The artefacts of radiochromic film dosimetry with flatbed scanners and their causation by light scattering from radiation-induced polymers

Abstract

Optical experiments and theoretical considerations have been undertaken in order to understand the causes of the 'orientation effect' and the 'parabola effect', the artefacts impairing the desired light absorption measurement on radiochromic EBT3 films with flatbed scanners. EBT3 films exposed to doses up to 20.9 Gy were scanned with an Epson Expression 10000XL flatbed scanner in landscape and portrait orientation. The horizontally and vertically polarized light components of the scanner were determined, and another Epson Expression 10000XL flatbed scanner was disassembled to examine its optical components. The optical properties of exposed and unexposed EBT3 films were studied with incident polarized and unpolarized white light, and the transmitted red light was investigated for its polarization and scattering properties including the distribution of the scattering angles. Neutral density filters were studied for comparison. Guidance was sought from the theory of light scattering from rod-like macromolecular structures. The drastic dose-dependent variation of the transmitted total light current as function of the orientation of front and rear polarizers, interpreted by light scattering theory, shows that the radiation-induced polymerization of the monomers of EBT3 films produces light scattering oscillators preferably polarized at right angles with the coating direction of the film. The directional distribution of the scattered light is partly anisotropic, with a preferred scattering plane at right angles with the coating direction, indicating light scattering from stacks of coherently vibrating oscillators piled up along the monomer crystals. The polyester carrier film also participates in these effects. The 'orientation' and 'parabola' artefacts due to flatbed scanning of radiochromic films can be explained by the interaction of the polarization-dependent and anisotropic light scattering from exposed and unexposed EBT3 films with the quantitative difference between the scanner's horizontally and vertically polarized light supply and with the limited directional acceptance of the scanner's light recording system.

Modeling the dose dependence of the vis-absorption spectrum of EBT3 GafChromic™ films

“…Another observation that can be made for dose levels over 10 Gy is the increase in net absorbance at wavelengths far from the absorption peaks, for instance in the region below 450 nm. This rise in absorbance can be attributed to light scattering by PDA polymers, deflecting light from the optical fiber‐coupled detector . Because of the limited sensitivity, and the increasing contribution of scattering, only dose levels from 0 to 10 Gy will be considered in the following dose dependence analysis.…”

“…Furthermore, light is scattered by the PDA polymers. The latter scattering effect can be described by the Rayleigh–Debye–Gans scattering theory as shown by Schoenfeld et al. Absorption and scattering by the protective layers, absorption by the marker dye, and light scattering by the monomer crystals can be considered as background features and can be eliminated by subtracting the UV‐vis absorption spectrum by the spectrum of a film that does not contain any polymers.…”

Technical Note: On GAFChromic EBT‐XD film and the lateral response artifact

“…Second, the LRA stems from the polarization of scanner illumination transmitted through RCF. 12,13 To reach the detector, the transmitted light is reflected from mirrors in the optical path. Owing to the dependence of reflectivity on the angle of incidence on the mirrors, there is less light reaching the detector from film located near the edge of the scan area than from film located in the center of the scan window.…”

Correcting scan‐to‐scan response variability for a radiochromic film‐based reference dosimetry system

“…Fortunately, processes and protocols have been developed and described to deal with these issues and they should be in routine use to eliminate the unreliabilities. 14,15,[31][32][33][34][35][36][37][38][39] However, there has been yet another factor not thoroughly addressed thus far, which is scan-toscan variability inherent to scanning or readout process. The current study seeks to build on, and reinforce, the previous work introducing the use of reference films 35 to address scanto-scan variability, but omitting the detailed investigation of the root causes.…”