ESTRO 36 Abstract Book

S932 ESTRO 36 2017 _______________________________________________________________________________________________

of agreement was noted for all tests (Table 1), of which for geometric accuracy (inside length=0.4mm; diameter=1.0mm), slice thickness accuracy (T1=0.3mm; T2=0.5mm), slice position accuracy (T1 slice 1=1.4mm; T1 slice 11=0.5mm; T2 slice 1=0.9mm; T2 slice 11=0.6mm), intensity uniformity (T1=0.0%; T2=0.1%), percent-signal ghosting (0.0003) and low-contrast object detectability (T1=2.9; T2=3.4) were all much smaller than the corresponding ACR criterion. As illustrated in Fig. 1, all data points fell within the 95% limit of agreement except for diameter accuracy, for which 3 out of 54 (~5.6%) data points fell outside the 95% limit of agreement. Furthermore, small measurement bias close to zero was also obtained for most tests. In terms of ICC, excellent inter-observer agreement (ICC>0.75) was achieved in geometric accuracy (ICC>0.99), spatial resolution (ICC = 1), slice position accuracy (ICC = 0.81), image intensity uniformity (ICC = 0.80), percent ghosting ratio (ICC = 0.85) and low-contrast object detectability (ICC = 0.89). A fair inter-observer agreement was seen in the slice thickness accuracy (ICC = 0.42). Based on both BA-analysis and ICC, excellent inter-observer agreement could be achieved in the ACR MRI phantom test under RT-setting.

department. This work aimed to optimise image quality and dose for CBCT clinical protocols using an Elekta XVI (R.5.0.2) machine for all clinically relevant treatment sites. Material and Methods The Elekta XVI system was fully calibrated and Acceptance Tests (AT) were performed for all FOVs before the optimisation procedure. Three different phantoms were used to complete the optimisation: CATPHAN 600, Phillips WEP Phantom Set (PWEPPS) (5 circular objects 15, 20, 26.5, 36.5, 50cm diameter – Fig.1.) and Rando phantom (RP). The optimisation methodology was designed to assess dose vs the following image quality parameters: spatial resolution (SR), uniformity (UN), contrast (CON), CNR, SD, SNR and artefacts. These parameters were evaluated as absolute values and compared to the “standard” image results. These “standard” images were taken for AT presets. The optimisation process was performed by setting the exposure parameters: mA per frame, ms per frame and gantry start/stop angles. The first step involved taking CATPHAN images using varying mA and ms settings. SR, UN, CON, CNR, SD and SNR values were recorded. Final mA and ms settings were chosen based on SNR and UN results, and were no worse than 20% and 5% respectively in relation to the “standard image”. Images were also compared using the same mAs but different mA and ms values. The second step involved taking PWEPPS images using the final mA and ms settings for each protocol. SNR and UN were evaluated for phantom diameters relevant to the treatment site in question. The RP was used to assess image quality for the finalised clinical protocols .

Results The optimisation process resulted in better image quality in relation to the original presets and “standard images”. Dose was reduced by a factor ranging from 2-4 times. For a given mAs, superior image quality was seen for a higher mA and lower ms, indicating that the detector response was better for a higher dose rate. Saturation artefacts (Fig.2) were visible for 64mA and 10ms when the images included the intersection between the test object and air. The worse UN was seen for LFOV. This was affected by “cutting” from the reconstruction 40 pixel rows at the edge of the panel. It was done because the bad pixel map correction algorithm could not effectively correct the bad pixels. Additionally, for 2D kV images, bad pixel artefacts were visible using the TOR18FDG phantom. The kV detector panel was replaced and the new one was calibrated to get similar gains so the optimisation process

Conclusion Our results showed that ACR MRI phantom test under RT- setting was highly reproducible and subject very little to inter-observer disagreement. EP-1723 Optimisation of an Elekta XVI (R.5.0.2) system for clinical protocols – image quality vs dose. D. Oborska-Kumaszynska 1 , D. Northover 1 1 Royal Wolverhampton NHS Trust, MPCE Department, Wolverhampton, United Kingdom Purpose or Objective The use of CBCT in radiotherapy has significantly increased in recent times, which has led to an increase in the concomitant dose received by some patients. Often, the generic preset protocols provided by the manufacturers are not optimised for a particular