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

Highlights

• Three new sources of uncertainty in radiochromic film dosimetry are presented.

• The variance of the scanner noise follows periodical patterns in both axes.

• Inter-scan variability depends on each autocalibration of the CCD detectors.

• Points in the film translate to different pixel coordinates in repeated scans.

• A novel method to correct for spatial inter-scan variations is proposed.

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.

Introduction

The system composed of radiochromic films and a flatbed scanner is the dosimeter of choice for many applications in radiology and radiation therapy [1]. This dosimetry system is affected by several sources of uncertainty. Some of them involve only the film: for example, the thickness variations of the active layer [2], the change in film darkening as a function of post-irradiation time [3], the influence of humidity and temperature [4], [5], the UV-induced polymerization [6], etc. Some other uncertainties are a consequence of the interaction of the characteristics of both the film and scanner: for example, the lateral artifact [7], [8], Newton rings [9], the dependency with the orientation of the film on the scanner bed [10], the cross talk effect [8], the dependency on film-to-light source distance [11], [12], etc. Finally, other uncertainties are intrinsic to the scanner: for example, noise [13], [14], the inter-scan variability of the scanner response [11], warming-up of the lamp [15], [16], differences between color channels [17], [18], [19], [20], etc.

Despite all those perturbations, GAFChromic films (Ashland Inc., Wayne, NJ) have been repeatedly found to be capable of delivering accurate dose measurements [20], [21], [22], [23]. Still, to further improve the accuracy of the dosimetry system, thorough knowledge of its uncertainties is necessary.

GAFChromic EBT3 films were used in this study, in combination with the Epson Expression 10000XL scanner (Seiko Epson Corporation, Nagano, Japan). In the literature, the Epson Expression 10000XL scanner has been selected numerous times [3], [7], [8], [11], [24], [25] for radiochromic film dosimetry. In this work, the repeatability of this scanner has been examined. As a result, three new artifacts have been identified and analyzed: grid patterns, spatial inter-scan variations and scanning reading repeatability.