Optimization of the Dose Rate Effect in Tetrazolium Gellan Gel Dosimeters

Abstract

Tetrazolium salts provide an appealing candidate for 3D gel dosimeters as they exhibit a low intrinsic color, no signal diffusion and excellent chemical stability. However, a previously developed commercial product (the ClearView 3D Dosimeter) based on a tetrazolium salt dispersed within a gellan gum matrix presented a noticeable dose rate effect. The goal of this study was to find out whether ClearView could be reformulated in order to minimize the dose rate effect by optimizing of the tetrazolium salt and gellan gum concentrations and by the addition a thickening agent, ionic crosslinkers, and radical scavengers. To that goal, a multifactorial design of experiments (DOE) was conducted in small-volume samples (4-mL cuvettes). It showed that the dose rate could be effectively minimized without sacrificing the integrity, chemical stability, or dose sensitivity of the dosimeter. The results from the DOE were used to prepare candidate formulations for larger-scale testing in 1-L samples to allow for fine-tuning the dosimeter formulation and conducting more detailed studies. Finally, an optimized formulation was scaled-up to a clinically relevant volume of 2.7 L and tested against a simulated arc treatment delivery with three spherical targets (diameter 3.0 cm), requiring different doses and dose rates. The results showed excellent geometric and dosimetric registration, with a gamma passing rate (at 10% minimum dose threshold) of 99.3% for dose difference and distance to agreement criteria of 3%/2 mm, compared to 95.7% in the previous formulation. This difference may be of clinical importance, as the new formulation may allow the quality assurance of complex treatment plans, relying on a variety of doses and dose rates; thus, expanding the potential practical application of the dosimeter.

Investigation of a 3D dosimetry system utilizing radio‐chromic gel and telecentric optical‐CT readout

“…The reconstruction techniques and parameters have been fully described elsewhere. 4,5 The 3D gamma analysis between the eclipse predicted and DLOS measured optical-CT dose distributions was performed by adapting the process detailed by Kevin Alexander utilizing 3D Slicer. 15 Figures 4 and 5 show dosimeter reconstruction, registration, and 3D Slicer volumes.…”

“…INTRODUCTION ClearView (ModusQA) is a novel single use radiochromic gel dosimeter designed for use with optical CT for relative dosimetry measurements with several published accounts of use. [1][2][3][4][5][6] When exposed to ionizing MV radiation, ClearView gel changes color from optically clear to purple through the reduction of a water-soluble tetrazolium salt into an insoluble formazan dye. Independent studies 1,2,4 and the User's Guide 7 have reported the change in optical attenuation to be linear with dose between 0 and 80 Gy, with negligible signal diffusion and change with time (aging of irradiated gel ∼3 × 10 −4 cm −1 /day), low optical scattering component, and near tissue equivalence (electron density 1.015 ± 0.3% relative to water).…”