Program Information
Comparison of EBT3 Radiochromic Film Densitometry with Narrow Bandwidth LED Source in Point, Slot and Full Aperture Geometries
K Jordan1,2*, (1) London Regional Cancer Program,(2)University of Western Ontario,London, ON, Ca
Presentations
SU-F-T-583 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:Film densitometers optimized for radiochromic EBT3 film are not commercially available and flatbed document scanners are limited by broad spectral overlap of the red green and blue channels and polarization effects. Non-polarized, narrow bandwith, intensity stabilized light emitting diode (LED) light sources were constructed and transmission measurements compared for point, slot and full aperture geometries.
Methods:LED’s with 10 nm bandwidths were focused to 1 mm spots for point scanning. The LED’s back-illuminated a 6 mm diameter diffuser near the focal point of a 25 cm diameter Fresnel lens for slot and full aperture transmission imaging. Films were located at the aperture plane, 15 cm from the lens and imaged with a 16 bit digital camera with 50 mm lens. Sheets, 10x8 cm, were irradiated to 25 Gy with a 4x4 cm 6 MV photon beam. Transmission measurements with a 633nm, linearly polarized laser were used as low-acceptance angle reference geometry.
Results:LED intensity stabilized to <1% within one minute of startup. The useful transmission range increased as the acceptance angle decreased, laser(~2%)> spot(~3%)> slot(~5%)> full aperture(~10%). The Fresnel lens introduced a 4-fold polarization symmetry that increased radially causing a 2% change in transmission at 10 cm from the optic axis for EBT3 film. Polarized spot densitometer and laser transmission were similar.
Conclusion:Spectrally filtered LED sources were effective for transmission measurements of radiochromic films. Because of polarization sensitivity of EBT3 film, the scanning spot geometry is the most general for measuring unpolarised transmission of larger film sizes, ie > 15x15 cm. Due to EBT3 film scatter, spot scanning should provide the most accurate profiles of high dose gradients, for a given acceptance angle. The spot scanning densitometer provided useful transmission to ~5, 25, >25 Gy with 635, 590 and 530 nm light respectively.
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