6. ScandiDos’s Discover system evaluation

Introduction ScandiDos present its Discover system as the first QA system that can verify irradiation parameters during treatments. Positionned on the head, Discover system equipped with a 4040 diodes matrix with a total thickness of 23 mm, is presented as beam transparent and highest resolution allowing detecting submillimeter MLC positioning errors. Discover system is FDA and CE approved. CGFL could realize a first evaluation of the system. Methods Discover was tested on a VARIAN TrueBeam with a 6 MV (TPR2010 = 0,666). The evaluation of the system was divided into 4 steps: -  Step 1: Handling (ergonomics, commissioning). -  Step 2: Beam attenuation measurements at 5 cm depth in water with DSD = 100 cm -  Step 3: Technical evaluation of Discover with 2 VMAT treatment plans. -  Step 4: Dosimetric evaluation of Discover by comparison of 13 VMAT treatment plans with ours reference measurements (PDIP, Varian and DELTA4, Scandidos). Gamma index used for our analysis are global, normalized to the maximum dose and have criteria of 3%, 3 mm and a 10% threshold. Results Phase 1: The commissioning of Discover is easy and fast without physical effort. Phase 2: The absolute dose measured decrease of 1.5% with Discover plugged. Phase 3: The system reproducibility, evaluated for 9 measurements, is respectively of σ gamma = 1.1% for plan 1 and σ gamma = 1.5% for plan 2. Phase 4: Average gamma acquired for the 13 treatment plans are respectively 0.19 for PDIP, 0.25 for Delta4 and 0.33 for Discover. Discriminating dose deviation and DTA, it appears those average gammas are mostly influenced by dose deviation. This deviation could be corrected by modeling Discover in the TPS. Discover provides a distribution of MLC positioning deviations. The analysis of the MLC positioning deviations show a progressive drift and permit to anticipate a preventive maintenance. This maintenance highlighted a premature wear of the MLC nuts. Conclusions The Discover system was tested through different technical and dosimetric tests. This first evaluation highlights the potential of the system but requires to get further with tests to make it a dosimeter monitoring the beam during treatment.