Effect of mirror system and scanner bed of a flatbed scanner on lateral response artefact in radiochromic film dosimetry

Physical and Engineering Sciences in Medicine https://doi.org/10.1007/s13246-024-01478-x SCIENTIFIC PAPER Effect of mirror system and scanner bed of a flatbed scanner on lateral response artefact in radiochromic film dosimetry Tarafder Shameem1,2 · Nick Bennie1 · Martin Butson2,3 · David Thwaites2 Received: 13 March 2024 / Accepted: 16 August 2024 © The Author(s) 2024 Abstract Radiochromic film, evaluated with flatbed scanners, is used for practical radiotherapy QA dosimetry. Film and scanner component effects contribute to the Lateral Response Artefact (LRA), which is further enhanced by light polarisation from both. This study investigates the scanner bed’s contribution to LRA and also polarisation from the mirrors for widely used EPSON scanners, as part of broader investigations of this dosimetry method aiming to improve processes and uncertainties. Alternative scanner bed materials were compared on a modified EPSON V700 scanner. Polarisation effects were investigated for complete scanners (V700, V800, on- and off-axis, and V850 on-axis), for a removed V700 mirror system, and independently using retail-quality single mirror combinations simulating practical scanner arrangements, but with varying numbers (0–5) and angles. Some tests had no film present, whilst others included films (EBT3) irradiated to 6 MV doses of 0–11.3 Gy. For polarisation analysis, images were captured by a Canon 7D camera with 50 mm focal length lens. Different scanner bed materials showed only small effects, within a few percent, indicating that the normal glass bed is a good choice. Polarisation varied with scanner type (7–11%), increasing at 10 cm lateral off-axis distance by around a further 6%, and also with film dose. The V700 mirror system showed around 2% difference to the complete scanner. Polarization increased with number of mirrors in the single mirror combinations, to 14% for 4 and 5 mirrors, but specific values depend on angles and mirror quality. Novel film measurement methods could reduce LRA effect corrections and associated uncertainties. Keywords Radiotherapy dosimetry · Radiochromic film · GafChromic · EPSON scanner · Film dosimetry · Mirror effect · Scanner bed effect Introduction Radiochromic film is often used for two-dimensional dose measurement in radiotherapy. Its high spatial resolution, weak energy dependence and near tissue equivalency make it popular for patient-specific quality assurance (QA) of complex radiotherapy treatment techniques (e.g. IMRT, VMAT, SABR/SBRT) [1–5]. Digitization of optical density of irradiated films is commonly done using commercial flatbed scanners, as they are inexpensive and can produce very Tarafder Shameem 1 North Coast Cancer Institute, Lismore, NSW, Australia 2 Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW, Australia 3 EPA, Sydney, NSW, Australia high-resolution images; for practical clinical work, EPSON scanners are commonly used [3], [12–15]. However, the dosimetry system of radiochromic film coupled with a commercial flatbed scanner has some drawbacks as for any other dosimetry system. The orientation effect, of film to scanner bed, and the lateral response artefacts (LRA) are the two main issues associated with the system [6–15]. A strict protocol of marking and placing the film in the same orientation throughout the dosimetry process eliminates the orientation effect. The LRA effect is the change of measured optical density from middle to side of the film, orthogonal to the scanner’s light source travel direction [8, 11], [16–19]. It remains as a major issue which has been investigated widely [11], [17–21]. The origin of the LRA effect comes from two different sources, the film itself and various components of the flatbed scanner. The needle-like crystal structure [11, 22, 23] in the active layer of radiochromic films introduces anisotropic scattering and polarization of light [23]. Upon 13 Physical and Engineering Sciences in Medicine irradiation, the neighbouring polymers create bonds and turn into even longer rods, which enhances both of these phenomena. The lens system, scanner bed and mirror system of flatbed scanners [13] are the components that contribute towards LRA in different ways. The lens system fails to collect all the light from distant parts of the film [10, 15]. The difference of refractive indices of film and scanner bed can cause a path length effect which reduces optical densities at the distant part of the film. The co-efficient of reflection changes with the incident angle of light on the mirror system components [12] and the mirror system introduces light polarization [8, 12, 13]. The magnitude of light polarization, introduced by the scanner and the film, increases with increasing lateral distance from the centre of the scanner [12]. Other studies also considered light polarization resulting from various films [14] and film-scanner combinations [8, 13]. There is little data in the literature on the relative effects from the different scanner components. The lens system was investigated by Shameem et al. [15] within the current project. Van Battum et al. [12] investigated the path length effect. They stated that “The film-induced optical path length variation becomes relevant if its refraction index differs from that of the glass plate of the flatbed scanner”. Schoenfeld et al. [13] provided a discussion of the theoretical background of the pathlength effect and the role of mirrors in LRA caused by light incident angle affecting the co-efficient of reflection. Van Battum et al. [12] investigated light polarisation caused by some film-scanner combinations, with most experiments reported for an EPSON XL10000 scanner. A schematic diagram of the mirror system used in the EPSON 10000XL is provided in [10] and is redrawn here (Fig. 1), showing the light travel path through the mirror system and the positions and angles of the mirrors to each other and to the incident light path. It may be noted that the 10000XL is a bigger scanner (A3) than the A4 scanners often used in practical dosimetry applications and has a five-mirror system, whereas the V700, V800 and V850 scanners considered in the current work are A4 scanners and use four-mirror systems. The purpose of this work was to investigate the scanner bed path-length effect and also the polarization effect caused by mirrors, but using independent novel approaches to those previously reported [12, 13] in the literature and for these often-used A4 scanners. The relative path length effect caused by the scanner bed was considered by comparing a range of materials having different refractive indices from that of film or glass. The measurement of the polarisation effect of mirror systems followed two approaches, one considering effects from the complete scanner system, using a method directly comparable to the previous EPSON XL10000 A3 scanner study [12], but here applied to currently-used A4 scanners, and the ot (...truncated)