ESTRO 37 Abstract book

S1216

ESTRO 37

1 Heidelberg University Hospital, Radiation Oncology, Heidelberg, Germany 2 German Cancer Research Center, Medical Physics in Radiation Oncology, Heidelberg, Germany 3 German Cancer Research Center, Radiology, Heidelberg, Germany 4 German Cancer Research Center, Radiation Oncology, Heidelberg, Germany Purpose or Objective This prospective study aimed to evaluate the feasibility of a shuttle-based MRI-guided radiation therapy (MRgRT) in the treatment of pelvic malignancies. Material and Methods The study included 20 patients with pelvic malignancies who received daily MRI in treatment position prior to RT. Feasibility analysis comprised an assessment of the patient compliance, positional inaccuracies caused by the shuttle transfer between the MR unit and the treatment room and the time required for the workflow. The limited space in the MR scanner influenced the selection of patients, as the bore of the scanner had to accommodate the immobilization device and the constructed stereotactic frame. As a consequence, very tall, obese or extremely muscular patients could not be enrolled in this trial. Results MRI for position verification could be performed without any problems in 78% of applied radiation fractions. Moreover, in only 9% of applied radiation fractions, MRI was omitted due to patient compliance issues and treatment-related side effects. The entire study workflow required a median time of 61 minutes (min) (range 47-99 min), whereby the median duration for RT alone was 13 min (range 7 – 26 min). Patient positioning, MR and CBCT imaging including potential patient repositioning and the shuttle transfer took median times of 10 min (range 7 – 14 min), 26 min (range 15 – 60 min), 5 min (range 3 – 8 min) and 8 min (range 2 – 36 min), respectively. To assess feasibility of shuttle-based MRgRT, the reference point coordinates for the x, y and z axis were determined for the MR images and CBCT obtained prior to the first treatment fraction and correlated with the coordinates of the planning CT. Therefore, MR and CT images were matched based on fiducials which showed median positional differences of 2 mm or less in all spatial dimensions. Conclusion The presented shuttle workflow proved to be feasible within certain limits to realize off-line MRgRT of pelvic malignancies. EP-2196 Component beam results of a versatile and compact linear accelerator for proton therapy G. De Michele 1 , G. D'Auria 1 , A. Degiovanni 1 , J. Farr 2 , M. Baelen 3 , A. Jeff 1 , M. Caldara 1 , Y. Ivanisenko 1 , S. Ballestrero 1 , M. Esposito 1 , R. Moser 1 , H. Pavetits 1 , F. Cabaleiro 1 , P. Stabile 1 , M. Giunta 1 , C. Mellace 1 1 ADAM SA, Accelerator technologies, Geneva, Switzerland 2 ADAM SA, Medical Physics, Geneva, Switzerland 3 Advanced Oncotherapy plc, Regulatory Affairs, London, United Kingdom Purpose or Objective A compact linear accelerator (LINAC) operating at 3 GHz has been built for proton therapy. Material and Methods Components of a proton therapy LINAC were modeled using multi-particle tracking code (TRAVEL, PATH) and 3D multi-physics codes (CST, Ansys). The components included the first high-frequency Radio-Frequency Quadrupole (RFQ) ever built and a Side-Coupled Drift Tube LINAC (SCDTL). The RFQ and SCDTL were evaluated for proton energy acceleration, beam current, and

emittance. The proton energy measurements were performed with a spectrometer, and a time of flight system. The beam current and emittance were measured with a two-slit aperture onto a Faraday cup. Results The RFQ was tuned to 750 MHz and the SCDTL to 3 GHz. Protons were injected into the RFQ at an energy of 40 keV and were accelerated to 5 MeV with a 50 µA beam current (Fig 1) and < 1 pi-mm-mrad emittance (Fig 2). The high current and small emittance values measured are characteristic of LINACs. A single SCDTL module directly coupled to the RFQ produced an additional 2.5 MeV energy gain, resulting in 7.5 MeV total. The resulting energies were in very good agreement (within few percent) with the simulations results. The testing of additional, sequential SCDTLs to increase the total energy is currently underway, and the latest results will be presented.

Fig 1: Comparison of measured and expected beam profiles at the spectrometer profile monitor.

Fig 2: Comparison of measured and expected emittances in vertical and horizontal planes. Conclusion A new high-frequency LINAC based proton therapy system has been developed, installed and tested with beam up to 7.5 MeV proton energy. The measured beam acceleration, current, and emittance match with simulations studies. The combination of high flux and small emittance suggests application to highly conformal, as well as hypo-fractionated proton therapy. This step has been crucial for the validation of two out of three types of accelerating structures which compose the LIGHT (LINAC for Image Guided Hadron Therapy) accelerator. EP-2197 Dose normalization in lung SBRT based on ICRU 91 and comparison to alterative normalization methods S. Tanadini-Lang 1 , L. Wilke 1 , K. Karava 1 , N. Andratschke 1 , S. Ehrbar 1 , M. Guckenberger 1 1 University Hospital Zürich, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective There is a large variability in dose prescription and normalization methods in stereotactic body radiation therapy (SBRT) for lung cancer. Historically an inhomogeneous dose was prescribed to a certain isodose- line enclosing the planning target volume and the normalization was done on the maximum dose (in