Abstract Book

S1105

ESTRO 37

the 6 MV VMAT FFF in the treatments to decrease the time of treatment. EP-2023 Commissioning of a surface imaging system for beam gating at an Elekta Linac V. Batista 1,2 , S. Albert 3,4 , B. Rhein 1,2 1 Heidelberg University Hospital, Clinic for Radiation Oncology and Radiation Therapy, Heidelberg, Germany 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany 3 Duale Hochschule Baden-Württemberg, Radiation Protection, Karlsruhe, Germany 4 Studsvik GmbH & Co.KG, Radiation Protection, Pforzheim, Germany Purpose or Objective Align RT is a clinical system used to detect the respiratory motion through the detection of breathing changes in the patient’s surface. Currently, the system is used for patients under deep-inspiration breath-hold. The aim of this work was to evaluate the system performance and limitations for respiratory beam-gating, either for volumetric modulated arc therapy (VMAT) or three- dimensional conformal radiotherapy (3DCRT). Material and Methods The measurements were carried out using the CIRS Dynamic Thorax Phantom at the Elekta VersaHD linear accelerator (Linac). The phantom provides a repeatable three-dimensional target motion inside a tissue equivalent phantom and a surrogate arm gives the external motion to be detected with AlignRT. Inserts for an ionization chamber and a radiographic film were used to measure the delivered dose in the static, no-gating and gating schemes. The consignment and validation of the respiratory gating was subdivided into four parts: 1) safety and precision of Align RT; 2) evaluation of the configuration parameters that affect the respiratory gating irradiation; 3) performance of the system for VMAT and 3DCRT plans; 4) definition of a patient verification quality assurance (QA) workflow. Results The combined use of AlignRT with Elekta Linacs showed to be feasible and led to an improvement of the delivered dose distribution (see figure 1 and 2). However, in order to achieve a robust, clinical significance and accurate treatment delivery some aspects need to be considered: a) stability of the system during long acquisition periods, b) gantry angles where the acquired signal is strongly affected by the limited coverage of some camera; c) for small breathing magnitudes the signal detection accuracy is reduced; d) for irregular breathing patterns the response time of the system affects the irradiation within the gating-window; e) the defined Region-of-interest should be large enough to result in a statistical relevant measure; f) for a VMAT irradiation the gantry speed should be reduced to include the system response time; and g) the relevance and definition of beam-gating for each patient need to be pre-treatment evaluated through a patientspecific QA.

Conclusion AlignRT with an Elekta Linac showed to be suitable and might result in an improvement of the delivered dose distribution. However, a commissioning program of the complete system (TPS, Linac, and AlignRT) should be conducted to identify the limitations and to define clinical protocols. EP-2024 4D evaluation of proton pencil beam scanning and double scattering for hepatocellular carcinoma T. Pfeiler 1,2 , C. Bäumer 1 , O. Blanck 3,4 , M. Chan 3,5 , E. Engwall 6 , D. Geismar 1,7,8 , S. Peters 1,7,8 , B. Spaan 2 , J. Wulff 1 , B. Timmermann 1,7,8 1 West German Proton Therapy Center Essen WPE, Essen, Germany 2 TU Dortmund University, Experimental Physics 5, Dortmund, Germany 3 University Clinic Schleswig-Holstein, Department of Radiation Oncology, Kiel, Germany 4 Saphir Radiosurgery Center, Güstrow and Frankfurt, Frankfurt, Germany 5 Imperial College London Healthcare Trust, Department of Radiation Physics, London, United Kingdom 6 RaySearch Laboratories AB, Stockholm, Sweden 7 University Hospital Essen, West German Cancer Center WTZ, Essen, Germany 8 University Hospital Essen, Clinic for Particle Therapy, Essen, Germany Purpose or Objective In proton therapy there is a rising trend towards pencil beam scanning (PBS) for the treatment of moving tumors. Contrary to passive delivery techniques such as double scattering (DS), active PBS offers a high conformal dose distribution both at the distal and proximal edge of the target, enables intensity modulation and does not require patient specific hardware. However, it suffers from the interplay of organ motion and pencil beam motion which might lead to serious distortions of the intended dose distribution. Modeling of interplay effects becomes thus a crucial ingredient of motion management in PBS [1]. In this study we investigate 4D dynamic accumulated dose distributions (4DDs) of PBS and DS plans regarding target coverage and sparing of organs at risk (OARs) for hepatocellular carcinoma (HCC), representative for targets with moderate motion amplitudes. Material and Methods 4D robust optimized PBS plans and DS plans with beam specific PTVs were created in RayStation 5.99 for 7 HCC patients. 2 mm setup error, 5% range uncertainty and 10