Retrospective of 300 delivery quality assurance of patients treated with the tomotherapy HI-ART 2 using the IBA matrixx-evolution

Abstract

Introduction The 2D-array ion chambers: MatriXX-Evolution (IBA) was set up at the North Hospital of Marseilles (AP-HM) in August 2011 for delivery quality assurance (DQA) of patients treated with Tomotherapy Hi-Art2 (Accuray). First, the detector performances were assessed with a Synergy-S accelerator (Elekta) and then with the Tomotherapy Hi-Art2. Despite some inaccuracies due to the 7 mm inter-diode spacing, a realization of 40 DQA had enabled to guarantee the quality of an absolute dose measurement, with an averaged gamma score of 96.7%. A comparison MattriXX-Evolution/ Gafchromic-EBT2 was performed with 20 DQA and pointed the benefits out, on both practical and dosimetrical plans, of using the MatriXX-Evolution. This study had finally led to the integration of the multi-detector into the clinical routine. Material and method The gamma index is computed with a dose tolerance and a distance to agreement of 3% (global maximum) and 3 mm respectively. Using linear interpolation, the resolution of the measured dose distribution is reduced to 1 mm. Until now, 300 DQA were performed using the MatriXX-Evolution. This retrospective enabled to reveal and analyze the different parameters affecting the quality of the results, and then to evaluate the operating limits of using this detector with the Tomotherapy Hi-Art2. Results The averaged gamma index is 97.9% ±2.4 (1 standard deviation). The analysis of the influence of the parameters highlighted that the field dimensions and the delivered dose do not seem to be correlated to the gamma index value. The gamma score decreases as the pitch increases and in the cases of reduced target volumes. With its homogeneity of detection and its accuracy for low doses measurement, the MatriXX-Evolution is very efficient for most of the pathologies. The Inter-diode resolution is a source of inaccuracies in the case of complex pathologies involving non-linear dose gradients. Conclusion The MatriXX-Evolution is adapted to the DQA with the Tomotherapy Hi-Art 2. Although this detector found to be reliable to measure absolute dose distributions, the physicists using this equipment have to be aware of its limits and the sources of discrepancy between calculation and measurement.