ESTRO 2020 Abstract Book

S417 ESTRO 2020

Abstract text In radiation therapy, the need to irradiate superficial lesions is common; these lesions are often at shallow depth or on the skin of the patient. However, one of the characteristics of the radiation used in radiotherapy treatments is that the maximum dose deposited lies at a given depth, with skin-sparing property. Therefore, a bolus, a natural or synthetically developed material, is placed on the surface area to be irradiated and acts as a tissue layer to provide a more effective treatment in the superficial lesions. Nevertheless, it is difficult for the commercially available flat-form boluses to make full contact with irregularly shaped patient skin, as it occurs in post-mastectomy chest wall, causing the presence of air gaps between the bolus layer and the patient's skin that may influence the planned dose distribution. In fact, this phenomenon is not considered by TPS and may cause serious changes between the planned dose distribution and the actual dose administered during the treatment. The decrease of the air gaps will give a better conformation of the dose distribution to the target volume, improving the accuracy of the radiation treatment. The use of 3D printing techniques to create a patient specific bolus facilitates correspondence with the patient skin, yielding agreement between the planned and delivered doses. This lecture aims to describe dosimetric properties and practical aspects of flexible materials for 3D printed bolus and to show how to produce efficient and more comfortable customized bolus for patients. SP-0752 Endoscope-guidable Multichannel Applicator – An in-house development for endoluminal brachytherapy of the oesophageal cancer B. Wisgrill 1 1 Universitätsklinik für Strahlentherapie, Brachytherapy, Wien, Austria The Bonvoisin-Gerard applicator or its modifications are used as standard for treatment in endoluminal brachytherapy of oesophageal cancer. The design of this applicator offers a transfer tube that is located in the centre of the applicator, this is called central-source design. The guidelines of DEGRO and ICRU regarding the planning of endoluminal brachytherapy using this applicator form state that the effective radiation length should extend over the macroscopically determined tumour length plus a safety margin of 2-3cm in the cranial and caudal direction and the resulting dose at a distance of 5mm from on the applicator surface or 5mm tissue depth should be regarded as the target volume dose. Considering the circular extension of the isodose lines which result from the central source design, it appears that also tumour-free parts of the oesophagus are irradiated with the target volume dose. In contrast to the Bonvoisin-Gerard applicator, the applicator developed for brachytherapy at the radiotherapy department of the General Hospital Vienna allows optimized dose coverage of the target volume while protecting the tumour-free oesophageal parts. Material and Methods: The basis of the applicator is a plastic catheter with a lumen of 6.5mm. This enables the insertion of a paediatric endoscope which is used for optimal positioning of the applicator. Ten pieces of 5french probes (lumenCare® Azure, Nucletron) are attached to the surface of the Abstract text Purpose:

addressed: (i) the fraction of patients that is referred for each treatment (sub)site after a plan comparison, challenges and optimization of the workflow in clinical practice, NTCP differences between the plan-comparison plan and the clinical treatment plan, and NTCP stability during treatment (plan adaptation). Finally, I will address some future developments, including plan comparison based on fully automated treatment planning or dose/NTCP prediction with machine learning. SP-0750 Problems solved and open questions in clinical practice of proton treatment planning. M. Schwarz 1,2 1 S. Chiara Hospital, Protontherapy, Trento, Italy ; 2 tifpa- Infn, Medical Physics, Trento, Italy Abstract text The recent past of proton treatment planning in clinical practice saw significant developments, as well as the beginning of new issues likely to be solved in the near future. Pencil beam scanning is the indisputed standard for most proton treatments, and this allowed large scale implementation of techniques based on either single field (SFO/SFUD) or multi-field (MFO/IMPT) optimization, showing how MFO is able in several cases to produce plans that are more robust, not less, than the SFO (and, even more, passive scattering) counterpart. The field of robustness optimization shifted its focus from algorithms development to the introduction of these approaches in clinical practice, where it is now in a phase of consolidation, where standardization of practices is both needed and highly sought by many centers. Plan robustness evaluation, which hasn't been the subject of research nearly as much as robust optimization, raises interesting questions that go beyond the practice of proton therapy and show the need of more efforts to tackle an issue related to photon therapy too. Dose calculation was associated with both bad news (e.g. the inadequacy of some dose calculation algorithms in challenging situations) and good news, such as the introduction of Monte Carlo dose calculation in routine planning. We can now say that dose calculation accuracy is not the weak link in the overall accuracy of proton therapy. On two aspects proton planning is seemingly running behind photon techniques: (online) adaptive planning and autoplanning. Fast adaptive planning is still far from being the norm, and at the same time large scale availability of high quality daily imaging and the use of hypofractionation regimes are still lacking. Albeit it is difficult to identify which are the causes and which are the consequences, demonstrating the feasibility in clinical practice of efficient replanning approaches is a priority. Protons have specific needs, such as model-based patient selection, which would greatly benefit from autoplanning. Instead, autoplanning for proton therapy is in its infancy. Methods for automatic beam selection are probably a "low hanging fruit" worth our attention to enable large scale adoption of autoplanning in the near future.

Symposium: Innovations in radiotherapy

SP-0751 3D bolus: practical aspects S. Goncalves 1 1 Instituto Português de Oncologia do Porto Francisco Gentil- EPE, Física Médica, Porto, Portugal