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

Abstract

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.

Dose–response linearization in radiochromic film dosimetry based on multichannel normalized pixel value with an integrated spectral correction for scanner response variations

“…Results showed that the use of a specific control mitigated the high variation seen with the global control. This observation agreed with conclusions from Lewis and Devic and Mendez et al where they recommended the inclusion of a reference material during any scan. Furthermore, with the “fingerprint” correction protocol was shown to reduce the scanner variation consistently at all examined dose levels, which could be an indication for the potential effect of continuous scanning medium on the film signal.…”

“…In this study, only results from the 12000XL scanner were reported since other studies already addressed xenon fluorescent lamp‐based scanners . The changes seen in Fig.…”

“…(), the same conditions during calibration and measurement have to be maintained. This is not always the case even if the user followed a specific protocol diligently . The reasons for this are mainly attributed to changes in the film and scanner responses.…”

PRIMO Monte Carlo software benchmarked against a reference dosimetry dataset for 6 MV photon beams from Varian linacs

“…A calibration curve (pixel value–dose) from 0 to 5 Gy was established for each measurement session. The unexposed pieces are used in https://www.radiochromic.com to account for inter–scan variations [ 17 ]. For each IMRT/SBRT case, doses at the central point indicated by a cross-hair passing through the four marks showing the linac cross-hair was obtained for the four exposed films.…”

Comparison of three film analysis softwares using EBT2 and EBT3 films in radiotherapy

“…10 Considerable warm up effect was not observed for our scanner, but before every scanning we waited at least fifteen minutes, and the first five scanned images were never used. 11 Every pixel in a colour image has three-channel (RGB) image signal. The scanned images can be evaluated by film analysis software.…”