ESTRO 36 Abstract Book

S159 ESTRO 36 2017 _______________________________________________________________________________________________

heterogeneous, and the EBRT dose distribution may also include significant dose gradients in the vicinity of the brachytherapy high dose region, e.g. in case of lymph node boosts. Organ motion between fractions as well as change in anatomy between brachytherapy and EBRT constitute specific challenges for dose summation. Deformable image registration (DIR) aims to match each tissue voxel irradiated by each fraction of external-beam radiation with the corresponding voxel irradiated by each fraction of brachytherapy. However, DIR is related with specific uncertainties and does not necessarily provide added value for dose accumulation. The alternative of accumulating dose through DIR is to perform direct addition of DVH parameters as recommended in the ICRU report 89. Direct addition assumes inherently that hot spots and cold spots remain in the same spatial region across suceeding fractions and is therefore also related with uncertainties or even not appropriate for certain scenarios. DIR can be carried out with deformation models based on image intensity, biomechanical models, or combinations of these. Biomechanical models take into account organ shapes and potentially biomechanical properties of organs or organ walls/surfaces, such as elasticity. Biomechanical models based on contours need to have these defined in both source and target images, and the correspondence of contours becomes part of the objective function which drives the optimisation. DIR models based entirely on image intensity do not take into account contours, and the objective function is based on correspondence in image intensity. The major questions with regard to DIR and dose accumulation in brachytherapy are: 1) Is it problematic to accumulate dose without DIR? I.e. what is the accuracy and limitations of dose accumulation with DVH addition? 2) Can DIR solve the problem? I.e. what is the accuracy of DIR-based dose accumulation? For summation of EBRT and brachytherapy, direct DVH addition is accurate if the EBRT dose distribution is homogenous in the region where the BT boost is going to be delivered. In case of a homogeneous EBRT dose, the EBRT dose contribution to the primary target D 90% and D 98% as well as D 2cm3 for organs at risk will be equal to the prescribed EBRT dose. Dose distributions from four-field -box techniques are normally homogeneous . Furthermore, it is also possible to control the homogeneity of IMRT and VMAT in the region the the BT boost through dose optimisation, e.g. by introducing help structures in the region of the primary target/GTV with specific constraints on homogeneity. However, in the case of lymph node boosts which are in close relation to the primary target and the BT boosted region, direct addition of EBRT and BT DVH parameters may not reflect the true accumulated dose. DIR has not been investigated for this purpose, but may provide an added value. For summation of dose from succeeding BT fractions, there are indications that DVH addition has an accuracy better than 5% for organs such as bladder and rectum, as hotspots are quite stable across brachytherapy fractions. For target structures, there have not been any systematic evaluations, but often cold spots are located in the same region of the target, and DVH addition is assumed to work well. For highly mobile organs such as sigmoid colon or bowel loops, it is well know that hotspots may end up in very different parts of the loops and DVH addition is expected to significantly overestimate the hotspot dose. DIR algorithms which are based on contours and biomechanical models have been demonstrated to work well for bladderand improves dose assessment although the improvement with DIR as compared to direct DVH addition is normally less than 5%. However, some DIR algorithms based on image intensities can be related with significant uncertainties and may provide dose assessments which are less accurate than with DVH addition, and DIR should therefore be used with great caution. Sigmoid and bowel are highly deformable organs and represent a significant challenge for DIR. There are

Symposium: Registration and fusion techniques

SP-0310 Rigid registration techniques for different imaging modalities N. Nesvacil 1 1 Medical University of Vienna, Department of Radiotherapy- CCC- Christian Doppler Laboratory for Medical Radation Research for Radiation Oncology, Vienna, Austria Image registration has become an important part of treatment planning and execution in 3D image guided external beam radiotherapy (EBRT) and brachytherapy (BT) during the last decades. In principle, the same algorithms for rigid or non-rigid image registration can be applied to either type of radiotherapy. However, for their application in brachytherapy the presence of the brachytherapy delivery device, i.e. the applicator, plays an essential role. This presentation will provide an overview of rigid registration techniques in brachytherapy, compared to external beam radiotherapy. In gynaecological brachytherapy, where the applicator and CTV might move in relation to the bony anatomy during the course of a (multi-fractionated) treatment, applicator-based rigid registration can be used to combine images for treatment planning acquired with the same or different modalities at different time points. One of the most useful applications of this technique is to transfer target contours defined on a reference image, e.g. an MRI at the time of the first BT, to subsequent CT image volumes for planning of further BT fractions, if MRI is not always available for dose plan adaptation to the anatomy of the day. Requirements and pitfalls for clinical applications of this technique will be discussed. In order to analyse interfraction variations for target and organs at risk (OARs) based on image volumes acquired at different time points, rigid image registration can provide a good estimate of the dosimetric impact of anatomical changes between applicator, CTV and OARs. Clinical examples will be discussed for different treatment sites. Limitations of the technique will be summarized and special focus will be given to prostate and gynaecological BT treatment planning. Multi-modal rigid image registration for brachytherapy is also used to improve target delineation and dose plan optimization for a single fraction. MRI acquired before brachytherapy can be combined with CT images for treatment planning of prostate cancer, in order to visualize and delineate intraprostatic lesions as boost target volumes in HDR or LDR brachytherapy. In the setting of gynaecological cancer brachytherapy where only CT is available for visualization of the applicator and surrounding organs after implantation, rigid registration of ultrasound images obtained with applicator in situ could be used in the future for dual modality dose planning of a single fraction. Challenges and solutions for the registration of ultrasound and CT images, such as determining the applicator position in the ultrasound image volume, will be explored to conclude this presentation. SP-0311 Deformable registration for dose summation K. Tanderup 1 1 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Dose summation across brachytherapy fractions and accumulation with external beam radiotherapy (EBRT) dose is essential for assessment of total dose to both targets and organs at risk in treatments with fractionated brachytherapy and in combinations with EBRT. Brachytherapy dose distributions are highly