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Purpose/Objective
Current Gafchromic EBT, EBT2 and EBT3 (GC-EBT-x) dosimetry protocol improvements focus on evaluation algorithms and streamlining dosimetry work-flow. At the same time the scanning part of the dosimetry process mostly remains unchanged. Focus of this work is to demonstrate improvements in the scanning part of GC-EBT-x Dosimetry. We identify undefined readout temperature and scan-timing as source of systematic uncertainty. We further demonstrate a substantial improvement in reproducibility when using wet scan fluid for scanning.
Materials and Methods
For film scanning an Epson V700pro flat bed scanner in transparency mode is used. The scanner bed temperature is monitored with a Voltcraft K202 data logger thermometer with two temperature probes situated on the head side and on the foot side of the scanner bed. Systematic control of absolute temperature and temperature balance between head and foot side of the scanner was achieved with defining rigidly timed surface- temperature based warmup- and scanning procedures. For the wet scan procedure commercial wet scan fluid was used.
Results
For EBT film already observed systematic and reversible temperature dependent readout change can be reproduced for EBT2 and EBT3 film. Systematic readout change of 0.3% for 10 scans on flat bed scanner occurs, when unirradiated film is evaluated. Timing of warmup and evaluation scans is observed to influence temperature distribution of the scannerbed. Fig.1 illustrates a scanner readout value change sole in the upper part of the scan area with an amount of 1% different readout for 10 min of waiting without moving the scanner lamp. When choosing a reference temperature for dosimetry and using an always moving lamp as part of the scanning protocol uncertainties can be minimised. Scan reproducibility of temperature controlled inter-day scan sessions we observe to be 0.7%. This compares well to previously published results for reproducibility for not temperature controlled scanning protocols (Fuss et. al. PMB 52 2007). With the use of scan fluid the reproducibility of inter-day scan sessions can be reduced to 0.31%
Conclusions
Systematic temperature influences translate into dose errors via absolute and relative temperature on the scanner-bed. The errors introduced by the unreproducible optical path when scanning dry override benefit of temperature control for a reference ROI in the middle of the scanner-bed. Nevertheless the described relative dose errors make the use of temperature as a dosimetry parameter necessary. The definition of a constant reference temperature and a strict scanning protocol with always moving lamp minimises the described sources of error. Wet scanning is demonstrated to bring clear improvement in reproducibility of GC-EBT-x dosimetry.