ESTRO 2021 Abstract Book

S538

ESTRO 2021

Armin Luhr Germany

Abstract not available

SP-0673 LET-guided treatment planning D. Wagenaar 1 1 University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands

Abstract Text Variations of the relative biological effectiveness (RBE) of proton therapy with the linear energy transfer (LET) are predicted from pre-clinical experiments. Treatment planning restrictions are frequently imposed to reduce the impact of this biological uncertainty at the end of range of proton beams. However, the LET is a physical quantity and can be calculated using a Monte Carlo dose engine and can be used to calculate the RBE. Using LET-based RBE calculations in clinical practice presents several challenges such as choosing between the different physical definitions of LET, differences between RBE models and validating the LET/RBE calculations. Additionally, there is the practical issue of safe integration into clinical workflows in absence of LET/RBE calculations in clinical treatment planning systems (TPS). Even so, our institute has over two years of experience with LET/RBE calculations for our clinical practice. These evaluations are an effective way to estimate the biological uncertainty in critical cases or to compare the biological uncertainty of different planning techniques. However, integration of LET/RBE into optimization objectives might prove a far superior method of managing the uncertainty for clinical patients. While several systems are available for both LET/RBE calculations and LET optimization, the clinical adaptability depends on their integration into clinically available TPSs. Even if technical challenges are overcome, the uncertainty of the relation between LET and RBE remains. Current LET-based RBE models are largely based on preclinical evidence. RBE models can potentially be derived from patients as more clinical LET calculations become available. Ideally this would come from relating clinically relevant endpoints to a combination of dose and LET parameters. However, differences in LET among patients are small for our clinical practice. Therefore, relating imaging changes to a combination of dose and LET parameters can be a helpful alternative for clinical validation of LET-based RBE models. SP-0674 Implementing SGRT in the clinic N. Weitkamp 1 , S. Perryck 2 , C. Linsenmeier 3 , R. Dal Bello 3 1 University Hospital Zürich, Radio-Oncology, Zürich, Switzerland; 2 Universtity Hospital Zurich, Radio- Oncology, Zurich, Switzerland; 3 University Hospital Zurich, Radio-Oncology, Zurich, Switzerland Abstract Text In July 2014 the first Surface Guided Radiation Therapy (SGRT) system was installed in the clinic for radiation- oncology USZ. Before using SGRT in a clinical setting, a dedicated team of clinicians, physicists and RTT`s was formed to implement the system. The team had the opportunity to see the SGRT system in a clinical setting and receive vendor training. After site visits, a workflow was created and clinical protocols were developed to safely implement the system in a clinical routine. First we started with monitoring all our free breathing (FB) breast patients. The SGRT core team trained the other staff members so they had a chance to get familiar with the system and we had the opportunity to collect as much monitoring data as possible. After being familiar with the system we started to use the SGRT system together with the RPM system (Respiratory Gating for Scanners RGSC system (Varian Medical Systems, Palo Alto (USA)), for our breast patients treated with deep inspiration breath hold (DIBH). We used both systems simultaneously in order to see if the CT scan, acquired with the RPM system, correlates with the SGRT system on the treatment machine. The next step in the implementation of the SGRT system was to ensure the safe treatment of DIBH patients with the SGRT system only. After establishing this workflow with the breast patients, we focused on setting up and monitoring thorax and abdominal patients with the SGRT system. The goal here was to go tattoo-less, creating a robust set up and to monitor patient movements during treatment. The SGRT core team initiated all these projects and new implementations. In addition, the core team set up new protocols, gave hands on training and ensured that all workflows and clinical protocols were available for all staff members. New projects and workflows were discussed in a team meeting before starting them and for the first patient one person of the SGRT core team was present. After being familiar with the system, convinced about the safety and accuracy and having well trained staff, we started in 2019 a new workflow for breast treatments, the so called imageless treatment. If the set up is accurate for the first three days of treatment, the next treatments were applied with SGRT only and kV/MV imaging was done once weekly. If, for some reason, the SGRT set up failed, imaging was done according to the shrinking action level protocol (sal-protocol). After installing our third SGRT system in 2020 and a turnover of staff members, we received and did more training. First, we started with a vendor training on site. Online anonymous questionnaires were send out to all RTT`s before this training. A member of the SGRT core team did then a second training session on the treatment machine. Here the results of the questionnaires helped us to know what the training sessions focus needed to be on. After the second session we sent out the same questionnaire again, the results showed that these trainings improved the self-assurance and understanding of the system for our staff members a lot. Currently the SGRT is routinely used to setup and gate breast DIBH treatments, setup and monitor SBRT treatments and its use will be extended to tattoo-less set up of patients at multiple sites. Symposium: Surface-guided radiotherapy