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Purpose: When comparing different scans of the same radiochromic film,several patterns can be observed. These patterns are caused by differentsources of uncertainty, which affect the repeatability of the scanner. Thepurpose of this work was to study these uncertainties.  Methods: The variance of the scanner noise, as a function of the pixelposition, was studied for different resolutions. The inter-scan variability ofthe scanner response was analyzed taking into account spatial discrepancies.Finally, the distance between the position of the same point in different scanswas examined.  Results: The variance of noise follows periodical patterns in both axes,causing the grid patterns. These patterns were identified for resolutions of50, 72 and 96 dpi, but not for 150 dpi. Specially recognizable is thesinusoidal shape with a period of 8.5 mm that is produced with 72 dpi.Inter-scan variations of the response caused systematic relative dosedeviations larger than 1% in 5% of the red channel images, 9% of the green and51% of the blue. No systematic deviation larger than 1% was found afterapplying response corrections. The initial positioning and the speed of thescanner lamp vary between scans.  Conclusions: Three new sources of uncertainty, which influence radiochromicfilm dosimetry with flatbed scanners, have been identified and analyzed in thiswork: grid patterns, spatial inter-scan variations and scanning readingrepeatability. A novel correction method is proposed, which mitigates spatialinter-scan variations caused by deviations in the autocalibration of theindividual Charge Coupled Device detectors.

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