ESTRO 2020 Abstract Book

S425 ESTRO 2020

Additional volumes with margins that is individually specified (field-specific) have to be used to account for uncertainties in range. Paganetti has suggested a margin recipes that is widely used in proton planning [2]. However, some treatment planning systems offers robust optimization with field-specific margins as suggested by the user [3]. Consequently, the range uncertainty in proton planning also has an influence on the number of beams as well as the selection gantry angles. Robust planning have had the potential of mitigate the impact of range uncertainties. In this phase of the treatment planning process, proton planning emphasise other considerations than the photon planning. Plan robustness should be considered during the optimization as well as during the treatment plan evaluation, as well as the comparison with a photon treatment plan to choose the “best” treatment plan. Thinking protons instead of photons can be a great challenge. How to achieve the best plan? This includes selecting robust beam angles and thinking about what the protons interact with on its way to the target volume. Discussions about target volumes are frequent, as the use of them. Delineation is a major issue, not only for CTV/PTV but for other structures the protons might interact with in its beam path, as well as optimisation structures to provide the best treatment plan. References 1. Prescribing, Recording, and Reporting Proton-Beam Therapy. J ICRU Report 782007;7:NP 2. Paganetti H. Range uncertainties in proton therapy and the role of Monte Carlo simulations. Phys Med Biol ; 2012; 57(11):R99-R117 3. Li Y, Niemela P, Li L, Jiang S, Li H, et al. Selective robust optimization: A new intensity-modulated proton therapy optimization strategy. Med. Phys 2015;42(8):4840-4847 SP-0780 Motion management: what are the differences between photon and proton therapy? H. Hentschel 1 1 EBG MedAustron GmbH, Medical Department, Wiener Neustadt, Austria Abstract text Due to the fundamental differences in dose deposition mechanisms comparing photons and charged particles, there are several topics which have to be addressed when treating patients with protons or carbon ions. The defined depth of the Bragg Peak, i.e. the point of dose deposition within a target volume, is both a blessing and a curse. A reliable and safe delivery of the prescribed dose with steep dose gradients between target volume and organs at risk can only be achieved by using precise and reproducible patient immobilization and daily meticulous radiographic verification of the patient position. Providing a comfortable and stable position for patients becomes even more challenging when instead of a gantry, fixed beamlines are used for treatment. Another crucial factor is the duration of treatment fraction, which usually takes longer in particle therapy, compared to conventional radiotherapy. There is a number of indications where in addition to the setup accuracy, organ motion has to be considered. Due to the different beam delivery technique in particle therapy (pencil beam scanning), motion in the irradiated volume may have a significant impact on dose distribution. To overcome the intrafractional interplay effect between target motion and scanned particle beams, there are approaches to reduce target mobility, for example by:

- patient preparation (drinking protocols, enema, using medication to reduce peristalsis) - means of compression - technical measures like respiratory or fluoroscopic gating and surface scanner guided solutions - repainting (i.e. repeated delivery of individual beams within one fraction). Different approaches and particle therapy specific requirements are presented, with a focus on the methods which are, or will be, applied at the MedAustron ion beam therapy center.