Po-Thur Eve General-28: Characterization and Real-Time Measurements of Optical Density with GafChromic EBT Film

Abstract

To establish whether GafChromic EBT medium is suitable for real‐time fibre‐based in vivodosimetry, we are investigating whether dose‐rate and temperature fluctuations introduce extraneous uncertainties to change in optical density (ΔOD) measurements. 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 fibres 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 of 5, 10, 25, 50, 100, 250, 500 or 1000 cGy at a dose‐rate of 14, 32, 65, 130, 260 or 520 cGy/min. Change 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. 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). This suggests that there is an increase in uncertainty of real‐time ΔOD measurements when variations in dose‐rate are introduced for the doses in 5–1000 cGy range, and dose‐rates between 14 and 520 cGy/min. The average increase in uncertainty is 4%.