Potential errors in optical density measurements due to scanning side in EBT and EBT2 Gafchromic film dosimetry.
OBJECTIVE
The purpose of this study is to determine the effect on the measured optical density of scanning on either side of a Gafchromic EBT and EBT2 film using an Epson (Epson Canada Ltd., Toronto, Ontario) 10000XL flat bed scanner.


METHODS
Calibration curves were constructed using EBT2 film scanned in landscape orientation in both reflection and transmission mode on an Epson 10000XL scanner. Calibration curves were also constructed using EBT film. Potential errors due to an optical density difference from scanning the film on either side ("face up" or "face down") were simulated.


RESULTS
Scanning the film face up or face down on the scanner bed while keeping the film angular orientation constant affects the measured optical density when scanning in reflection mode. In contrast, no statistically significant effect was seen when scanning in transmission mode. This effect can significantly affect relative and absolute dose measurements. As an application example, the authors demonstrate potential errors of 17.8% by inverting the film scanning side on the gamma index for 3%-3 mm criteria on a head and neck intensity modulated radiotherapy plan, and errors in absolute dose measurements ranging from 10% to 35% between 2 and 5 Gy.


CONCLUSIONS
Process consistency is the key to obtaining accurate and precise results in Gafchromic film dosimetry. When scanning in reflection mode, care must be taken to place the film consistently on the same side on the scanner bed.
Desroches
Joannie
J
Département de Radio-Oncologie, Centre hospitalier de l'Université de Montreal, Montréal, Quebec H2L 4M1, Canada.
Bouchard
Hugo
H
Lacroix
Frédéric
F
D002138:Calibration; D003198:Computer Simulation; D004307:Dose-Response Relationship, Radiation; D019544:Equipment Failure Analysis / Q000379:methods; D005373:Film Dosimetry / Q000379:methods*; D006801:Humans; D055095:Optics and Photonics; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D015203:Reproducibility of Results; D012984:Software