GAFCHROMIC EBT photospectral dose response dependence on temperature and implications for flat bed scanning.

PURPOSE: Various forms of GAFCHROMIC film have been used for several years as radiographic media for measuring dose distributions. GAFCHROMIC EBT (GC-EBT) film is particularly useful for clinical dose ranges. This thin film dosimeter develops a blue color (lambdamax approximately 635 nm) when irradiated with ionizing radiation.
METHODS: Temperature controlled photospectrometry was used to assess temperature related readout changes in GC-EBT type film dosimetry. For observing clinical impact of findings, multiple scan studies with rising flat bed scanner temperature were performed.
RESULTS: The whole optical spectrum in the observed wavelength range of 450-700 nm shows a distinct spectrum shift linear with temperature toward lower wavelengths when readout temperature is increased. In addition, absorption decreases in maximum regions and increases in minimum regions of the absorbance spectrum. The most pronounced wavelength dependent readout differences occur in the slopes of the spectrum curves. Absorption readout differences of -1%/degree C for a 2.7 Gy irradiated film piece at a readout wavelength of approximately 650 nm can be found. The readout difference is strongly dependent on readout light spectral characteristic, irradiation dose, and temperature. Readout difference can be positive or negative. Characteristic temperature behavior patterns are present for each color channel of a flat bed scanner. All described effects are reversible within the measurement accuracy of this study.
CONCLUSIONS: When using unsuitable readout light, careful control of the readout temperature is necessary in order to obtain consistent and accurate results. Adapted GC-EBT type film dosimetry guidelines are presented. Temperature dependent readout differences on a flat bed scanner can be avoided when using scanner bed temperature as a fixed dosimetry parameter.