ESTRO 37 Abstract book

S963

ESTRO 37

measurement uncertainty and its influence on gamma results. As an alternative we propose to use non- measuring way for testing known errors during delivery of the IMRT plan. Material and Methods Three QA devices were investigated: ArcCHECK (SunNuclear), Octavius (PTW) and EPID (Varian). The Python script changing the dose distribution calculated in TPS into the artificial measurement file has been prepared for each device. This artificial measurement had the same detectors resolution as in the real measurement. We introduced MLC errors into 40 IMRT plans for 4 cancer sites (10 plans each). Head & Neck and Brain with 6MV beams while Prostate and Gynecology plans with 15MV beams. There were introduced two types of systematic MLC errors: gap width and gap position (±0.5, ±1, ±2 and ±3mm for both banks of MLC). DVHs were calculated in order to detect MLC errors with the clinically relevant influence on dose distribution. Artificial measurements were prepared for each plan with MLC error. We performed gamma analysis with criteria: 1%/1mm, 2%/2mm, 3%/3mm both for local and max (global) gamma. Dose threshold was set to 5% for gamma analysis despite of the device. Results Artificial measurements preparation lasts between 1 to 5min. depending on device type and number of fields. We assumed 2% change in PTV mean dose, PTV D98% lower than 95% and PTV D2% greater than 107% as clinically relevant. In Fig. 1 Octavius 2D percent of passing points for gamma analysis is shown. The shaded area shows MLC errors with not clinically relevant change in PTV dose distribution. For different cancer sites different percent of passing points level is required in order to detect errors with clinical impact on PTV dose. In Fig. 2 the comparison of percent of passing points for 2mm/2% max gamma analysis is shown for all tested devices.

Artificial measurements allow us to compare different pre-treatment verification methods in relatively quick way without occupancy of the linac time for measurements. Another benefit from our methodology is that the result is independent of the measurement’s uncertainties. Based on artificial measurements comparison we can decide on clinically adequate gamma criteria and percent of passing points levels for different devices and cancer sites. EP-1793 Establishing local confidence limit and the optimization of VMAT patient specific quality assurance J. Kunnanchath 1 , T. Dufan 1 , N. Malik 1 , A. Salah 1 , V. Knight 1 , K. Singh 1 , T. Smith 1 1 American Hospital Dubai, Radiation Oncology, Dubai, United Arab Emirates Purpose or Objective Optimization of VMAT patient specific quality assurance (QA) through establishing the institutional local confidence limit by statistical analysis of gamma results. Material and Methods The VMAT patient specific plans for fifty VMAT cases were calculated in Varian Eclipse treatment planning system (TPS) and all the plans were executed in Varian Clinac Trilogy machine. Patient specific QA was performed using portal dosimetry (Varian) and Arc check device (Sun nuclear). The gamma criteria of 3% dose difference and 3mm distance to agreement was used to find the difference between TPS calculated and measured dose distribution. Area gamma, mean gamma and maximum gamma were calculated and tabulated for the TPS vs. measured planar dose using portal dosimetry. The percentage of pixels passing gamma of one was calculated and tabulated for the TPS vs. measured planar dose using Arc check device. The mean and standard deviation of the gamma results were calculated and the local confidence limit was derived by using the concept of ǀ mean ǀ ±1.96σ Results In portal dosimetry, the area gamma ≤1, average gamma and maximum gamma were 99.1±1.06, 0.26±0.05 and 2.05±0.53 respectively with gamma criteria of 3% dose difference and 3mm distance to agreement. In Arch check, the average percentage of pixels passed the gamma of one was 99.3±0.7. Local confidence limit derived for portal dosimetry area gamma, average gamma and maximum gamma were 97%, 0.37 and 3.10 respectively. For Arc check, the confidence limit derived was 98% against the gamma criteria of 3%- 3mm. Conclusion The local confidence limit for gamma analysis using portal dosimetry and Arc check device was established. Every institution should establish their local confidence limit in order to optimize the patient specific QA based on their machine, QA device and type of plans. EP-1794 Cavity sensitivity correction factors for alanine dosimetry in Bruker EMX-micro EPR spectrometers H.L. Riis 1 , S.J. Zimmermann 1 , J. Helt-Hansen 2 , C.E. Andensen 2 1 Odense University Hospital, Department of oncology, Odense, Denmark 2 Technical University of Denmark, Center for Nuclear Technologies, Roskilde, Denmark Purpose or Objective Alanine is a passive solid-state dosimeter material with potential applications for remote auditing and dosimetry in complex fields or non-reference conditions. Alanine has a highly linear dose response which is essentially independent of dose rate and energy for clinical MV photon beams. Alanine is available as pellets with a 5 mm

Fig. 1 Percent of passing points for Octavius 2D, different gamma criteria and cancer sites.

Fig. 2 Percent of passing points for different devices and the same gamma criteria. Conclusion