ESTRO 36 Abstract Book

S161 ESTRO 36 _______________________________________________________________________________________________

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

currently not any deformable registration algorithms which have shown performance in sigmoid and bowel which is sufficent for dose accumulation. In conclusion, DVH addition is currently recommended by the ICRU 89 report for dose summation in brachytherapy, and provides in most scenarios a good accuracy for assessment of total dose in targets and in organs such as bladder and rectum. Dose summation in highly mobile organs such as sigmoid and bowel is currently related with significant uncertainties, and there could be potential to improve this with appropriate DIR algorithms. SP-0312 Imaging and fusion techniques for focal brachytherapy L. Beaulieu 1 1 Laval University - Faculty of Science and Engineering, Université laval Cancer Research Centre, Québec City, Canada Over the last decade, numerous technological developments have made brachytherapy one of the most precise needle-based procedures on the market. The cornerstone of interstitial brachytherapy for many years now has clearly been real-time ultrasound (US) image- guidance and more recently real-time 3DUS image- guidance. From whole gland prostate cancer treatments to focal boosts and now focal therapy, brachytherapy is head of the curve of any other prostate focal therapy modality at this time in terms of precision and accuracy. However, current standard US-guidance is not sufficient for focal therapy; our real-time image-guidance technique needs to be supplemented with more information. This presentation will look at the role of multi-parametric MRI in prostate focal therapy as well as US-augmented with MRI for real-time guidance. This brings the notion of augmented reality as well as the challenge of image fusion among two very different imaging modalities and image sets also taken under very different conditions. We will also discuss the topic of merging tissue information (e.g. biopsy) with imaging data to provide a complete cancer burden maps for targeting purposes. Finally, we will provide a forward-looking view of real-time multi- parametric 3DUS guidance and targeting for such procedures. OC-0313 What is the effect of axillary treatment on patient reported outcomes in breast cancer patients? M.L. Gregorowitsch 1 , H.M. Verkooijen 1 , N. Fuhler 1 , D.A. Young Afat 1 , A.N.T. Kotte 1 , M. Vulpen van 1 , C.H. Gils van 2 , D.H. Bongard van den 1 1 University Medical Center, Radiation Oncology, Utrecht, The Netherlands 2 Julius Center for Health Sciences and Primary Care- University Medical Center, Epidemiology, Utrecht, The Netherlands Purpose or Objective In breast cancer patients with limited (sentinel) lymph node involvement, axillary lymph node dissection (ALND) is increasingly being replaced by axillary radiotherapy. Since ALND is associated with a high risk of upper-body morbidity, axillary radiotherapy might be favorable in patients with limited lymph node involvement. However radiation-induced morbidity can also influence quality of life, the extent of which may depend on the irradiated volumes. We compared patient reported outcome measures (PROMs) of breast cancer patients at the start adjuvant radiotherapy, during and after radiotherapy according to the extent of axillary treatment. Proffered Papers: Breast and gynaecology