ESTRO 2021 Abstract Book

S234

ESTRO 2021

Materials and Methods This work was conducted using Elekta Versa HD linear accelerators and clinical plans were calculated in Raystation v.7.0 (Raysearch, Sweden). Energies studied were 6MV, 10MV, 6FFF & 10FFF with clinical plans 6 MV & 6FFF. The detector was integrated into the myQA SRS phantom or in water-equivalent Scanplas. Field and plan measurements were compared to Raystation calculations using gamma analyses and compared with commissioned plan verification systems e.g. radiochromic film & the MatriXX 2D array (IBA Dosimetry). Chamber measurements were used to assess dose linearity, dose-rate dependence and field size dependence. Planning system calculations & radiochromic film were used to assess off-axis square fields and step & shoot off-axis stripes. Measurement of clinical plans for a variety of beam energies and sites was assessed. Errors were simulated within a selection of delivered treatment plans to assess the detector’s sensitivity. This included removal of segments, and introduced errors in monitor units, MLC positions and energies. Results The response to dose showed good linearity (r 2 =1). The mean normalised detector signals across the range of dose rates were 96.3% ± 1.9% and 96.0% ± 2.1%, for 6FFF and 10FFF, respectively, and 97.1% ± 2.0% and 97.9% ± 1.3%, for 6MV and 10MV, respectively. Field size dependence agreed within 1.1% of chamber measurements above 1cm field sizes; and the detector showed good short term stability. For small off-axis squares and stripes, the gamma analysis pass rate was typically 100% (1%, 1mm). The measurement of clinical plans with the detector in the Scanplas demonstrated gamma pass rates of 99.7 – 99.9% (2%, 2mm), and 97.6% - 98.6% (1%, 1mm) when compared to Raystation dose distributions.

Fig. 1 shows the calculated (top) and measured (below) doses for a prostate plan and Fig. 2 shows the profiles across the calculated (top) and measured (below) dose distributions with the corresponding gamma map for a prostate plan (1%, 1mm).