Investigation of cyanine‐based infrared dyes as calibrants in radiochromic films

Abstract Background Radiochromic material such as lithium pentacosa‐10,12‐diynoate (LiPCDA) has been suggested as the radiation‐sensitive material for real‐time in vivo fiber‐optic dosimetry. In this configuration, micron‐thick radiochromic coating would measure the absorbed dose, where a major challenge is the uncertainty in the active material thickness, necessitating calibration. A homogeneously incorporated inert infrared (IR) dye, which must also be stable in ambient conditions and against radiolysis, can be added to the radiochromic film to enable optical calibration. Purpose This study investigates four commercial cyanine‐based dyes (IR‐783, IR‐806, IR‐868, and IR‐880) for use as an optical calibrant in fiber‐optic radiochromic dosimeters. Methods All dyes were dissolved in water to confirm solubility. IR‐783 and IR‐806 were dissolved in 10% w/w gelatin solution and coated onto a polyester substrate, which were then sandwiched between two layers of adhesives forming IR‐783 and IR‐806 films. A second batch of IR dyes in gelatin incorporated the LiPCDA, and was coated onto substrate and sandwiched between adhesive to form IR dye + LiPCDA films. The absorbance spectra of the films were measured periodically (176 and 102 days for IR‐dye films, and IR dye + LiPCDA, respectively). The average percentage absorbance, normalized to day 1, was fit to either a single or a double exponential decay model to calculate the spectral stability lifetime (τ1, τ2). Films were irradiated using a 6 MV LINAC beam with a standard setup of 100 source to axis distance (SAD), 10 cm × 10 cm field size and 1.5 cm depth. The change in absorbance of the IR‐dye + LiPCDA films were measured after they were irradiated to 1, 2, 5, 10, and 20 Gy at 3 Gy/min. Results Only IR‐783 and IR‐806 were sufficiently water soluble. In gelatin matrix, these dyes demonstrated a decrease in absorbance with time for IR‐783 and IR‐806 dyes, with IR‐783 films having an average τ1 = 73 ± 7 days and IR‐806 films τ1 = 7 ± 3 days. When combined with LiPCDA, IR‐806 degraded, losing its original peak at ∼820 nm. Similarly, IR‐783, combined with LiPCDA, showed signs of degradation; however, its original absorbance peak was still observed at ∼800 nm. In the IR‐783 + LiPCDA films, the IR‐783 dye had a τ = 4 ± 1 days, an order of magnitude faster than the IR‐783 with no LiPCDA films. When exposed to x‐ray irradiation, the IR‐783 dye in the IR‐783 + LiPCDA films showed no change in absorbance with increasing absorbed dose. In contrast, the LiPCDA in the films responded as expected, increasing in optical density with increased absorbed dose. Conclusions IR‐783 and IR‐806 dyes were observed to degrade over time following exponential decay curves. IR‐806 could not be combined with the LiPCDA without degrading. The combination of IR‐783 with LiPCDA demonstrated single exponential decay behavior at a comparatively faster rate than films that did not have LiPCDA. IR‐783 was insensitive to ionizing radiation and thus may be suitable for thickness correction, but an alternative manufacturing procedure may need to be developed.

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