PurposeHigh modulated VMAT plan can be achieved by the last generation of Treatment Planning System (TPS). The aim of this study is to evaluate if a 2D chamber array is adequate to check this complex plans. The response function of the ion chamber and the spatial frequency of the dose distribution are analyzed to quantify the loose of information due to the aliasing effect [1]. Methods and materialsAlmost 200 VMAT plans of different anatomic district (Table 1) were performed with TPS Monaco; the plans were delivered with Elekta agility LINAC and for the patient pre-treatment QA a PTW OCTAVIUS Detector 729 was employed [2]. Gafchromic EBT3 were used as reference dosimetric system due to the higher spatial resolution. A home made program in Matlab was elaborated to analyze the spatial frequency of each line and column of the dose distribution maps. ResultsEach patient plan was recalculate in a specific phantom for pre-treatment verification. This method often involves a loss of spatial frequencies (Fig. 1). The highest detectable spatial frequency that can be obtained with the “OCTAVIUS Detector729” is 0.05 mm−1 (single acquisition). The large sampling distance may result in under-sampling of VMAT fields with high frequency contents. Head & Neck and Lung stereotactic treatment maps show frequency contribution beyond the limit 0.05 mm−1 with loss of information and possible artefacts in case of reconstructed distribution (aliasing). ConclusionThe 2D Array is a dosimetrically accurate and sensitive tool when used for QA and verification of most clinical radiotherapy beams. However, verification for Head and Neck or SBRT treatment plans require more closely spaced ion chambers or multiple merged acquisition should be considered in order to fulfil Nyquist theorem.