ESTRO 37 Abstract book

S11

ESTRO 37

the gantry head or close to the patient surface if a moveable snout is available. It can be used to irradiate the entire lesion volume or part of it (the deepest part of the lesion is irradiated without RS). This led to different accuracy of dose distribution calculation in Treatment Planning system. The aim of this discussion is to understand in what configuration the dose calculation is more reliable and what is the effect of the type of dose algorithm on the calculation accuracy. Apertures are tools that let to reach a better lateral conformity of the dose in pencil beam scanning. They are useful to improve the dose quality delivered but not necessary for the treatment. They became necessary when uniform scanning or passive beam scattering delivery techniques are used. For this reason and for the fact that they are not so widespread in PBS treatments they will be marginally discussed. It has been demonstrated that heterogeneities in patient anatomy can have a strong impact on dose calculation accuracy. Many methods of quantifying the lateral and longitudinal heterogeneities have been proposed. New solutions to decrease their impact on dose calculation accuracy and improve in dose algorithm have been published. An overview of all of these works will be shown to understand where we are and what we have to do to improve the reliability of dose calculation in patient anatomy. SP-0032 Enabling proton dose calculations on CBCT images G. Landry 1 , C. Kurz 2 , F. Kamp 2 , C. Thieke 2 , C. Belka 2 , K. Parodi 1 1 Ludwig-Maximilians-Universität München, Department of Medical Physics, Garching, Germany 2 University Hospital LMU Munich, Department of Radiation Oncology, Munich, Germany Abstract text The finite range of protons in matter entails both potential for dose conformity, and sensitivity to changes in the water equivalent thickness (WET) between the patient’s skin and the distal edge of the PTV. Anatomical changes stemming from weight loss, tumour growth or regression, bladder filling or stochastic motion of the digestive track are frequently associated with WET changes and may thus degrade the dose conformity achieved at the treatment planning stage. Detection and correction of such dose distribution degradation is best achieved by performing a dose recalculation on an updated computed tomography (CT) image, ideally acquired in-room at the treatment position. While some proton therapy centres have opted for in-room CT-on- rails imaging, which provide dose-calculation-quality images without correction, several have preferred to employ at-isocenter gantry-, nozzle-, C-arm- or couch- mounted cone beam CT (CBCT) imaging solutions (see Figure 1).

CBCT images suffer from severe artefacts, partly caused by a high scatter-to-primary ratio due to the flat panel imaging geometry which captures a significant proportion of scattered photons. It is generally acknowledged that the image quality of CBCT is not sufficient to allow the conversion to stopping power required for performing accurate proton therapy dose calculations, and that correction techniques must be employed to achieve this objective. Correction strategies are generally based on deformable image registration (DIR) of the planning CT data (Figure 2A) to the anatomy observed in the CBCT images (Figure 2B), essentially combining the image quality of conventional CT scanning with the ability of CBCT to image the anatomy in the treatment position to create a virtual CT image (Figure 2C). The technique has been shown to work well for head and neck cases where 3D images are considered when compared to a control or re- planning CT (Figure 2E). A variant of such correction methods employs the virtual CT to generate an estimate of the scatter distribution reaching the flat panel imager and correct the CBCT projections before image reconstruction (Figure 2D), which has been shown to perform well for sites such as the pelvis where less predictable motion than for head and neck cases can be expected. The latter approach has been compared to Monte Carlo simulations of scatter and found equivalent.