Impact of backscattered radiation from the bunker structure on EPID dosimetry

Abstract

Amorphous silicon electronic portal imaging devices (EPIDs) have been investigated and used for dosimetry in radiotherapy for several years. The presence of a phosphor scintillator layer in the structure of these EPIDs has made them sensitive to low‐energy scattered and backscattered radiation. In this study, the backscattered radiation from the walls, ceiling, and floor of a linac bunker has been investigated as a possible source of inaccuracy in EPID dosimetry. EPID images acquired in integrated mode at discrete gantry angles and cine images taken during arcs were used with different field setups (18×18 and 10×10cm2 open square fields at 150 and 105 cm source‐to‐detector distances) to compare the EPID response at different gantry angles. A sliding gap and a dynamic head‐and‐neck IMRT field and a square field with a 15 cm thick cylindrical phantom in the beam were also investigated using integrated EPID images at several gantry angles. The contribution of linac output variations at different angles was evaluated using a 2D array of ion chambers. In addition, a portable brick wall was moved to different distances from the EPID to check the effect at a single angle. The results showed an agreement of within 0.1% between the arc mode and gantry‐static mode measurements, and the variation of EPID response during gantry rotation was about 1% in all measurement conditions.PACS numbers: 87.56.bd; 87.56.J‐; 87.53.Bn; 87.56.Da

A Monte Carlo calculation model of electronic portal imaging device for transit dosimetry through heterogeneous media

“…Such model, therefore, in principle could be vulnerable to changes in phantom and air gap thicknesses and the related scattering contributions. 20,21 A simplified and fast calculation model of EPID based on MC transport for transit dosimetry was introduced by Jung et al 22 The model incorporates structural and elemental compositions of EPID (aS1000) by homogenized (i.e., water) layers in the XVMC code. 23 To account for heterogeneity in the actual composition of EPID structures, each homogeneous layer was assigned the physical density of the corresponding structure.…”