ESTRO 36 Abstract Book

S414 ESTRO 36 2017 _______________________________________________________________________________________________

contribution of microscopic lung parenchyma (below CT resolution). PO-0788 First assessment of Delivery Analysis tool for pre-treatment verification on the new Radixact system A. Girardi 1 , T. Gevaert 1 , C. Jaudet 1 , M. Boussaer 1 , M. Burghelea 2 , J. Dhont 1 , T. Reynders 1 , K. Tournel 1 , M. De Ridder 1 1 Universitair Ziekenhuis Brussel, Department of Radiotherapy- Universitair Ziekenhuis Brussel- Vrije Universiteit Brussel- Brussels- Belgium, Brussels, Belgium 2 Brainlab AG, BRAINLAB AG Feldkirchen Germany, Brussels, Belgium Purpose or Objective To evaluate the accuracy of the Delivery Analysis (DA) tool for patient-specific pre-treatment verification and the sensitivity to detect discrepancies in dose delivery in comparison with widespread detectors. Material and Methods The Radixact machine is equipped with the DA device for pre-treatment Quality Assurance (QA) and interfraction verification. This tool is designed to assess the consistency of the delivered treatment through the detector data and to show anatomical changes of the patient. The latter representing a powerful tool to be coupled with Adaptive Radiotherapy. The idea is to use the detector: a) to measure the Multileaf Collimator (MLC) leaf open time, b) to compare the planned sinogram to the delivered one and c) for dose reconstruction purposes. In this study, we performed pre-treatment verification on the very first twenty heterogeneous patients treated worldwide (target volumes ranging between 98 to 4179 cc) using the DA, the Sun Nuclear MapCheck2 (MC2) and the ScandiDos Delta4 (D4).The Gamma Index was used to show the agreement between dose planning calculations and measurements. To compare the three methods, criteria were set to 2% and 3% in local dose and to 2mm and 3mm in distance, respectively, excluding doses lower than 20% of the maximum doses. The performances of the systems were analysed with a single factor ANOVA test, with a significance level of α=0.05. A possible dependence of the results from the target volume was furthermore explored with a simple linear regression analysis. Results The ANOVA test showed no statistically significance differences between the performances of the three systems, both for the 2%/2 mm and the 3%/3mm criteria (p-values equal to 0.351 and 0.660 respectively). The linear regression indicated a variation of performance as a function of target volume for the MC2 (R 2 2%/2mm =0.819 and R 2 3%/3mm =0.979) and the D4 detectors (R 2 2%/2mm =0.991 and R 2 3%/3mm =0.990), which is not highlighted for the DA system (R 2 2%/2mm =0.283 and R 2 3%/3mm =0.290). This difference could be related to the missing data due to the larger dimension of the dose map with respect to the detection area of the MC2 and D4 systems.

Conclusion This study showed that the performances of the Delivery Analysis tool for the new Radixact machine is not different from those of two other widespread detectors for pre- treatment verification. Moreover, the linear regression test showed that the performances of the system are not correlated with the target volume, as is the case for two other detectors used in the study, proving its sensitivity as a patient specific QA tool. PO-0789 Demystifying failed VMAT PSQA measurements with ArcCHECK P. Kinsella 1 , L. Leon-Vintro 2 , B. McClean 3 1 St Luke's Radiation Oncology Center, Physics, Dublin, Ireland 2 University College Dublin, School of Physics, Dublin, Ireland 3 St Luke's Radiation Oncology Network, Physics, Dublin, Ireland Purpose or Objective A means of reducing PSQA measurements for VMAT is currently a popular topic of discussion due to the resource burden it generates and the increased use of VMAT. The reluctance to reduce or replace PSQA may be partly due to the difficulty in identifying the cause/s of plan failures. Plans may fail due to a large number of potential factors caused by the TPS, linac or measurement device. The goal of this study was to uncover the reason/s why a selection of VMAT plans have failed. Material and Methods Five ‘bad’ plans yielding low (failing) gamma pass-rates and high average gamma-values were s elected for analysis. Two ‘good’ plans yielding high gamma pass-rates and low average gamma values were also used for comparison. The plans were measured with SNC ArcCHECK (1220 Model) cylindrical detector diode array and analysed with gamma analysis in SNC Patient software. The institutional tolerance was ≥95% of the points must pass a gamma analysis with 3% and 2mm gamma criteria, with a