  Purpose: When comparing different scans of the same radiochromic film,
several patterns can be observed. These patterns are caused by different
sources of uncertainty, which affect the repeatability of the scanner. The
purpose of this work was to study these uncertainties.
  Methods: The variance of the scanner noise, as a function of the pixel
position, was studied for different resolutions. The inter-scan variability of
the scanner response was analyzed taking into account spatial discrepancies.
Finally, the distance between the position of the same point in different scans
was examined.
  Results: The variance of noise follows periodical patterns in both axes,
causing the grid patterns. These patterns were identified for resolutions of
50, 72 and 96 dpi, but not for 150 dpi. Specially recognizable is the
sinusoidal shape with a period of 8.5 mm that is produced with 72 dpi.
Inter-scan variations of the response caused systematic relative dose
deviations larger than 1% in 5% of the red channel images, 9% of the green and
51% of the blue. No systematic deviation larger than 1% was found after
applying response corrections. The initial positioning and the speed of the
scanner lamp vary between scans.
  Conclusions: Three new sources of uncertainty, which influence radiochromic
film dosimetry with flatbed scanners, have been identified and analyzed in this
work: grid patterns, spatial inter-scan variations and scanning reading
repeatability. A novel correction method is proposed, which mitigates spatial
inter-scan variations caused by deviations in the autocalibration of the
individual Charge Coupled Device detectors.
