164. Dose evaluation and image quality assessment of mega-voltage CT Tomotherapy acquisitions

Abstract

PurposeTo assess dose and image quality in phantom with HiArt TomoTherapy and to evaluate the feasibility to acquire pre-treatment images with a pitch Coarse with a 3 mm slice thickness (ST) instead of the currently used 6 mm ST. MethodsMultiple Scan Average Dose (MSADw) was calculated according to the literature using a Exradin ion-chamber in a TomoPhant placed at the isocenter. Measurements were repeated 5 times for each combination pitch -ST reconstruction (Fine 1 and 2 mm, Normal 2 and 4 mm and Coarse 3 and 6 mm) for 2 different scan lengths (5 and 10 cm).Image quality was assessed scanning a Catphan500 for 5 times with each combination of pitch and ST and the collected data were evaluated in terms of contrast linearity, signal uniformity, noise, Signal-to-noise (SNR) ratio. In addition, spatial resolution was evaluated both qualitatively and quantitatively. ResultsMSADw for the 10-cm scan length was of 1.03 ± 0.01 cGy for pitch Coarse, 1.48 ± 0.01 cGy for pitch Normal and 2.83 ± 0.01 cGy for pitch Fine. We found slight lower values for the 5-cm scan length (0.94 ± 0.01 cGy, 1.36 ± 0.01 cGy and 2.67 ± 0.01 cGy for pitch Coarse, Normal and Fine, respectively) due to the lower scattered radiation.Considering image quality, signal uniformity was comparable among the different pitch-ST combinations; noise ranged from 2.4% to 2.5% with no significant differences changing pitch or ST. Signal-to-noise ratio ranged from 40.0 ± 2.0 from pitch Coarse-6 mm to 42.1 ± 1.2 for Normal-4 mm but we found no statistical significance in these differences.MTF50% was of 3.3 lp/cm for Coarse, 2.7 lp/cm for Normal and 2.5 lp/cm for Fine. Anyway these differences were not empirically detectable on the images, in which we observed up to 4 lp/cm for all the pitch-ST combinations. ConclusionsThe acquisitions of pre-treatment images with pitch Coarse and 3 mm ST reconstruction is feasible and justified according to our results.