How flatbed scanners upset accurate film dosimetry

Abstract

Film is an excellent dosimeter for verification of dose distributions due to its high spatial resolution. Irradiated film can be digitized with low-cost, transmission, flatbed scanners. However, a disadvantage is their lateral scan effect (LSE): a scanner readout change over its lateral scan axis. Although anisotropic light scattering was presented as the origin of the LSE, this paper presents an alternative cause. Hereto, LSE for two flatbed scanners (Epson 1680 Expression Pro and Epson 10000XL), and Gafchromic film (EBT, EBT2, EBT3) was investigated, focused on three effects: cross talk, optical path length and polarization. Cross talk was examined using triangular sheets of various optical densities. The optical path length effect was studied using absorptive and reflective neutral density filters with well-defined optical characteristics (OD range 0.2-2.0). Linear polarizer sheets were used to investigate light polarization on the CCD signal in absence and presence of (un)irradiated Gafchromic film. Film dose values ranged between 0.2 to 9 Gy, i.e. an optical density range between 0.25 to 1.1. Measurements were performed in the scanner's transmission mode, with red-green-blue channels. LSE was found to depend on scanner construction and film type. Its magnitude depends on dose: for 9 Gy increasing up to 14% at maximum lateral position. Cross talk was only significant in high contrast regions, up to 2% for very small fields. The optical path length effect introduced by film on the scanner causes 3% for pixels in the extreme lateral position. Light polarization due to film and the scanner's optical mirror system is the main contributor, different in magnitude for the red, green and blue channel. We concluded that any Gafchromic EBT type film scanned with a flatbed scanner will face these optical effects. Accurate dosimetry requires correction of LSE, therefore, determination of the LSE per color channel and dose delivered to the film.

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams

“…A frame, cut out from a transparency sheet was employed to place films in a reproducible and cantered position on the scanner. Whenever there was a gap between the frame and the film pieces along the axis parallel to the lamp, it was closed with idle film pieces in order to minimize the cross‐talk effect . Before acquisitions and after long pauses, five empty scans were taken to stabilize the lamp.…”

Auditing local methods for quality assurance in radiotherapy using the same set of predefined treatment plans

“…For absolute dose calibration using three colour channels [20], quarters of a film were irradiated in the audit array with 0, 200, 400 and 600 MU (∼0-3.8 Gy) with a 10 × 10 cm 2 field. The films were converted to dose according to the wellestablished local protocol of the VU University Medical Centre (Amsterdam, the Netherlands) [21,22].…”

Grid patterns, spatial inter-scan variations and scanning reading repeatability in radiochromic film dosimetry

“…Some of them involve only the film: for example, the thickness variations of the active layer 2 , the change in film darkening as a function of post-irradiation time 3 , the influence of humidity and temperature 4,5 , the UV-induced polymerization 6 , etc. Some other uncertainties are a consequence of the interaction of the characteristics of both the film and scanner: for example, the lateral artifact 7,8 , Newton rings 9 , the dependency with the orientation of the film on the scanner bed 10 , the cross talk effect 8 , the dependency on film-to-light source distance 11,12 , etc. Finally, other uncertainties are intrinsic to the scanner: for example, noise 13,14 , the inter-scan variability of the scanner response 11 , warming-up of the lamp 15,16 , differences between color channels 17-20 , etc.…”

“…GAFChromic EBT3 films were used in this study, in combination with the Epson Expression 10000XL scanner (Seiko Epson Corporation, Nagano, Japan). In the literature, the Epson Expression 10000XL scanner has been selected numerous times 3, 7,8,11,24,25 for radiochromic film dosimetry. In this work, the repeatability of this scanner has been examined.…”