Quality assurance of rapid arc treatments: performances and pre-clinical verifications of a planar detector (MapCHECK2).

The purpose of this study is to evaluate the use of MapCHECK2 in a patient-specific quality assurance (QA) procedure for Rapid Arc (RA) radiotherapy and to obtain reference values of gamma index (γ) for different irradiation geometries. Dose distributions of 386 patients, optimized with Eclipse treatment planning system for RA on Varian Clinac 2300IX were exported and measured with MapCHECK2 detector array inserted in a MapPHAN dedicated phantom. Gamma index analysis was used to evaluate differences between calculated and delivered doses; collected data were analysed on the basis of complexity of plans and type of disease. Angular dependence and dose drift of detectors were estimated. The overall mean passing rate (percentage of points with γ < 1) was equal to 97.0% ± 3.1%; plan-specific evaluation of %γ < 1 showed significant differences among different treatment protocols (pancreas 98.6% ± 1.5%, prostate 96.6% ± 2.8% and rectum 92.8% ± 5.7%). MapCHECK2 demonstrated a strong angular dependence over a restricted range of angles (90° ± 5° and 270° ± 5°); this condition affected the result of pre-treatment QA only for plans with intense lateral fluence. The drift of diode array response due to the accumulated dose was found to be comparable with the manufacturer's declaration (0.5% per 1000 Gy). MapCHECK2 proved to be a useful and accurate tool in the process of patient-specific QA for RA treatment. Excluding the irradiation geometries with a high contribution of lateral fluence, the mean percentage of cases with γ-index > 95% equal to 90.3% was obtained instead of 81.7% including all plans.

PERFIDI filters to suppress and/or quantify relaxation time components in multi-component systems: an example for fat-water systems.

Parametrically Enabled Relaxation FIlters with Double and multiple Inversion (PERFIDI) is an experimental NMR/MRI technique devised to analyze samples/voxels characterized by multi-exponential longitudinal relaxation. It is based on a linear combination of NMR sequences with suitable preambles composed of inversion pulses. Given any standard NMR/MRI sequence, it permits one to modify it in a way which will attenuate, in a predictable manner and before data acquisition, signals arising from components with different r rates (r=1/T1). Consequently, it is possible to define relatively simple protocols to suppress and/or to quantify signals of different components. This article describes a simple way to construct low-pass, high-pass and band-pass PERFIDI filters. Experimental data are presented in which the method has been used to separate fat and water proton signals. We also present a novel protocol for very fast determination of the ratio between the fat signal and the total signal which avoids any time-consuming magnetization recovery multi-array data acquisition. The method has been validated also for MRI, producing well T1-contrasted images.