210. GLAaS absolute dose calibration algorithm with an Elekta iViewGT Electronic Portal Imaging Device (EPID). (December 2018)

• Record Type: Journal Article
• Authors: Vanetti, E.; Esposito, M.; Nicolini, G.
• Is Part Of: Physica medica. Volume 56(2018)Supplement 2
• Page Start: 189
• Page End: 190
• Publication Date: 2018-12
• Subjects: Medical physics -- Periodicals; Biophysics -- Periodicals; Biophysics -- Periodicals; Imagerie médicale -- Périodiques; Radiothérapie -- Périodiques; Rayons X -- Sécurité -- Mesures -- Périodiques; Physique -- Périodiques; Médecine -- Périodiques; 610.153
• DOI: 10.1016/j.ejmp.2018.04.221
• Languages: English
• ISSNs: 1120-1797

Abstract

Abstract : Purpose: Radiotherapy with intensity modulation (IMRT and VMAT) implies the setup of adequate QA programs to guarantee safe treatments. Concerning this issue, amorphous silicon EPID technologies are particularly interesting due to their high spatial resolution, large area, stability, dynamic range, and real-time acquisition capabilities. In this context, the GLAaS[1] algorithm has proved capable to meet a variety of applications with a simple direct calibration process to convert raw measurements into absorbed-dose-to-water; nevertheless, GLAaS was never validate with a different technology apart from Varian-EPIDs. To test its feasibility out of single manufacture environment, GLAaS was applied to Elekta iViewGT images. Methods: GLAaS accounts for pixel-by-pixel response changes in time differentiating on field and/or segment sizes and beam quality, i.e. primary or transmitted radiation (e.g. below MLC), basis. Dosimetric data from a 6 MV Elekta-Synergy were collected at dmax and SDD = 160 cm (EPID position), for primary and transmitted radiation to be correlated with iViewGT pixel reading (R). For open and modulated fields, comparison to TPS dose maps (Monaco-XVMonteCarlo) was assessed through profiles and gamma analysis. Results: For a given beam, the response of iViewGT was confirmed to be linear (D(Gy) = m ∗ R + q), as already established for Varian EPIDs. Similarly, the variable slope response to primary radiation was modelled according to the following empirical algorithm: m pr ( OF ) = a · OF + b; OF ( EwwF ) = [ c + d · ln ( EwwF ) - 1 where EwwF is the equivalent field size of each segment based on IC data and OF is the EPID reading measured output factor. For transmitted radiation the relation mtr = k · mpr was adopted. Conclusions: The extension of GLAaS to iViewGT offers the opportunity to easily set-up flexible and reliable verification, also in highly demanding conditions as for SRS and SBRT treatments; the compatibility with different technologies could encourage multi-centers studies concerning treatment delivery and dose calculation problematics.