ESTRO 2021 Abstract Book

S92

ESTRO 2021

pencil beam scanning (PBS) technique in the community. Although previous dosimetric studies have demonstrated arc therapy's potential clinical benefits in varieties of disease sites, the technique has never been tested in any clinical proton beam therapy system until 2018 because of the technical and engineering challenges to rotate the heavy gantry and switching numerous energy layers efficiently. This talk will cover the roadmap and development of Spot-scanning Proton Arc therapy (SPArc) technique in both planning and treatment delivery mechanism that enabled the proton arc therapy on a clinical proton system. Additionally, I will review the performance of the prototype dynamic proton arc module from IBA ProteusONE and introduce an experimental method to quantitatively model the dynamic delivery sequence of the arc therapy. We will also go through some latest technological breakthroughs in the optimization algorithm to further improve the proton arc therapy's efficiency such as the intergration of the energy layer sorting and machine-specific arc delivery sequence into the SPArc optimization algorithm. At last, we will review some potential benefits in certain clinical indications, simplification of the existing clinical workflow, and future direction of the proton arc therapy clinical applications. Through the talk, the audience will: 1. Understand the background engineering and technical development that enables the Spot-scanning arc therapy 2. Learn the latest breakthrough in the prototype arc therapy treatment delivery, QA, and novel proton arc planning algorithms 3. Explore the potential clinical applications utilizing arc therapy such as interplay effect mitigation, hypofractionation, and dose escalation .etc. 4. Explore potential benefits of simplifying the clinical workflow and increasing the daily treatment throughput. Abstract Text The transition from 3D conformal radiotherapy (3DCRT) to multifield intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT) in photon-based radiotherapy has improved high-dose conformity and normal tissue sparing, at the expense of a large low-dose ‘bath’ to the irradiated area. Taking advantage of the unique pattern of energy deposition of protons, proton therapy has been shown to reduce the dose delivered to the surrounding healthy tissues while delivering adequate dose to the target volume. Proton therapy delivery has followed the same history than photon-based radiotherapy techniques, with Intensity Modulated Proton Therapy (IMPT) nowadays the state-of-the-art technique. The next step is therefore logically the introduction of proton arc irradiation in the clinic, a concept that has raised increasing attention in the past years. This presentation will focus on the potential of proton arc irradiation for tailoring of the dose distribution. First, a review of the current literature on the achievable dose distributions with proton arc irradiation will be provided, focusing on the benefits of proton arc therapy compared to IMPT for several tumor sites. A comparison of the pros and cons of proton arc therapy compared to photon-based rotational therapy will then be proposed, both in terms of dose distribution and expected clinical impact; i.e. the potential of proton arc irradiation for reducing normal tissue complication probability. Of note, the evaluation of secondary cancer risk due to increased low-dose bath of rotational photon/proton therapy will be covered. Finally, future perspectives of rotational delivery will be explored, with e.g. the concept of rotational hadron therapy. SP-0145 Improving proton therapy dose distribution through the use of arc irradiation L. Toussaint 1 1 Danish Centre for Particle Therapy, Medical Physics, Århus N, Denmark

SP-0146 Can proton arc therapy offer the opportunity to increase the therapeutic index? A. Carabe 1 1 Hampton University Proton Therapy Institute, Medical Physics & Dosimetry, Hampton, USA

Abstract Text Proton arc therapy (PAT) has been proposed as a possible evolution for proton therapy. This commentary uses dosimetric and cancer risk evaluations from earlier studies to compare PAT with intensity modulated proton therapy. It is concluded that, although PAT may not produce better physical dose distributions than intensity modulated proton therapy, the radiobiological considerations associated with particular PAT techniques could offer the possibility of an increased therapeutic index.

Symposium: The use of MR in radiotherapy

SP-0147 Does MR-only treatment planning live up to its expectations? T. Nyholm 1 1 Umeå University, Department of radiation sciences, Umeå, Sweden

Abstract Text MR-only treatment planning, meaning the exclusion of CT from the preparation phase of radiotherapy, has been shown feasible and is in clinical use. The presentation will describe the rational for MR-only treatment planning for external radiotherapy, technical and workflow issues and solutions, and finally attempt to answer the question if MR-only treatment planning is living up to the expectations.

SP-0148 Real-time adaptation under MR-guidance J. Bertholet 1 1 Inselspital, Bern University Hospital, Department of Radiation Oncology, Bern, Switzerland