Validating Acuros XB with heterogeneous phantoms: can film orientation have an impact?

Abstract

The aim of this study was to measure doses using films placed parallel and perpendicular to a 6MV radiation beam in heterogeneous phantoms, and to compare these doses to those calculated using the dose calculation algorithms Acuros XB and Analytical Anisotropic Algorithm (AAA). A heterogeneous slab phantom composed of SB5 bone equivalent material sandwiched between WTe solid water, and another composed of cork between WTe solid water were studied. Dose measurements were carried out using Gafchromic EBT-3 and EBT-XD films placed parallel to the radiation beam through the centre of the phantom setup. Each film was then placed perpendicular to the beam, at four depths along the beam axis, with a similar method reported by Delbaere et al. Each phantom was CT scanned and a 3x3cm2 beam was calculated using Acuros XB dose-to-medium/ dose-to-water and AAA. The results showed agreement between the film measurements and dose calculations in the solid water and cork regions of the phantoms. However, deviations were observed in the bone where vertical film measurements agreed with doses calculated by Acuros dose-to-water, whereas horizontal film measurements agreed with AAA calculations. Horizontal and vertical film doses differed by up to 8%. Acuros dose-to-medium calculations were not in agreement with either film orientation. In conclusion, the orientation of the film when measuring the dose delivered by a radiation beam does have an impact on the measured dose. However, for the horizontal film measurements there were air gaps on either side of the film which may have affected the results.Reference: Delbaere A, Younes T, Vieillevigne L. On the conversion from dose-to-medium to dose-to-water in heterogeneous phantoms with Acuros XB and Monte Carlo calculations. Phys Med Biol. 2019 Oct;64.