Evaluation of the reconstructed dose from the three-dimensional dose module of a helical diode array: factors of influence and error detection

The 3D-dose module (3DVH) of the ArcCHECK-phantom reconstructs the dose distribution in the phantom volume and transfers it to the patient geometry. Our aim was to evaluate the 3DVH-reconstructed dose systematically building up from simple to complex cases. Therefore, the influence of different field sizes without and with blocking the isocenter was tested. The dose distributions of different radiation techniques, error-free and error-induced VMAT-plans were verified by measuring with films and other detectors in the phantom. It was checked how the inclusion of the dose measured separately in the ArcCHECK-isocenter affects the reconstruction. Thus it was also investigated which detector should be used for the dosimetry in the isocenter. Without including the isocentrically measured dose, the reconstruction for the smallest field (2 × 2 cm²) was 5% (6 MV) and 3.7% (10 MV) higher than measured with an ionization chamber. With increasing field size, the deviation decreased. For fields with blocked isocenters, the reconstructed dose was between −10.6% and −24% lower than determined with a microDiamond. Measurements with the Semiflex of the spinal plan resulted in higher doses than calculated by the treatment planning system (TPS) and measured with the film and the other detectors. Through the inclusion of the isocentric dose in the reconstruction its accordance with the film increased mostly. With exception of an error-induced head and neck plan, the induced errors in the reconstructed dose volume histogram became visible, but were underestimated. With the 3DVH-algorithm not every induced-error was detected. The 3DVH underestimated the dose in blocked areas. To protect organs at risk (OAR), these are often blocked. Consequently, there is a risk that a clinical decision is based on a 3DVH that underestimated the dose for the OAR. We recommend including the isocentric dose in the reconstruction. The detector used for the isocentric measurements should be carefully chosen.