Development and characterisation of a high-sensitivity X-ray CT polymer gel dosimeter

Polymer gel dosimeters have shown potential for clinical 3D dosimetry; however, their use has been limited due to low sensitivity and reliance on scarcely available magnetic resonance imaging. This study aimed to optimise a PASSAG (Poly AMPS Sodium Salt And Gelatin) polymer gel dosimeter for X-ray computed tomography, to enhance its clinical feasibility. The total monomer concentration was increased to improve sensitivity, and different cosolvents were tested to enhance the limited solubility of N, N'-methylenebisacrylamide, the crosslinker. n-propanol was identified as the optimal cosolvent, allowing for an 18.3% monomer concentration, 30% crosslinker to total comonomer mass gel, at a 3:7 cosolvent-to-water ratio. The optimised formulation, PASSAG-N (PASSAG- n-propanol), consisted of 54.4% w/w deionised water, 23.3% n-propanol, 12.8% 2-acrylamido-2-methylpropane sulfonic acid sodium salt, 5.5% N, N'-methylenebisacrylamide, 4.0% gelatin, and 0.089% (4.65mM) tetrakis (hydroxymethyl)phosphonium chloride. The dosimeter was irradiated within a standard timeframe to assess its sensitivity, and theoretical calculations confirmed its equivalence to water, soft tissue, brain, and muscle. Compared to a cosolvent-free formulation, PASSAG-N exhibited a 250% increase in Hounsfield unit (HU) change, demonstrating enhanced sensitivity. The optimised gel showed a linear response over a 1-12 Gy dose range, with an average sensitivity of 1.072 ± 0.041 HU Gy⁻¹ and a dose resolution ≤ 0.31 Gy, making it a promising alternative for clinical X-ray computed tomography-based dosimetry. This study highlights the potential of PASSAG-N as a highly sensitive and potentially practical polymer gel dosimeter for clinical applications.