Characterisation of Fricke-xylenol orange-Pluronic gel dosimeters for end-to-end three-dimensional radiotherapy dosimetric quality assurance

OBJECTIVE: Fricke-xylenol orange (XO)-Pluronic F-127 (FXP) gels have demonstrated favourable dosimetric properties, but their investigation has hitherto been limited to small sample volumes. This study characterised large-volume FXP gel dosimeters combined with optical computed tomography (OCT) readout for three-dimensional 6MV radiotherapy dosimetry measurements. APPROACH: An FXP gel formulation proposed in the literature (25% w/w Pluronic F-127, 0.165mM XO, 0.5mM FAS, 50mM H₂SO₄) and two reduced XO formulations (0.08mM and 0.04mM) were investigated using OCT readout at 589nm and 633nm. The effects of XO concentration and scanning wavelength on dose sensitivity, dose resolution, and [XO-Fe]³⁺ ion diffusion were evaluated. MAIN RESULTS: All FXP formulations exhibited linear dose-response behaviour over the studied dose range of 0.2-4.15Gy. OCT readout at 589nm provided higher dose sensitivity but was associated with increased optical attenuation and ring artefacts. Reducing the XO concentration to 0.08mM preserved dose sensitivity, whereas further reduction to 0.04mM resulted in reduced sensitivity without a noticeable impact on imaging artefacts. OCT readout at 633nm mitigated imaging artefacts at the expense of dose sensitivity, while achieving dose resolutions below 5% at doses above 1.5Gy (95% confidence level). The diffusion of [XO-Fe]³⁺ ions was found to decrease with increasing XO concentration and to be anisotropic with respect to gravity, supporting a post-irradiation readout time window of 60 to 120min to limit spatial blurring. An optimised FXP-OCT configuration employing 0.08mM XO, a 633nm scanning wavelength, and readout at 60min post irradiation, was applied to an end-to-end (E2E) radiotherapy quality assurance protocol. Results exhibited good agreement with treatment planning system calculations for an intracranial stereotactic radiotherapy plan, with a 3D global gamma index passing rate of 96% (3%/2mm criteria). SIGNIFICANCE: These findings support the feasibility of large-volume FXP gel dosimetry for E2E dosimetric quality assurance in complex radiotherapy applications.