Monte Carlo simulations of different CT X-ray energy spectra within CTDI phantom and the influence of its changes on radiochromic film measurements.

PURPOSE:In this work we use Monte Carlo simulations to investigate change in Computed tomography (CT) X-ray energy spectra between exposures in air and within CT dose index (CTDI) phantom. While the results of these simulations will be relevant when measuring CTDI with any dosimeter, we apply the appropriate beam quality change correction for CTDI measurements using XR-QA2 model GafChromic™ film. METHODS:Dose profiles were measured with film strips, sandwiched between acrylic rods cut in half, placed within CTDI phantoms and scanned before and after irradiation with document scanner in reflective mode. Reference dosimetry system was calibrated in terms of air kerma in air, which was converted into absorbed dose using ratio of mass-energy absorption coefficients water-to-air for a given beam quality, following the AAPM TG-61 protocol. RESULTS:Beam qualities for all film positions within CTDI phantom show beam softening for HVLs above 6 mm Al and beam hardening for HVLs bellow 6 mm Al. Calculated CTDI values using HVL in air for all CTDI positions, and those calculated using the appropriate calibration curves based on beam quality correction show for Head CTDI phantom differences ranging from 0.3% to 2.1% and for Body CTDI phantom from 2.5% to 5.7%. CONCLUSIONS:We describe method for CTDI measurements using radiochromic film dosimetry protocol corrected by the beam quality change within the phantom. Our results show differences in CTDI measurements of up to 5.7% when compared to using film calibration curves for beam quality in air.