SU‐FF‐T‐121: Characterization and Real‐Time Measurements of Optical Density with GafChromic EBT Film

Abstract

Purpose: Over the last few decades, various groups have investigated optical fiber‐based dosimeters for in vivo measurements. Recently, two radiochromic films have also been considered for real‐time in vivo point‐based optical dosimetry, and both GafChromic MD‐55 and GafChromic EBT films faired well in several criteria. However, GafChromic MD‐55 was shown to have the change in optical density depend both on dose rate and on temperature, as measured immediately at the end of a given exposure. To continue with the search for a suitable medium for real‐time fiber‐based in vivo dosimetry, GafChromic EBT film is also being investigated for dose rate and temperature effects. Method and Materials: 1 cm × 1cm pieces of film were irradiated with a 6 MV beam within a 30 cm × 30 cm × 10 cm Solid Water phantom fitted with optical fibers for real‐time measurements, using a 10 cm × 10 cm field at SAD, 100 cm SAD, and 1.5 cm depth. Each exposure delivered a dose in 5 to 1000 cGy range at a dose rate in 14 to 520 cGy/min range. Changes in OD obtained immediately at the end of each exposure were compared. The percent uncertainty in ΔOD for a given dose was calculated by using all the values obtained, regardless of the dose‐rate used. Results: The percent uncertainty ranged between 1.8% and 3.6%, with an average of 2.8%. The expected variation in ΔOD within a single sheet of GafChromic EBT film for a given dose is 1.5% (two standard deviations). The average increase in uncertainty is 4%. Conclusion: An increase in uncertainty of real‐time ΔOD measurements is observed when variations in dose‐rate are introduced for the doses in 5 to 1000 cGy range, and dose‐rates between 14 and 520 cGy/min.