Dosimeters

“…Inaccuracies arise when small calibration vials are used to produce calibration curves for computing 3D dose distributions in larger phantoms like the one shown in Figure 2. These inaccuracies arise from a variety of sources including different temperature histories during gel manufacture and different levels of oxygen contamination 42–44. Some medical physicists prefer an alternative calibration technique that relies on depth‐dose profiles, because preparing and irradiating a single large container of gel is simpler than using a series of small vials and this approach may provide more reliable results 12, 36, 40.…”

Comment on ‘The influence of antioxidant THPC on the properties of polymer gel dosimeter’

“…With growing THPC concentration the scope of post irradiation changes decreases. ' We had reported such observations earlier (Sedaghat et al 2011).…”

Basic investigation on performance of low-density polymer gel dosimeter

“…Bayreder used 2 mM of THPC as antioxidant, while, in Haraldson et al study, 94mM of THP was recommended to remove any probability of oxygen contamination in LD gel. Sedaghat et al claimed, oxygen and antioxidant both act as radical scavengers that affect the amount of polymer formed in the gel and modifying the radiation dose response of the dosimeter (17) . In LD gel, excess oxygen that is released by styrofoam beads, react with the extra antioxidant and somehow neutralize its inverse effect, but the amount of unreacted antioxidant left in the LD gel dosimeter can have an impact on the polymerization reaction.…”