SU‐E‐J‐125: Classification of CBCT Noises in Terms of Their Contribution to Proton Range Uncertainty

Abstract

Purpose:This study assesses the potential use of CBCT images in adaptive protontherapy by estimating the contribution of the main sources of noise and calibration errors to the proton range uncertainty.Methods:Measurements intended to highlight each particular source have been achieved by adapting either the testbench configuration, e.g. use of filtration, fan‐beam collimation, beam stop arrays, phantoms and detector reset light, or the sequence of correction algorithms including water precorrection. Additional Monte‐Carlo simulations have been performed to complement these measurements, especially for the beam hardening and the scatter cases. Simulations of proton beams penetration through the resulting images have then been carried out to quantify the range change due to these effects. The particular case of a brain irradiation is considered mainly because of the multiple effects that the skull bones have on the internal soft tissues.Results:On top of the range error sources is the undercorrection of scatter. Its influence has been analyzed from a comparison of fan‐beam and full axial FOV acquisitions. In this case, large range errors of about 12 mm can be reached if the assumption is made that the scatter has only a constant contribution over the projection images. Even the detector lag, which a priori induces a much smaller effect, has been shown to contribute for up to 2 mm to the overall error if its correction only aims at reducing the skin artefact. This last result can partially be understood by the larger interface between tissues and bones inside the skull.Conclusion:This study has set the basis of a more systematical analysis of the effect CBCT noise on range uncertainties based on a combination of measurements, simulations and theoretical results. With our method, even more subtle effects such as the cone‐beam artifact or the detector lag can be assessed.SBR and JOR are financed by iMagX, a public‐private partnership between the region Wallone of Belgium and IBA under convention #1217662