ESTRO 2023 - Abstract Book

S1478

Digital Posters

ESTRO 2023

particularly for dosimetrically sensitive sites such as the lung, and dosimetric criteria may not fully capture the impact on outcome.

PO-1759 Verification of ZAP-X treatment plans with the Octavius 1600 SRS detector array

K. Büsing 1 , J. Harmsen 2 , P.D. Klassen 3 , H.K. Looe 1 , B. Poppe 4 , D. Poppinga 5

1 Carl-von-Ossietzky University, University Clinic for Medical Radiation Physics, Oldenburg, Germany; 2 Practice for Radiation Therapy Nordhorn-Meppen, Radiation Therapy Meppen, Meppen, Germany; 3 St. Bonifatius Hospital , ZAP-X Center, Lingen, Germany; 4 Carl-von-Ossietzky University, University Clinic for Medical Radiation Physics, Oldenburg , Germany; 5 PTW Freiburg, PTW, Freiburg, Germany Purpose or Objective The ZAP-X is a novel self-shielding system designed for stereotactic radiotherapy of brain lesions. The linear accelerator with 3 MV nominal photon beam is mounted gyroscopically on two axes allowing 4 pi radiation angle. The irradiation is performed with collimators between 4 mm and 25 mm in diameter. As with other radiotherapy modalities, the treatment plans require independent verification to ensure patient safety. However, this task is complicated by the use of multiple isocenters and the large number of non-coplanar beam angles. The aim of this work is to establish a clinical workflow for patient plan verification based on the liquid-filled ionization chamber array OCTAVIUS 1600 SRS (PTW Freiburg, Germany). Materials and Methods A planning CT was performed with the OCTAVIUS 1600 SRS array positioned between 3 cm RW3 solid water beneath and above is. The treatment plan to be verified was recalculated on this planning CT, where the target was placed in the middle of the array’s sensitive area, without changing the relative position of the isocenters to each other, the gantry positions, cone s and MU per beam. During the measurement, the array with the RW3 was placed on the patient couch using the same setup as in the planning CT. All measurements were performed in clinical mode, so that the workflow started with the patient positioning based on 2D kV images. Thereby, the setup with the array was automatically aligned to the first isocenter. For each isocenter, the treatment delivery was measured and verified separately. The patient positioning workflow was repeated between the isocenters to check the array alignment. The measured and calculated dose distributions were compared using global gamma-index criteria of 1 mm / 3%. Results By using the measurement setup in this study, a realistic clinical workflow including the automatic positioning using the 2D kV images is possible. For more than 70% of the measured isocenters, the comparison between the measured and calculated dose distribution resulted in a gamma-index passing rate of over 90%. Close inspections on the results of isocenters with lower gamma index passing rates indicated a directional dependency of the 1600 SRS array. Conclusion This study demonstrated the first clinical feasible workflow of patient plan verification using the high-resolution OCTAVIUS 1600 SRS array at the ZAP-X stereotactic platform. The initial results indicated the importance to account for the angle dependent detector’s response. In a further step, correction strategy taking this aspect into account will be studied. 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany; 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO , Heidelberg, Germany; 3 National Center for Radiation Research in Oncology NCRO , Heidelberg Institute for Radiation Oncology HIRO , Heidelberg, Germany; 4 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany; 5 University Hospital Heidelberg, Radiation Oncology, Heidelberg, Germany Purpose or Objective To investigate the requirements for safety, accuracy and quality assurance in stereotactic irradiation of multiple brain metastases with a single isocenter, simple and highly reproducible production of polymer dosimetry gel (PAGAT) is required. The aim of this work was to develop a cost-effective system that allows producing the polymer gel in reproducible quality with simple handling. Materials and Methods The production of PAGAT gel has been described in detail in Elter et. Al. [1]. The developed apparatus (Fig.1) consists of two reactors, in which anchor stirrers are installed that are suitable for mixing highly viscous media. A water bath originating from the gastronomy sector is used to heat the two reactors. A BBQ thermostat equipped with four thermocouples is used to control the temperature within 32° C and 42°C. The reactors are equipped with lids that have four access points through which connections for nitrogen supply, pressure control, insertion of temperature sensors and a connection of both reactors are feasible. The substrates to be mixed together later are prepared in the reactors. Reactor 1 contains the pork gelatin and reactor 2 the monomer solution of the two acrylamide substances. After preparation, the reactors are placed in the preheated water bath and closed with the lids. After the substrates have dissolved in both vessels, the acrylamide solution is transferred to the gelatin by means of negative pressure and mixed there for some time. After driving out the oxygen with nitrogen and then applying a negative pressure of 800mbar, a most of of the dissolved PO-1760 A low budget synthesis apparatus for reproducible production of dosimetry gels A. Runz 1,2 , R. Eboue Teto 1,3 , F. Dinkel 1,4 , W. Johnen 1,4 , A. Elter 1,4,5 , S. Dorsch 1,4,5 , G. Echner 1,4 , C.P. Karger 1,4