Effect of scanner lens on lateral response artefact in radiochromic film dosimetry

Abstract

Radiochromic film is a good dosimeter choice for patient QA for complex treatment techniques because of its near tissue equivalency, high spatial resolution and established method of use. Commercial scanners are typically used for film dosimetry, with Epson scanners being the most common. Radiochromic film dosimetry is not straightforward having some well-defined problems which must be considered, one of the main ones being the Lateral Response Artefact (LRA) effect. Previous studies showed that the contributing factors to LRA are from the structure of the active ingredients of the film and the components and construction of the flatbed scanner. This study investigated the effect of the scanner lens on the LRA effect, as part of a wider investigation of scanner design effects and uncertainties. Gafchromic EBT3 films were irradiated with 40 × 40 cm2 field size 6 MV beams. Films were analysed using images captured by a Canon 7D camera utilising 18 mm, 50 mm and 100 mm focal length lenses compared to images scanned with a conventional Epson V700 scanner. The magnitude of the LRA was observed to be dependent on the focal length of the lens used to image the film. A substantial reduction in LRA was seen with the use of the 50 mm and 100 mm lenses, by factors of 3–5 for the 50 mm lens and 4–30 for the 100 mm lens compared to conventional desktop scanner techniques. This is expected to be from the longer focal length camera lens system being able to collect more light from distant areas compared to the scanner-based system. This provides an opportunity to design film dosimetry systems that minimise this artefact.

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

“…With a smaller number of mirrors, the effect was observed to be less; two or three mirror systems, showing about 5% light polarisation, whereas a one mirror system had no signi cant effect on light polarisation. This further supports the conclusions from the work evaluating lens effects in lm dosimetry [15], that a re-designed direct imaging system could potentially improve overall lm dosimetry by signi cantly reducing the need for LRA effect corrections, although this would increase system size.…”

“…Path Length Effect: The lm preparation, handling and irradiation methods were similar to those described in our previous work [15], [24]. Full detail can be obtained there, but essential detail is repeated here for completeness.…”

“…In ImageJ an average pro le was generated across each lm for each combination using a rectangular ROI cropped 1mm in from the lm edge [15,24]. The average pro les for each dose level, each orientation, each bed type and each colour channel were analysed in MS Excel where they were normalised to the mean of the central 100 data points.…”

“…The lens system, scanner bed and mirror system of atbed scanners [13] are the scanner components that contribute towards LRA in different ways. The lens system fails to collect all the light from distant parts of the lm [10], [15], the difference of refractive indices of lm and scanner bed causes a path length effect which reduces optical densities at the distant part of the lm [12] and the mirror system introduces light polarization [12], [13]. The magnitude of light polarization, introduced by the scanner and the lm, increases with increasing lateral distance from the centre of the scanner [12].…”

“…The effect of the lens system was investigated by Shameem et al [15]. Van Buttom et al [12] investigated the path length effect and light polarisation caused by the scanner.…”